CSEC GEOGRAPHY

Oral Johnson
CSEC GEOGRAPHY
Prepared by: Oral Johnson

Oral Johnson
INTERNAL FORCES
The layers of the Earth
The earth is made up of three main layers: the core, the mantle and the crust. These layers
become denser towards the center of the Earth. Density is the degree of compactness, which
increases with depth as a result of higher temperature and greater pressure.
The crust is the thinnest layer of the Earth. The crust is usually between 10km and 60km thick.
The crust thickness is often referred to as the relative thickness of an apple skin (when compared
to the size of an apple). There are two types of crust, oceanic and continental. Continental crust
is made up of silica (Si) and aluminium (Al) while oceanic crust is made up of silica (Si) and
Magnesium (Ma). Continental crust is called (SIAL) and oceanic crust is called (SIMA).
Oceanic crust is between 6 and 10 km thick. Continental crust can be up to 70km thick. Oceanic
crust is denser than continental crust.

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The mantle is the thickest layer of the Earth at 2,900km thick. It makes up nearly 80% of the
volume of the Earth. The mantle itself is divided into 2 layers, the upper and lower mantles. The
mantle is often described as being semi-solid or molten. Here we have magma that flows slowly
due to the convection currents. The rocks in the upper mantle are cool and brittle enough to
break under stress. Rocks in the lower mantle are hot and soft and flow rather than break.
Differences in behaviour separate the upper from the lower mantle.
The upper most part of the mantle and the entire crust makes up the rigid lithosphere. Below
the lithosphere is a more mobile lower layer called the asthenosphere. At the centre of the earth
is the core. The outer core is made of liquid iron and nickel. Heat from the core powers the
convection currents in the mantle. The inner core is the hottest part of the Earth reaching
temperatures between 4,000-4,700°C, which are as hot as the surface of the sun. It contains the
centre of the earth which is about 6,378km from the surface. It is made of solid iron and nickel
that are under so much pressure they cannot melt.
The crust is very thin compared with the diameter of the Earth as whole. If a guava represented
the earth, the skin of the guava would be about the thickness of the crust. However, the crust is
not a continuous layer like the skin of a guava. Instead it is broken up into a number of large and
small segments known as plates. The word tectonics comes from Greek; it means ‘building’. So
plate tectonics means ‘plate building’.

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History of theories
For most of human history people had no idea that the positions of the continents had slowly
changed over time. However, in 1912, Alfred Wegener published his theory of continental drift.
He said that the continents had slowly drifted apart from one super-continent called Pangaea
which existed 200 million years ago. The evidence for this included:
 The fit of continent- the ‘jigsaw’ effect
 Similar plant (India and Antarctica) and animal (South Africa and Brazil) fossils found in
neighbouring continents now separated by water
 Rocks of similar type and age found at the edges of continents that could have once fitted
together.
The American Harry Hess then suggested that deep convection currents would force molten rock
to well up just under the crust. Eventually the increasing pressure would crack the crust and force
it apart. Research on rocks on the bed of the Atlantic Ocean in the 1960s supported Hess’s ideas.
It became clear that the newest rocks were in the centre of ocean at the underwater mountain
range known as the Mid-Atlantic Ridge, which is made up of volcanic rocks. The age of the
rocks steadily increases with distance from the Mid-Atlantic Ridge.

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In 1965 the Canadian geologist J. Wilson linked the ideas of continental drift and seafloor
spreading. He introduced the idea of moving belts and rigid plates which formed the basis of the
theory of plate tectonics.
Why plates move
Plates move because of what happens in the mantle below. The intense heat coming from the
earth’s core causes the magma in the mantle to move very slowly in giant convection currents.
These movements of magma are in places:
 Upwards towards the crust
 sidewards or horizontal to the crust
 downwards toward the core

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These very powerful convection currents cause the plates of the earth’s crust to move. Where the
movement is upwards plates are forced apart and new crust is formed. Where the movement is
downward plates are brought together and plate material may be destroyed. Plate movement is
usually continuous and it causes no problems on the surface of the earth. However, sometimes
movement can be very sudden, causing earthquakes. Most earthquakes are small and have little
effect on people. However, some are of great magnitude and have terrible consequences.
Global distribution of plate boundaries
There are seven very large or major plates:
 Pacific
 North American
 South American
 Eurasian
 African
 Indo-Australian
 Antarctic
The smaller plates include the Nazca, Cocos, and Caribbean plates

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Plates do not overlap. Where they are brought together by convection currents, once plate is
either is forced down into the mantle and destroyed or they are both pushed upwards to form
mountains. The SIMA which forms oceanic crust is denser than the SIAL of continental crust.
The continental curst is permanent.
In contrast, oceanic crust is always being formed in some places and destroyed in others.
Oceanic crust is therefore younger than continental crust. In Greenland the continental crust is
more than 3500 million years old but oceanic crust is nowhere older than 250 million years. The
formation of new oceanic crust and the destruction of old oceanic crust is in balance as the Earth
is neither shrinking nor expanding in size.
Types of plate boundaries
Transform plate boundaries
Two plates slide last each other at a transform plate margins (also known as conservative plate
boundary). Crust is either formed or destroyed nor there any volcanic activity. However major
earthquakes can occur. Usually the plates slide past each other very slowly without any impact
on the surface. But now and then the plates stick. When this happens huge pressure can build up.
If the pressure is released suddenly an earthquake occurs. The plate margin is therefore
conservative because crystal rocks are neither being destroyed or created.

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This has happened many times along the San Andreas Fault in California. In 1906 San Francisco
earthquake the surface moved by 6 meters. It measured 8.3 on the Richter scale. Over 450
people were killed and almost 30 000 buildings were destroyed.
Convergent plate boundaries
Convergent plate boundaries are also called destructive plate boundaries. This happen when two
plates move towards each other. There are three types of convergence:
 Oceanic-Continental
 Continental-Continental
 Oceanic- Oceanic
Oceanic-continental
When an oceanic and a continental plate collide the denser, oceanic plate subducts beneath
the lighter continental plate. A deep-sea trench or subduction occurs when the oceanic plate
is forced downwards into the mantle.
 The increase in pressure along the plate boundary causes the descending plate to
crack. This can cause large earthquakes.

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 The oceanic crust breaks up and melts to form new magma as it descends to great
depths. This is due to friction and the very high temperatures as it enters the mantle.
 The newly formed magma is lighter than the mantle. Some of it may rise to the
surface along lines of weakness in the continental crust
 If a lot of magma rises upwards volcanoes may be formed.
The same process happens when the two plates are oceanic plates. The difference is that the
older oceanic plate is going to be the denser one. There is still subduction and creation of
magma. However the newly formed magma will rise through an oceanic crust to form volcanic
island arcs (which are volcanic islands in water). The Windwards and Leeward Island are a
good example of an island arc.
The Peru-Chile trench is 8050 metres deep. The deepest in the world is the Mariana trench is the
west Pacific Ocean which is 11 022 meters deep.
Continental- Continental (Collision Zone)
Sometimes two plates of continental crust come together. This is called a collision zone. Because
continental crust cannot sink, the crust is forced upwards to form Fold Mountains. An example is
the formation of the Himalayas Mountains. Here the Indian plate is still moving into the Eurasian

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plate at 5cm a year. At times this movement causes major earthquakes. A long time in the past
the sea of Tethys lay between the two land masses. But as the land masses slowly moved
together the sea was squeezed out. The rock strata on the seabed were folded up to form
mountains. Marine fossils found high in the Himalayas prove that these rocks were formed on
the sea floor. The world’s highest mountains are in the Himalayas.
Divergent plate boundaries
Divergent plate boundaries (also known as constructive plate boundaries) occur when two plates
move away from each other. New crust is formed at the boundary as magma moves up from the
mantle below. When this happens underwater, it is described as sea-floor spreading. This
happens at a number of places around the world, for example along the Mid-Atlantic Ridge. This
huge underwater volcanic mountain range has been formed from magma coming from the mantle
below. The lava has an unusual rounded shape and is called a pillow lava. As it oozes out along
the plate boundary it cools quickly on the ocean bed. In places volcanic cones have built up
along the ridge.
Over time these submarine volcanoes may become large enough to reach the surface. This has
happened in Iceland in the North Atlantic Ocean. The main island of Iceland was formed a long

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time ago but in recent times two new small islands have appeared from below the sea. Surtsey
was formed between 1963 and 1967, and Heimaey in 1973. Other islands along the Mid-
Atlantic Ridge include the Azores, Ascension Island and Tristan da Cunha. Because of sea-floor
spreading the Atlantic Ocean is being widened by 2-5cm a year. Almost three quarters of the
lava that pours out onto the earth’s surface each year is found in mid-oceanic ridges. The other
major mid-oceanic ridges are:
 East Pacific
 Pacific Antarctic
 Atlantic-Indian
 Carlsberg
 Mid-Indian.
Where plates move apart on land, rift valleys are formed. In East Africa the African plate is
splitting to form the Great African Rift Valley. It extends for 4000km from the Red Sea to
Mozambique. Its width varies between 10 and 50 km and its sides are up to 600m above the
floor. This rift valley is possibly the start of the formation of a new ocean as east Africa splits
away from the rest of the continent.

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The table below summarizes the relationship between earthquakes and volcanoes and the
different types of plate boundary. Both can occur at convergent and divergent boundaries.
However, earthquakes and volcanoes are at their most violent at convergent plate boundaries.
Only earthquakes occur at collision zones and transform plate margins.
Earthquakes
Earthquakes result from a slow build- up of pressure along plate boundaries. This occurs where
the plates ‘stick’. If this pressure is suddenly released, a violent jerking movement may occur on
the surface. This is an earthquake. The point below the surface where the pressure is released is
known as the focus. The point directly above the focus on the surface is the epicenter. The
epicenter usually experiences the greatest shock or seismic waves. The vibrations due to seismic
waves cause both vertical and lateral movements. These movements can create faults and cause
partial or total destruction of buildings. The impact of an earthquake generally reduces with

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distance from the epicenter. The energy released by an earthquake, described as the magnitude,
is measured on the 10-point Richter scale. A large earthquake can be preceded by smaller
tremors known as foreshocks and followed by numerous aftershocks. The man earthquake may
last less than a minute but aftershocks can continue for several weeks afterwards. Following the
earthquake in Kobe, Japan in 1995, which was 7.2 on the Richter scale, hundreds of aftershocks
were recorded over a three-week period.

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VOLCANOES
The name of the Roman god of fire, Vulcan, gave rise to the English word ‘volcano’. A volcano
is a vent, or opening in the crust, from which pour molten rock, rock debris, gases and
steam. When magma penetrates the surface it is known as lava.
Most volcanoes are found along convergent and divergent plate boundaries. Here there is molten
rock or magma to supply the volcanoes. There are about 1300 potentially active volcanoes in the
world today. A small number of volcanoes are a long way from plate boundaries. These are
found at hot spots. Here the temperature at the boundary of the mantle and crust is unusually
high, and there are lines of weakness in the crust which the magma can follow to reach the
surface. The Hawaiian Islands, in the middle of the Pacific Ocean, have been formed in this way.
Hot spots can also be found beneath continents; an example is the Yellowstone Basin in the USA
Diagram showing a volcano
Crater: This is the opening at a top of a volcano through which lava, ash and gases emerge.

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Conelet / secondary cone/ parasitic cone: This is a small cone which often forms on the side of
an existing volcano.
Pipe: The tunnel like structure which links the vent at the base of the volcano to the crater at the
top.
Magma chamber: a reservoir of magma within the earth’s crust beneath a volcano.
Vent: This is an opening in the earth’s crust from which molten rock, gases, and rock debris
escape.
Ash cloud- A cloud of ash formed by explosive eruptions.
Stages of a volcano
Volcano usually pass through three stages in their life cycle. Volcanoes are:
 Active when they’re currently erupting or eruptions occur at frequent intervals.
OR
Oregon State University- An active volcano is a volcano that has had at least one eruption
in the past 10,000 years. For e.g. Kick em Jenny north of Grenada
 Dormant when eruptions are infrequent and one has not occurred for some time. They are
called sleeping volcanoes.
OR one that hasn’t erupted in the past 10,000 years, but which is expected to erupt again
 Extinct when it is not expected to erupt again.

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Types of Lava/Magma
There are two basic types of lava: basic and acidic.
Basic/Basaltic Acidic
Low silica content High silica content
Darker color Lighter color
It is often very hot reaching temperatures of
10000
C-12000
C
Often has lower temperatures (8000
C-
10000
C)
Rich in Iron and Magnesium Low in Iron and Magnesium
Low viscosity (very fluid and runny e.g. water High viscosity ( very thick e.g. molasses or
toothpaste)
Flows over long distance before solidifying Flows for a short distance then solidifies
Forms gentle sided cones or volcanoes e.g.
shield volcanoes
Forms high, steep and dumpy cones
Eruptions are frequent and gentle Eruptions are infrequent and very explosive
Lava and steam eruptions Ash, rock, gases and lava ejected, pyroclastic
flow likely
It is associated with divergent plate margins
and hot spot volcanoes
It is associated with convergent plate
boundaries ( subduction zones and island
arcs)
Formation of volcanoes
Sometimes magma reaches the earth’s surface through a vent or a fissure (elongated crack).
When magma emerges on the surface it is called lave. If lava emerges through a vent it builds a
cone shape mound (typical volcano we are associated with). Successive eruptions overtime will
build up the cone. If magma emerges through a fissure, it builds up a lava plateau.
A volcanic cone is made of either lava, or a mixture of lava and ash, or ash and cinders only
(small fragments of lava). There are three types of volcanic cone:
 Lava cone ( basic and acidic) – (ONLY LAVA)

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 Composite cone ( both lava and ash)
 Ash and Cinder Cone (ash alone)
Lava cones
Basic lava cones
Some volcanoes erupt a type of lava which flows very easily and tends to flow quite some
distance before solidifying. This is known as basic lava. This volcano is composed of basic lava
which spreads over a wide area before solidifying. This type of lava tends to form broad
volcanoes with very gentle slopes. An example is Mauna Loa on Hawaii. Since these volcanoes
resemble a warrior’s shield, they are called shield volcanoes.
Acidic Lava
This lava cone is made up entirely of acidic lava. Because the lava has a high viscosity it flows
for a short distance then solidifies. Acidic lava produces steep sided and dumpy cones.

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Ash and Cinder Cone
In a violent volcanic eruption lava can be thrown to great heights where it cools and breaks into
small fragments of lava known as volcanic ash/cinders. The ash falls to the surface building up a
cone. Successive eruptions will build the cone overtime. An example is Paracutin in Mexico

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Composite cones/Stratovolcanoes
Some volcanoes produce different types of eruptions. Sometimes there are massive ash eruptions
which produces layers of ash. At other times there are eruptions of lava which produces layers of
lava. The ash is the result of a violent eruption while the lava is produced by more gently
eruption. The result of a series of eruptions is a steep sided cone composed of successive layers
of ash and lava. This type of volcano is known as a composite cone. Composite cones are found
near convergent plate boundaries. The Soufriere Hills volcano in Montserrat is an example of a
composite cone.

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Extrusive volcanic features
When lava cools and solidifies on the earth’s surface it forms extrusive volcanic features.
Examples include ash and cinder cone, basic lava cone, acidic lava cone, composite cone caldera,
spine, lava plateau and spine. These are all landforms are features above the earth’s surface.
Caldera
A volcanic eruption may be so explosive that the whole top of the volcano sinks into the magma
below. A huge crater is left which may by many kilometers in diameter. Later eruptions may
form new cones inside a caldera. Lakes may form in the lowest parts of a caldera. An example is

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Krakatoa in Indonesia. The Qualibou caldera in St. Lucia is 3.5 km x 5 km in size and was
formed more than 30,000 years ago. The town of Soufriere is located inside of it.

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Lava plateau/ Basalt plateau
Lava does not always reach the surface in a spectacular way. Sometimes large quantities of lava ooze out
slowly onto the surface from fissures (surface cracks). These fissure eruptions can cause lava to spread
out over a very wide area. Over time, a number of fissure eruptions in the same area can build up a high
plateau. Basalt plateaux are very large features, covering hundreds of thousands of square
kilometers and they usually “drown” the pre-existing landscape. In India the Deccan is a lava
plateau which covers almost 650 000 km2
Volcanic Spine
Sometimes acidic lava is so viscous that it cools and solidifies in the crater to form a spine or plug.
However spines are rare because they often break up rapidly on cooling.

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Intrusive volcanic features
Only a small amount of the magma that moves up from the mantle and through the crust reaches the
surface. Most magma cools and solidifies (hardens) before it reaches the surface. As the magma moves
upwards it forces its way into lines of weakness in the rock. Bedding planes, joints and faults are all lines
of weakness followed by the magma. Once magma gets into a crack in the crust the huge force behind it
can cause the crack to widen.
Although intrusive volcanic features are formed underground, they may be exposed millions of years later
if the rocks at the surface are eroded. Because volcanic rocks are hard they are often more resistant to
erosion than the rocks around them and they stand out in the landscape as higher ground.
 A dyke is formed when magma moving upwards towards the surface cool and solidifies. The
magma cuts across the bedding planes of sedimentary rock. Sometimes a large number of dykes,
called a dyke swarm, can occur together in an area. An example is the Moule a Chique headland
found at the southern tip of St Lucia.
 A sill is formed when magma flows horizontally between rock layers, roughly parallel to
the surface. This molten material cools forming a horizontal sheet of solid rock called a
sill. This may be horizontal or angled towards the surface.

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 A laccolith is the result of large amounts of magma moving between rock layers causing
overlying rock strata to arch upwards.
 A bathlolith is much larger than the other intrusive volcanic features. It forms when a
giant underground reservoir of magma cools and hardens to form granite. Batholiths can
be several hundred kilometers in diameter. A batholith may form the roots of a mountain.
A good example in the Caribbean is the Tobago Batholith

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Positive and negative effects of volcanic activity
Positive effects
Fertile Soils
Volcanic materials produce fertile soils. Ash and cinder are natural fertilizers as they are rich in
minerals. As lava cools and is eroded by the elements it also adds to soil fertility. For this reason
agriculture often thrives in volcanic regions, e.g. near Mt Vesuvius, which is an important
vegetable production are in southern Italy.
Land surface
Erupting volcanoes are thought to be the source of the first land as the hot planet cooled.
Eruptions continue in the sea creating new land. All the volcanic islands are created in this way.
There are so many islands that were created as a result of volcanic activity. The Eastern
Caribbean islands, the Aleutian Islands and the Marianas islands in the pacific are all volcanic
island arcs and were formed as a result of volcanic activity
Minerals
Many useful materials are formed directly or indirectly from volcanic activity. Building
materials such as granite and marble, precious minerals such as diamonds as well as silver and
copper are formed from magma and in and on the earth’s crust. Other mineral resources such as
gold, silver, nickel, copper, and lead are sometimes found around volcanic activity.
Geothermal energy
Geothermal energy production in Iceland is another positive of volcanic activity. Iceland is on
the Mid-Atlantic ridge (plates separating) and for this reason has a lot of volcanic activity.
Magma rises close to the surface of the crust and this heats the groundwater. This water is heated
to well beyond boiling point (up to 200 degrees Celsius) and becomes “super-heated”. Wells are
drilled into the rock and the hot water is pumped out. As this hot water reaches the surface it
does so as steam due to the intense heat. This steam is then used to drive turbines and create
electricity

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Tourism
Volcanic peaks. Whether active or formant are natural tourist attractions in many parts of the
world. From the majestic Mt Fuji in Japan to the ever flowing Hawaiian cones, tourist visit and
wonder at this evidence of the earth’s interior. Volcanoes are also important tourist attractions.
Mount Vesuvius in Naples and Mount Etna in Sicily are major tourist attractions in southern
Italy. In Iceland, the geysers and hot springs caused by volcanic activity bring many tourists to
the island. This tourism generates jobs and money in areas that may not have many other sources
of employment. Jobs are generated in areas such as accommodation, transportation, sight-seeing
and retail (shops).Geysers are also used as tourist attractions such as the Yellowstone National
Park.
Building Construction
When lava/magma is cooled it form igneous rocks. Some igneous rocks such as granite are being
used in the construction industry for thousands of years. Granite is very durable and strong
igneous rocks that is used for all kinds of structures. Because pumice is so light it is used quite
often as a decorative landscape stone. Basalt is also quarried in some part of the world and is
also widely used in the construction industry.
Negative Effects
Poisonous gases - Although the predominant gas erupted from volcanoes is H2O vapour, other
gases are erupted can have disastrous effects on life. Some of these gases are hydrogen sulfide
(H2S), sulphur dioxide (SO2), hydrogen fluoride (HF), carbon dioxide (CO2), and hydrogen
chloride ( HCL). Gases such as carbon dioxide, carbon monoxide, and sulfur dioxide can travel
down a volcano and asphyxiate (suffocating) wildlife and humans.
Lava flows
A river of molten rock 1000 degrees centigrade that can travel at 40mph. lava flows
can cause extensive damage or total destruction by burning, crushing, or burying everything in
their paths. Lava flows can erupt relatively non-explosively and move very slowly (a few meters
to a few hundred meters per hour) or they can move rapidly (typically down steep slopes.

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Tephra
All materials ejected from a volcano are called Tephra. These occurs when there is an
explosive eruption. They are classified according to their sizes. Materials the sizes of a football
can be ejected from a volcano. Ash, lapilli and volcanic bombs are tephra. The largest pieces of
tephra (greater than 64 mm) are called blocks and bombs often fall close to the volcano but
smaller size tephra such as ash and lapilli ( lapilli 2-64 mm) and ash (<2 mm) can be carried
downwind and affect areas far from the volcano. Millions of tons of ash can bury buildings.
Problems associated with tephra
 If ash builds up on the tops of roofs, it will often cause collapse. This is especially
common on flat topped buildings. Most deaths resulting from the eruption of Mount
Pinatubo in 1991 were due to collapsing roofs (Wolfe, 1992).
 Ash can disrupt electricity, television, radio, and telephone communication lines, bury
roads and other manmade structures, damage machinery, start fires, and clog drainage
and sewage systems
 Ash is also a great hazard to airplanes. Ash from the 1982 eruption of Galunggung
Volcano in West Java, Indonesia caused engines in two jet airplanes to fail. Both aircraft
dropped 25,000 feet before they could get their engines to start again.
 Tephra can also destroy vegetation which can result in famine. Famines are the largest
indirect hazard produced by volcanic eruptions. In 1815, after the eruption of Tambora
which ejected 151 cubic kilometers of ash into the atmosphere, 80,000 people died due to
famine (Bryant, 1991 and Francis, 1993)
 Ash can produce poor visibility and cause respiratory problems.
Pyroclastic flows
Pyroclastic flows are very hot, fast moving clouds of gases and

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tephra moving down the side of a volcano after an eruption column collapse. They are
also called nuée ardentes. They are extremely dangerous because they can travel up to
500 km/hr., reach distances of 30km and can be over 700 degrees Celsius in temperature.
They will burn, knock over or bury anything in their path. A pyroclastic flow from
Vesuvius volcano killed about 20,000 people in Pompeii in 79 CE.
Lahars (resembles wet concrete)
A volcanic eruption usually leaves lots of loose unconsolidated fragmental debris. When this
loose material mixes with water from rainfall, melting of snow or ice, or draining of a crater lake,
a mudflow results. Volcanic mudflows are called lahars. These can occur accompanying an
eruption or occur long after an eruption. Lahars are very dangerous because they do not require a
volcanic eruption yet can travel hundreds of miles. All that is required is loose pyroclastic
material on the volcano that mixes with precipitation or melting snow. In general, they destroy
anything in their path, carrying away homes, buildings, bridges, and destroying roads, and killing
livestock and people.

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TYPES OF ROCKS
Rocks
A rock is any hard, naturally occurring substance which is composed of minerals and which is
formed by geological processes. A mineral is a substance which is normally crystalline and is
formed by geological processes. Crystalline means the atoms forming the crystals are arranged in
a definite manner. Some minerals are non-crystalline, i.e. the atoms forming the mineral are not
arranged in any definite order. Most minerals are compounds of several elements, e.g. Silica
(SiO). A few minerals are themselves elements. A geological process is any natural process
which modifies (changes) geological features. People who study rocks have found it useful to
classify them according to the processes by which they were formed. There are three major ways
in which rocks are formed and hence there are three major classes of rock; igneous rocks,
sedimentary rocks and metamorphic rocks.
Igneous Rocks
These rocks are formed when molten rock from deep within the Earth’s crust (magma) finds its
way into or onto the Earth’s crust where it cools and hardens. They do not occur in layers and
most of them are crystalline (contain crystals)
Intrusive igneous rocks
When magma cools and hardens within the Earth’s crust the resulting rocks are called intrusive
igneous rocks. Intrusive igneous rocks are also called plutonic igneous rocks. Because these
rocks were formed by the slow cooling of magma, they often contain large crystals, making them
coarse grained. Examples of intrusive igneous rocks are granite and gabbro. These igneous are
usually made up of large crystals which can be seen with the naked eye. They only reach the
earth surface when the rocks above them are eroded.

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Extrusive igneous rocks
When magma reaches the Earth’s surface it is known as lava. This lava cools and hardens
quickly, forming what are known as extrusive igneous rocks or volcanic igneous rocks. These
rocks are usually fine grained as the lava cools too quickly to allow large crystals to develop.
Basalt and obsidian are examples of extrusive igneous rocks. The crystals are usually so small
that is almost impossible to see with the naked eye. Obsidian (also known as volcanic glass) is
formed when lava cools so quickly that crystals are unable to form.
Sedimentary rock
These rocks are formed when material which has been deposited by agents such as rivers is
compacted over time until it hardens into rock. This usually happens in bodies of water such as
the sea. The material is deposited in layers and these layers are often clearly visible in the
resulting rock. Examples of sedimentary rocks include limestone and sandstone. All sedimentary
rocks are non-crystalline (contains no crystals). However they usually contain fossils. There are
three main groups:
 Mechanically formed or clastic sedimentary rocks- They are made from pieces of other
rocks damaged by weathering and erosion. They form when layers of sediment
containing this debris accumulate and cement together into a sedimentary rock. Examples
include: Breccia, siltstone, conglomerate, sandstone and shale
 Chemical sedimentary rocks are made from tiny particles of minerals that precipitate
from a liquid in which they were originally dissolved. Limestone forms when calcium
precipitates out of water to form a layer of tiny particles that eventually cement together
to form rock. Examples include: rock salt, iron ore, chert, flint, some dolomites and some
limestones

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 Organic sedimentary rocks are form the accumulation of plant and animal remains.
Examples from animal accumulation include chalk and coral. Examples from plants are
peat, lignite and coal
Metamorphic rock
When igneous or sedimentary rocks are subjected to intense pressure and heat over a long period
of time, their structure and mineral composition may change a great deal. These rocks
metamorphose into a completely different type of rock. The heat and pressure required to bring
about this change in the rock exists far below the earth’s surface. Examples of metamorphic
rocks are marble (which is metamorphosed limestone) Examples
-Slate from clay
-Gneiss from granite
-Quartzite from sand
-Graphite from coal

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EXTERNAL FORCES
Weathering
Everywhere in the world rocks are worn away by many different processes. Depending on the
exact location the processes that operate vary because they are affected by climate, rock type
relief and slope angle. For example, rock types are different in hardness (resistance); the more
resistant ones wear away more slowly than softer ones. The hotter and wetter the climate the
faster weathering and erosion happen
Denudation
Denudation simply means the wearing away of the land by weathering and erosion. It includes
all natural agencies, for example sun, rain, frost, wind, rivers, sea, ice, temperature change and
even the actions of plants and animals. This set of major processes is responsible for the creation
of the Earth’s varied landscapes.
Weathering
Weathering is the wearing away (disintegration and decomposition) of rocks by the effects of the
weather and the atmosphere. No movement is involved in this, so the breakdown of the rock is
said to be ‘in situ’- in other words, ‘in that place’. Sometimes, after the break-up of the rock,
fragments are moved but only by gravity, for instance slipping down a slope.
Erosion
Water, ice and wind also wear away the earth’s surface. Water can mean either rivers or the sea.
Ice is in the form of glaciers. Wind erodes especially when it is carrying something to help it
wear away rock, usually sand. (Imagine how a blast of wind carrying sand would feel against
your skin!).
The difference between erosion
The key difference here is movement. In the case of weathering, no movement is involved
(remember the term in situ). The agent that weathers the rock does not move the debris. Any
movement of loose fragments happens due to gravity. However, in erosion, the agent breaking
up the rock also removes the debris. For example, the sea attacks cliffs and moves the fragments

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out to sea or along the beach. A glacier erodes the rock it moves over and carries fragments
within it, depositing them many kilometers away when it melts.
There are three different types of weathering:
 Physical ( or mechanical) weathering
 Chemical weathering
 Biotic ( or biological) weathering
Physical weathering
Physical weathering – This involve rocks breaking apart but without any chemical change taking
place.
Frost shattering/Freeze-Thaw
Globally, frost shattering, or the freeze-thaw process, is the most important and widespread type
of weathering, although it is less common in the Caribbean due to this region’s tropical latitude.
Only the highest areas could be affected. For this type of weathering to occur the temperature
must fluctuate (change or vary) either side of 00C. There must also be bare rock exposed at the
surface, with little vegetation cover to protect the rock from the weather conditions.
If it rains during the day, or there is moisture from dew or melting ice or frost, water can trickle
into cracks, crevices or pores in the rock. Daytime temperatures are more than 00C. If at night
the temperature drops below zero, the water in the cracks or pores freezes. Ice take up 9% more
space than equivalent amount of water and so exerts pressure on the rock. Repeated freeze-thaw
conditions continue throughout the winter in the cool temperate zones of the world, slowly
widening the cracks until pieces of rock break-off. Frost shattering is most likely to happen on
steeper bare rock slopes, so the broken pieces will slip downhill easily under the force of gravity.
They collect at the bottom of the slope in a fan-shaped pile. This is called scree or talus. Where
the land is more level then boulders ad smaller stones litter the surface. These too have been
broken off by frost shattering. They are known as blockfields or felsenmeer.
In colder regions frost shattering is less common, as temperatures are too low; the fluctuation
around 00C does not happen very often. It rarely becomes warm enough, even during the day,
for melting to occur. In Polar Regions it is very unusual for temperatures even to rise above zero.

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Temperature changes
Rocks expand when heated and contract when cooled. Regular heating and cooling occurs in
some climates. In hot deserts the diurnal range of temperature (that is temperature differences
within a 24-hour period) can be as much as 500
C
At midday over 400C is not uncommon, but at night temperatures may drop below freezing
(O0
C). The outside layer of rock therefore expands and contracts regularly. This weakens it until

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eventually it peels off. This process is known as exfoliation or onion peeling. After one layer has
broken off the next one is attacked by temperature change.
Rocks are made up of different minerals such as granite. These minerals can expand and contract
at different rates which causes the rock to disintegrate- a process known as granular
disintegration.
Pressure release
Pressure release also results in exfoliation, but here the cause is different. Rocks deep
underground, such as igneous intrusions for e.g. granite, have the weight of other rocks above
pressing down on them. The granite of Dartmoor in south-west England was formed 6km deep
below the surface, so had a huge mass of rock pressing down on it. Such pressure sets up stresses

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within the lower rock. Over time, the upper rocks are weathered and eroded away, so the
pressure is reduced and the stresses are less. The rock expands as joints and other cracks open
up, almost as if they were breathing a sigh of relief! This widening of cracks is called dilation.
The cracks formed are roughly parallel to the surface, so the layers of rock peel away one at a
time, like the skin of an onion. This is called exfoliation. Pressure release is responsible for
causing large-scale rounded landforms, such as Half Dome in Yosemite National Park, USA. In
the Caribbean, pressure release has been partly responsible for some of the granite landforms on
Tobago.

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Chemical Weathering
Chemical weathering is where a chemical change causes rocks to dissolve or decay.
Carbonation
Carbonation is the most important form of chemical weathering in limestone areas, whether they
are in tropical or temperate zones of the world. Rainwater containing carbon dioxide absorbed
from the atmosphere is able to dissolve calcareous (calcium–based) rocks. Carboniferous
limestone and chalk are the types of limestone affected most because they are made of almost
pure calcium carbonate.
Here are the stages of the process:
1) Rainwater dissolves carbon dioxide from the air as it passes through it
H2O + CO2 = H2 CO3
Rain water + Carbon dioxide = Mild carbonic acid
2) The mildly acidic rainwater reacts chemically with the calcium carbonate in the rock
H2 CO3 + CaCO3 = Ca (HCO3)2
Mildly acidic rainwater + calcium carbonate = Calcium bicarbonate
3) Calcium bicarbonate is soluble in water so it is easily washed away
The higher temperatures in tropical areas increase the speed of carbonation. Chemical reactions
are faster as temperature increases. Higher rainfall also plays a part because the rain is the
weathering agent.
Few minerals are soluble in pure water. It is important to understand than it is the weak acid that
allows this natural process of carbonation to take place. However, the carbon dioxide in
rainwater is not the only source of acid that can attack limestone. Any acid will attack calcium
carbonate, including those coming from industrial pollution. Nitrates and sulphates make nitric

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and sulphuric acids and both of these can dissolve in rainwater too. The carboniferous limestone
of the Pennine Hills in Northern England may have been dissolved faster due to the pollution
from the nearby industrial areas in that part of the country.
The Cockpit country in Jamaica is a world-renowned Limestone area. Features such as cockpits,
cones, stalactites and stalagmites were formed due to carbonation. The high annual rainfall and
high temperatures combine to cause extremely rapid limestone weathering.
Solution
Solution simply means the dissolving of minerals in water. Rocks other than limestone can be
affected by solution weathering. Many minerals are soluble in water, especially if the water is
slightly acidic as explained in carbonation. The greater the acidity (the lower the pH) the more
effective the rainwater will be in dissolving minerals in rocks. Rock salt (halite) is one mineral
that is particularly vulnerable to this process and can be dissolved easily ‘in situ’.
Rocks made up of a mixture of different minerals can be weathered by one or more of them
being dissolved, leaving the rock structure weaker. Other agents of weathering and erosion can
then attack more easily.
Oxidation
Oxygen occurs when rocks are exposed to oxygen in air or water. It is chemical addition of
oxygen to compounds in the rock and it can weaken the structure of the rock. The process of
oxidation has a similar effect on iron minerals within rocks. The chemical changes turn the solid
iron minerals into powdery red or brown clay. This then causes the rock to crumble and break
apart. This weathering process is also known as rusting, and the rock discolours to the reddish-
brown colour seen on rusty metal.
Iron can combine with oxygen to make two different compounds, ferrous iron oxide (FeO) and
ferric iron oxide (Fe2O3). Ferric oxide clearly contains more oxygen per atom of iron than
ferrous oxide.
4FeO + O2 = 2Fe2O3
Ferrous iron oxide + oxygen = Ferric iron oxide

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Evidence of oxidation can be seen in many regions of the world, particularly in hot deserts and in
parts of the tropics where iron-rich rocks are readily weathered in the warm and wet conditions.
Hydrolysis
Hydrolysis is the chemical breakdown of a substance when combined with water. The prefix
‘hydro’ means ‘water’ and the suffix ‘ lysis’ means to ‘break down’. With chemical weathering
of rock, we see a chemical reaction happening between the minerals found in the rock and rain
water.
The most common example of hydrolysis is feldspar, which can be found in granite changing to
kaolinite clay. When it rains, water seeps down into the grounds and comes in contact with
granite rocks. The feldspar crystals within the granite react with the water and are chemically
altered to form clay minerals, which weaken the rock.
Biotic weathering
Biotic weathering refers to the role plants and animals play in breaking down rocks. Plants and
animals can promote both the mechanical and chemical breakdown of rocks.
Biotic (physical)
By plants
Plants can grow anywhere as long as there is water. Plant roots can enter joints or cracks in order
to find moisture and nutrients. As the tree grow the roots become larger. Overtime when the root
size increase it exerts pressure or force on the crack. Overtime the small joint/crack will become
wider and deeper. This will eventually cause the rock to break apart.
You may have observed before tree roots can split into sections or driveways sufficiently to raise
and crack the concrete.
By animals
a) Burrowing animals like rabbits, moles, earthworms and even ants can contribute to biological
weathering. These animals can move rock fragments to the surface. As a result, these fragments
become more exposed to other environmental factors that can further enhance their weathering.

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b) These burrowing animals also create holes which makes an easy passage of water and other
weathering agents deeper into the soil into the ground where it can cause further disintegration of
rocks.
Biotic (chemical)
Animals living on the surface of the rocks on the coast secrete chemicals that dissolve rocks.
Some micro-organisms get nutrients by taking minerals from rocks. By removing these minerals
the rock becomes weaker. An animal called the Piddock shell drill into rocks in order for it to
protect itself. The Piddock shell secrete an acid that helps to dissolve the rock. Overtime the rock
gets weaker due to this process.
Another example is Lichen. Lichen is fungi and algae living together in a symbiotic relationship.
The fungi release chemicals that break down the minerals in the rock. The minerals released
from the rock are consumed by the algae. Over time the rock becomes weaker due to this
process.

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MASS WASTING
Mass wasting (also known as mass movement) refers to the movement of weathered material
downslope because of the influence of gravity. Two of the major form of mass wasting includes
soil creep and landslide
Landslide
A landslide is a sudden movement of rock or soil downslope under the influence of gravity. They
can have devastating consequences on the environment and all who inhabit it. Landslides can
block roads and railways, destroy buildings or even bury entire villages. All slopes are affected
by gravity, so loose material will eventually slip or fall. On gentler slopes, around 5 degrees, soil
creep operates which is so slow that is has relatively little effect on human life, property or
activity. Landslides are natural events and would occur without people, but human activities do
increase the risks, scale and frequency of these hazards.
Physical causes of landslides
Unconsolidated material on the slope
In 1998 the rains from Hurricane Mitch caused devastating mudflows from the unconsolidated
ash slopes of the unconsolidated ash slopes of the Caista volcano which killed 1900 people and
destroyed the town of Posoltega in Nicaragua
On steep slopes
All slopes are affected by gravity, so loose material will eventually slip or fall. The steeper the
slope the likelihood of experiencing a landslide increases. Landslides usually occur on steeper
slopes while soil creep occur on gentler slopes.

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Type of rocks
Shake and clay are both slippery, especially when wet. Wet clay acts as a lubricant so rock beds
above it slip down more easily. Shale is made of several very thin layers, so it slides easily,
taking any rocks above with it
The block of limestone slides on clay as it becomes slippery when wet
Angle of bedding planes
Bedding planes being roughly parallel to the slope surface makes it easy for material to slip
downward along the bedding planes. Gravity can exert its force easily on the rock beds. If the
underlying rock layers are impermeable a rock is more susceptible to slide. This keeps all the
moisture in the top layer of rock so it becomes saturated quickly.
Erosion of the base of a slope by rivers or ocean waves (Basal cutting)
Slopes on a coast are subjected to coastal erosion. Erosion takes place at the base of the slope.
The base provides stability or support. If the slope above is made of unconsolidated or very wet

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material and this becomes unsupported it is much more likely to collapse. When a river valley
erodes it produce a similar effect.
Precipitation
Heavy rains can cause slopes to become saturated which further weakens the slope. When the
material becomes saturated it becomes heavier and friction is reduce. When this happens a
landslide can take place. This is why so many slides occur after heavy and short intense period of
rainfalls especially in the tropics from tropical storms or hurricanes. Water does two things:
 It adds weight to the material, making the slope less stable
 It decreases friction which helps movement downslope
The torrential rain from hurricane Mitch in October 1998 caused flooding and landslides which
affected 3 million people in the Caribbean side of Nicaragua and Honduras. .
Volcanic eruptions
When a volcano erupts it ejects several types of materials. These include materials such as ash,
tephra and bombs. When the ejected material especially ash is mixed with water/ice slurry like
liquid is formed. These are term lahars. Perhaps the best known example is the one that occurred
in Colombia in 1985, when 21 000 of the
22000 population of Armero died following the eruption of the volcano Nevado Del Ruiz.

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Earthquakes
The vibrations from an earthquake can destabilize slopes. The vibrations and shaking can cause
the soil to lose strength and may cause an unstable slope to collapse. Earthquakes of magnitude
4.0 and greater have been known to trigger landslides
In the 1970s a powerful earthquake caused the partial collapse of Huascaran mountain in Peru.
The avalanche of rock and ice, travelling at speeds in excess of 300km/h destroyed the town of
Yungay in the Rio Santa Valley and killed some 20, 000 persons.
Landslides are also influenced by human factors
Building on unstable slopes
It is normal for a human being to build his house on a terrain without knowing the geology. At
times they may build their houses on unstable slopes. The weight of the building adds pressure to
already unstable slope. Over time the increase pressure my result in a landslide.
In December 1999 there were hundreds of landslides in and around Caracas the capital of
Venezuela. These major causes were heavy rain and by the activity of humans. The other two
causes involved human factors: steep sided valleys with unstable slopes had been used for high
rise buildings and vegetation had been removed to make way for these. This disaster left 30000
dead and 200 000 homeless.
Removal of Trees/ Deforestation
Human activities, such as deforestation, can make landslides more likely. In a forest, tree roots
help to bind the soil together. When trees are removed the soil become exposed and there are no
forest trees to hold the soil together. Any intense rainfall can easily saturate the soil making it
heavier and unstable. Deforested slopes are therefore prone to landslides.
Undercutting the base of a slope by road building
When humans undercut the base of a road to facilitate road building the slope is left unsupported,
the materials above it will collapse. It has the same effect as physical erosion.
Dumping waste material

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Waste from activities such as coal mining can be difficult to dispose of. In the village of Aberfan
in South Wales, UK, the material was piled up on the slopes of 25 degrees above the village. A
wet autumn in 1966 saturated the coal tip. The tip collapsed on the morning of 21 October,
engulfing the local junior school, killing 116 children and 5 teachers. This was one of the worst
disasters in the UK in the 20th
century.
Building Dams
One of the worst disasters of the 20th
century in Europe was in the north-east Italy in 1963. The
Vaoiant dam had been built across the narrow, steep-sided Piave valley. The rock beds of
alternate limestone and clay sloped towards the reservoir. The pressure of the weight of water in
the reservoir caused a small earthquake. A block of limestone slipped on the clay beneath, falling
into the water and forcing a 100 meter wave over the dam. Within seven minutes it hit three
small towns including Longarone. The speed of the disaster and the fact it happened at night
caused the high death toll of 3000. This could have been avoided through better planning and
quicker reaction to earlier ground tremors.

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Soil Creep
Soil creep is the slow, gradual movement of soil and rock particles down a slope under the
influence of gravity. This is the slowest of downhill movements and is difficult to measure as it
takes places at a rate of less than 1cm a year. However unlike faster movements, it is an almost
continuous process. Soil creep occurs mainly in humid climates where there is a vegetation
cover. More materials end up at the bottom of a slope by this process than in any other way. It
can take place on gradients as slight as 2 degrees but more usually on those of over 5 degrees.
There are two major causes of creep both resulted from repeated expansion and contraction.
Freeze-thaw when regolith (weathered material) freezes, the presence of ice crystals increases
the volume of the soil by 9 percent. As the soil expands, particles are lifted at right angles to the
slope in a process called heave. Sediments can expand when they freeze get wet or heated up in
the sun. When the ground later thaws and the regolith contracts (shrinks), the particles fall back
vertically under the influence of gravity and so move down slope. Creep takes a long time
because of each particle only move a millimeter to a few centimeters at a time
Wetting and Drying -Wet dry periods during times of heavy rainfall, moisture increases the
volume and weight of the soil, causing expansion and allowing the regolith to move downhill
under gravity. In a subsequent dry period, the soil will dry out and then contract.

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Evidences of soil creep
 There are many indicators that show a soil creep is happening. The clearest one is the
formation of terracettes. These are step like features, often 20-50 cm in height which
develops as the vegetation is stretched and torn.
 Trees also clearly show the effects of soil creep. As they slip gradually down a slope
they try to grow vertically as before, resulting in the bending of the trunk.
 Tilting of utility poles along the slope
 Build up accumulation of soil behind walls built along the slope.
 Walls bulge or break

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Conditions influencing soil creep
1. Climatic environment
In temperate environments soil can creep downhill at between 1 and 2 mm per year, but in
tropical regions it is quicker , perhaps 3-6 mm . In cold semi – arid areas it is even faster.

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RIVERS
THE WATER CYCLE
Over 97% of the world’s water is stored in oceans and seas. These water bodies make up about
70% of the surface of the Earth. The remaining stores of water are:
 2.1% as ice and snow ( most of this is Antarctica and Greenland)
 0.6% as ground water (held in rocks)
 0.1% in rivers and lakes
 0.001% held in the atmosphere as water vapor and clouds (water droplets). This amounts
to about 10 days’ supply of average rainfall around the world. If evaporation and
transpiration from the Earth’s surface suddenly stopped the world would run short of
water very quickly!
The three main processes in the water cycle are evaporation, condensation and precipitation.
 Evaporation is the process in which liquid water is changed into water vapor which
is a gas. Evaporation takes place mainly from surface water. Energy is needed for it to
occur. The energy comes from the sun’s heat and from wind. Look how quickly water
evaporates from concrete or tarmac on a very hot day compared with a cooler day!
Evaporation is also faster on a windy day compared with a calm day. Evaporation
from water surfaces on land would not be enough to keep rivers and lakes full and
provide the human population with drinking water. Fortunately, large amounts of
water evaporated from the seas and oceans are carried by air masses onto land where
condensation and precipitation take place.
 Condensation is the process by which water vapour changes into water droplets. It
happens when water vapour is cooled to a level known as the dew point.
Condensation forms cloud and can also occur at the surface as fog.
 Precipitation occurs when water in any form falls from the atmosphere to the surface.
This is mainly as rain, snow, sleet, and hail. Thus, water is constantly recycled
between the sea, air and land.

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The Drainage Basin System
When precipitation reaches the surface it can follow a number of different pathways.
A drainage basin is an area of land drained by a river and its tributaries. A drainage basin may
be described as an open system and it forms part of the hydrological or water cycle. If a drainage
basin is viewed as a system then its characteristics are:
 Inputs: how water is introduced into the drainage basin system. This is known as
precipitation.
 Stores: How water is stored or held for a period of time within the drainage basin system-
interception ( by vegetation), soil moisture, surface storage (lakes) and groundwater
 Transfers/flows : a process or flow of water from one place to another in the drainage
basin system- surface run-off/ overland flow, infiltration, percolation, through-flow,
throughfall and groundwater flow
 Outputs: How the water is released either back to the sea or back into the atmosphere-
evapotranspiration and river carrying water to the sea.

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Elements of the drainage basin system
Precipitation forms the major input into the system. Precipitation occurs when water in any form
falls from the atmosphere to the surface. This is mainly as rain snow, sleet and hail. When
precipitation reaches the surface it can follow a number a different pathways. A small amount
falls directly into rivers as direct channel precipitation. The rest falls onto vegetation or the
ground. If heavy rain has fallen previously and all the air pockets in the soil (pore spaces) are full
of water, the soil is said to be saturated. Because the soil unable to take in any more water, the
rain flows on the surface under the influence of gravity. This is called surface runoff or
overland flow. Overland flow is common in urban areas where the surface is made up of
impermeable materials such as tarmac and concrete.
Rainwater can be intercepted by vegetation. Interception is the precipitation that is collected and
stored by vegetation. Interception is greatest in summer when trees and plants have most leaves.
Some rainwater is stored on leaves and then evaporated directly into the atmosphere. The
remaining intercepted water either drips to the ground from leaves and branches ( throughfall)
or trickles down tree trunks or plant stems (stemflow) to reach the ground.
The water that reaches the ground may then enter the soil as infiltration. Infiltration is the
passage of water into the soil. The maximum rate at which water can pass through the soil is
called its infiltration capacity and is expressed in mm/hr. Some of the water will flow laterally
through the soil (roughly parallel to the surface) as through flow.
If the soil is not saturated, rainwater will soak into it. If the rock below the soil is permeable
(allows water into it), the water continues to soak down deeper into the rock. This continuous
downward vertical movement of water into the rock is called percolation. The water eventually
comes to an impermeable rock (which does not allow water into it). The underground water level
builds up towards the surface from here to create a groundwater storage. Here all the pore spaces
are filled with water and is sometimes called a zone of saturation. The underground water does
not remain stationary but flows downslope under gravity (laterally). The upper boundary of
underground water or the upper level of the saturated material is known as the water table.
Water contained in rocks is known as ground water. The groundwater may then be slowly

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transferred laterally as groundwater flow or base flow. The flow of groundwater is much slower
than runoff with speeds usually measured in centimeters per day, meters per year or centimeters
per year. Rock that holds ground water is known as an aquifer.
Water is lost from the system by evaporation and transpiration. Vegetation takes moisture
through its root system. It loses some of this into the air by transpiration. Surface water is also
lost by evaporation. The combination of the two is known as evapotranspiration. Once in the
river, water flows toward the sea and is lost from the drainage basin system.
Drainage basins
A drainage basin (or catchment area) is the area drained by a river and its tributaries. The
boundary of a drainage basin. The boundary of a drainage basin is called a watershed. This is a
ridge of high land that separates one drainage basin from another. The point where a river begins
is its source. A river reaches the sea at its mouth. A tributary joins the main river at a confluence.
A main river and all its tributaries form a river system. For example, the Mississippi and its
tributaries drain over one-third of the USA. Watershed in the Caribbean islands are typically
‘pear-shaped’: they are broad along the upstream divide and relatively narrow near the sea. In the
volcanic Windward Islands, watershed are steep and deeply dissected.
When small stream begin to flow they act under gravity, following the fastest route downslope.
Along the way water is added to them from tributaries, groundwater flow, throughflow and
overland flow.

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The source of a river
The starting point of a river may be:
 An upland lake

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 A melting glacier
 A spring in a boggy upland area where the soil is saturated that recognizable surface flow
begins
 A spring at the boundary between permeable and impermeable rocks
Drainage Density
Some big rivers have a large number of tributaries so that no place in the drainage basin is very
far from a river or stream. Such an area is said to have a high drainage density. Where a main
river has few tributaries the drainage density is low. High drainage densities occur where:
 The bedrock is impermeable
 The soils are easily saturated
 Precipitation is high
 Slopes are steep
 Interception by vegetation is limited
Where drainage density is high, water reaches streams quickly. It moves rapidly through the
basin. Therefore the flood risk is high compared with basins with low drainage densities. In the
Windward Islands, for example, drainage density is relatively high due to the steep slopes and
the volcanic nature of the islands. However, in the coralline/limestone Leeward Islands, slopes
within watersheds tend to be gentle with relatively low drainage densities. In these watersheds on
limestone there is significant percolation into rocks which builds up groundwater reserves.

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Drainage patterns
River systems often form a distinct pattern which is due to the structure of the rocks in the
drainage basin. The point at which one river or stream flows into another is known as the
confluence. Three distinctive patterns can be recognized, dendritic, trellis and radial.
Dendritic
This pattern looks like tree branches. This pattern develops in gently sloping basins with fairly
uniform rock type. The tributaries flow into the river at random forming a pattern like the veins
of a leaf. Examples of dendritic drainage are in the Caroni River in Trinidad and the Bruce Vale
river basin in Barbados. This type of drainage pattern is the most common in the Caribbean
region.
Trellis
This drainage pattern has an appearance of a rectangular grid. Rivers and their tributaries flows
almost perpendicular to each other with confluence of almost 90o. Trellis drainage takes place
where there is an alternate band of hard and soft rock at right angles to the main direction of the
slope. The main river has the power to cut though the hard rock while the tributaries cut though

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the softer rock at more or less right angles. This pattern can be seen in some areas of western
Barbados and is also present in the Northern range of Trinidad.
The principal river which flows down the slope is called a consequent river (C) next the
tributaries which cut out the vales and which do not flow down the main slope are called
subsequent rivers (S).
Radial
Radial drainage patters happens on a dome or volcanic cone. This pattern resembles the spokes
of a wheel. The river radiates outwards in all directions from a high central point or dome. The
volcanic islands in the eastern Caribbean have radial drainage pattern. The southern half of St.
Lucia and Nevis are good examples of where radial drainage takes place.

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Rivers: energy and processes
Energy is needed for transfers to occur. Around 95% of rivers energy is used to overcome
friction. The remaining 5 percent or so is used to erode the river channel and transport material
downstream. The amount of energy in a river is determined by:
 The amount of water in the river
 The speed at which it is flowing
Near the source, river channels are shallow and narrow. Also the bed is often strewn with
boulders and very uneven. High levels of friction upstream can cause considerable turbulence.
The water flows more slowly here than further downstream where the channel is wider, deeper
and less uneven. Although the river is unique, most show similar changes from source to mouth.
Three sections can be recognized along rivers: the upper course, middle course and the lower
course. Figure 3.3 show these sections which combine to form the long profile of the river. In
each section the main process taking place and the shape of the valley are different

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From source to mouth the rivers
 Gradient decreases
 Depth increases
 Width increases
 Volume increases
 Velocity increases
 Discharge increases
The volume is the amount of water in the river. The velocity is the speed of the water. The
discharge is the volume times the velocity. Discharge is defines as the amount of water passing a
specific point at a given time. It is measured in cubic meters per second m3/ sec. The discharge
rate can also show big variations between dry and wet seasons. The Amazon has the world’s
highest discharge at around 219000m3/sec
Erosion
There are four processes of erosion
Hydraulic action

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The pressure of water breaks away rock particles from the river bed and banks. The force of the
water hits river banks, and then pushes water into cracks. Air becomes compressed, pressure
increases and the riverbank may, in time collapse. Where velocity is high e.g. the outer bend of
meaner, hydraulic action can remove material from the banks which may lead to undercutting
and river bank collapse
Corrasion (or abrasion)
This is the wearing away of the bed and banks by the rivers load. This is the main type of erosion
in most rivers. Where depressions exist in the channel floor the river can cause pebbles to spin
around and turn hollows into potholes.
Attrition:
When pieces of rocks are broken away from the bed and banks the edges are usually sharp.
However, in swirling water rocks and stones collide with each other and with the bed and banks.
Over time the sharp edges become smooth and the pieces of rock become smaller in size.
Corrosion or Solution
Some rocks, such as limestone, dissolve slowly in river water which contains dissolved carbon
dioxide from the air. This process is common where carbonate rocks such as limestone and chalk
are evident in a channel

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Most erosion occurs when discharge is high and rivers are said to be in flood. Erosion acts on the
landscape in three ways:
 Near its source a river cuts down into its bed, deepening the valley. This is vertical
erosion
 In the middle and lower courses sideward or lateral erosion is most important. This
widens the valley
 Headward erosion takes place at the source. it causes the valley to grow very slowly
upstream.
Transportation
The load is the total amount of material being carried by the river. There are four processes by
which a river can transport its load: traction, saltation, suspension and solution.
Traction
Traction occurs when the largest cobbles and boulders roll or slide along the bed of the river. The
largest of these may only be moved during times of extreme flood.
Saltation
Saltation occurs when pebbles, sand and gravel are temporarily lifted up by the current and
bounced along the bed in a hopping motion. They are too heavy to carry in suspension
Suspension
Suspension is when material made up of very fine particles such as clay and silt is lifted as the
result of turbulence and transported by the river. Faster-flowing, turbulent rivers carry more
suspended material. The material held in suspension usually forms the greatest part of the total
load; it increases in amount towards the river’s mouth, giving the water its brown or black color.
Solution

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Solution is when dissolved material is carried by a river. Water flowing within a river channel
contains acids (e.g. carbonic acid from precipitation). If the bedrock is soluble, like limestone, it
is constantly dissolved in the running water and removed in solution.
Deposition
When the velocity of a river begins to fall, it has less energy and so no longer has the
competence or capacity to carry all its load.
Deposition occurs when:
 Discharge is reduced following a period of low precipitation
 Velocity is lessened on entering the sea or a lake (resulting in a delta)
 The gradient decreases significantly
 The current slows on the inside of a meander
 The river overflow its banks so that the velocity outside the channel is reduced.
When a river loses energy the first part of the load to be deposited is the large, heavy material
known as the bedload. Lighter material is carried further. The gravel, sand and silt deposited is
called alluvium. This is spread over the flood plain. The solution load- the lightest suspended
particles which include clay- is carried out to sea. Some rivers get their name from the colour of
the silt that they carry, for example the Yellow River in China.

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THE UPPER COURSE
The many features/landforms in the upper course of a river are:
 V-Shaped Valleys
 Interlocking spurs
 Potholes
 Rapids
 Waterfalls and Gorges
Rapids
Sometimes very thin alternating bands of hard and soft rock cross the course of a river. The
softer rocks wear away/erodes faster than the harder rocks. This is known as differential erosion.
The softer rocks are then on a lower level compared to the harder rocks. This creates an uneven
river bed and the river falling in a series of steps along the bands of the hard rock to form a zone
of turbulent water known as rapids

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Potholes
Where the bed is very uneven, pebbles carried by fast, swirling water can become temporarily
trapped by obstacles in the bed. The swirling currents cause the pebbles to rotate in a circular
movement, eroding circular depressions in the bed (abrasion). These are potholes. They general
increase in size only very slowly.
Interlocking Spurs
The river wind its way (meanders) around obstacles of hard rock. Erosion is concentrated on the
outside banks of these small meanders. This eventually creates spurs which alternate on each
side of the river, so they interlock. A spur is a ridge of high land which project towards a river at
right angles, decreasing in height towards the river.

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Waterfalls
Waterfalls are the most spectacular feature of the upper course, but they can also be found in the
middle course. This occur when there is a sudden change in the course of the river. This may be
due differences in rock hardness along the valley or for several other reasons:
 A steep drop at the edge of a plateau has been formed by uplift of the land
 A lava flow crosses the path of the river which pours over its edge as a waterfall
Waterfalls can form when the rock is horizontal, vertical or dipping upstream. The lower softer
rock is eroded more quickly causing the hard rock to overhang. The undercutting is caused by
corrosion and hydraulic action, with water swirling around in the plunge pool and spray hitting
the soft rock as the water plunges over the waterfall. The overhang steadily becomes larger until
finally it collapses. The rocks that crash down into the plunge pool are swirled around by the

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currents. This increases erosion and makes the plunge pool deeper. The rocks in the plunge pool
are eroded mainly by attrition.

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This process, beginning with the collapse of a layer of hard rock, is repeated multiple times. As a
result the waterfall retreats upstream, leaving a steep-sided gorge.

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V-shaped valleys
In the upper course much of the rivers energy is needed to overcome friction. The rest is used to
transport the load. The river in this section contains large boulders which can erode the bed
rapidly when the river is in flood. This results in the river cutting downwards into its bed, a
process known as vertical erosion. It forms steep V-shaped valley. Soil and loose rock on the
valley sides are washed down the steep slopes into the river. This adds to the load

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THE MIDDLE AND LOWER COURSES
In the middle course of the river profile the gradient is much less than in the upper course. The
volume of the water increases, with more tributaries joining the main river. More water is added
by through flow and, if the rock is permeable, by groundwater flow. Lateral erosion takes over
from vertical erosion as the most important process. Channel is much wider.
The lower course is nearest to the sea. The gradient is gentler. This section is characterized by
an even greater volume of water and higher velocity. Deposition is now much more important
than erosion. Meanders are more pronounced. The valley has the shape of an Open V in cross
section.
River cliffs and Point bars
Meanders occur in the middle course and are the result of erosion and deposition processes
operating in the river. The current is fastest and most powerful on the outside of the meander.
Within the river the fastest current is on the outside of the bend and the slowest current on the
inside of the bend. The concave or outside bend is much deeper so less friction and a higher
velocity.
Erosion is relatively rapid and the outside bank (concave bank) is undercut. Eventually the bank
collapses and retreats, causing the meander to spread across the valley. If the meander has
already reached the side of the valley, erosion on the outside bend may create a very steep slope
or river cliff. The current on the inside (convex bank) of the meander is much slower. As the
river slows it drops some of its load and deposition occurs. This builds up to form a gently
sloping slip-off slope, or point bar. Thus the water is shallow on the inside of the meander and
deep on the outside.
Diagram showing processes operationg on the inside and outside banks of a meander

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Diagram showing erosional and depositional actions of a river as it flows around a
meander

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Diagram showing a cross section of a meander
Meander migration
Because of the power of lateral erosion in the middle course, meanders slowly change their shape
and position. As they push sideways they widen the valley. But they also move or migrate
downstream. This erodes the interlocking spurs, giving a much more open valley compared with
that in the upper course.
Diagram showing how meanders migrate downstream

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Flood plain
A flood plain is the area of almost flat land on both sides of a river. It is formed by the movement
of meanders explained above. Meanders are more pronounced in the lower course. The
floodplain is constantly build up by flooded alluvial deposits. After each flood new layers of
alluvium are formed. This gradually builds up the height of the flood plain. The flood plain is
much more pronounced in the lower course as the river develops a very wide and flat valley
floor.
Diagram below show the flood plain

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Levees
When discharge is high the river is able to transport a large amount of material in suspension. At
times of exceptionally high discharge the river will overflow its banks and flood the low-lying
land around it. The sudden increase in friction as the river water surges across the flood plain
reduces velocity and causes the material carried in suspension to be deposited on the flood plain.
The heaviest or coarsest material will be dropped nearest to the river. This can form natural
embankments alongside the river called Levees. Levees are sometimes strengthened by engineers
to control flooding.
The lightest material is carried towards the valley sides. Each time there is a flood a new layer of
alluvium is formed. This gradually builds up the height of the flood plain.
Diagram showing the development of a Levee

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Meander necks and Ox-bow lakes
As a river flows from its middle course to its lower course, meanders become even more
pronounced and the valley becomes wider and flatter. Oxbow lakes are shallow, crescent shaped
lakes found on the flood plain and are the remains of a former course of a river.
An oxbow lake develops when a meander becomes so pronounced that only a narrow neck of
land separates the two ends of the meander. Erosion continues to cut into outside bends of the
meander and a meander neck is form. With continuous erosion the meander neck becomes
narrower and narrower. Eventually, when the river is in flood and discharge is high, it may cut
right across the meander neck following a more direct route and shortening its course. For a
while water will flow along both the old meander route and along the new straight course.
However, because the current slows down at the entry and exit points of the meander, deposition
will occur. After a time the meander will be cut off from the new straight course, leaving behind
an Ox-bow lake. When cut-off occurs the only sources of water for the ox-bow lake will be

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precipitation and flooding from the river. If evaporation is greater than these additions of water
the ox-bow lake will eventually dry up
Diagram the development of an oxbow lake
Braiding or braided rivers
Braiding is when a river divides for various distances into two or more channels. The channels
are separated by islands of sediment. Braiding occurs when:
 A river carries a very large load, particularly of sand and gravels, in relation to its
velocity.
 The discharge changes rapidly from season to season.
During a dry period or by increase load the river may not be capable of carrying its full load, and
so a great deal of deposition takes place on the bed of the river and the river channels become

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choked. This give rises to sandbanks and small islands in the bed. The river is forced to split up
into several channels (known as braided and its way through its own deposits. This is known as a
braided river.
Diagram showing a braided channel
Deltas
Deltas are formed by the deposition of sediments at the mouth of a river as it enters a sea or lake.
Deltas only form under certain conditions and most rivers do not end in a delta.
Large rivers in the lower course have the energy to transport a great deal of material in
suspension. As a river enters the sea its speed of flow is reduced, sometimes very suddenly,
causing deposition. The coarsest materials (like sand) is deposited first because of a greater
weight, while finer material (like clay) are carried out further into the sea. Thus layers of
different sediments are built up on the sea floor until they reach the surface. This happens first at

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the landward end of a delta, extending gradually out to sea. This huge platform of river sediment
is called delta.
A delta is therefore an accumulation of sediments at the mouth of the river which has been
formed by deposition of successive layers of sediments. When a river flows into a delta it has to
flow over its own deposit. This causes the river to braid. Each channel in a delta is called a
distributary.
The two main conditions required for deltas to form are:
 The river must have a large amount of sediment
 Coastal currents and waves must not be so strong as to remove sediment faster than the
river can deposit it- if this happens the sediments are spread over a much wider area of
sea floor beyond the mouth of the river.
There are three main types of delta:
 Fan-shaped or arcuate: This is triangular in shape with a slightly rounded outer margin.
The Nile and Yallahs River in Jamaica are examples
 Bird’s foot or digitate: distributaries flanked by sediment extend out to sea like the
claws of a bird’s foot. The Mississippi delta is a good example.
 Estuarine or cuspate: the delta forms an islands in the river’s mouth. The Amazon, and
Essequibo river in Guiana are examples
Major deltas are not common in the Caribbean. This is because it usually takes a large river to
build out into the sea.
Diagram showing the structure of delta

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LIMESTONE
Limestone is the name given to rock which is composed mainly of calcium carbonate. Calcium
carbonate is a chemical compound of calcium, carbon and oxygen. The chemical formula for
calcium carbonate is CaCO3. Limestone is a sedimentary rock which is formed underwater.
Some limestone consists mainly of coral or the shells of other small marine creatures. Limestone
may also be precipitated from seawater.
Limestone is a permeable rock. This means that water can enter limestone through pores, joints
or cracks in the rock. Another characteristic of limestone is that it can be slowly dissolved by
water. When groundwater or rainwater absorbs carbon dioxide, it becomes a weak acidic
solution which is especially effective at dissolving limestone. Because of these simple facts,
wherever limestone is in contact with rainwater or groundwater, some interesting limestone
features tend to develop.
Carbonation
Carbonation is the most important form of chemical weathering in limestone areas, whether they
are in tropical or temperate zones of the world. Rainwater containing carbon dioxide absorbed
from the atmosphere is able to dissolve calcareous (calcium–based) rocks. Carboniferous
limestone and chalk are the types of limestone affected most because they are made of almost
pure calcium carbonate.
Here are the stages of the process:
1) Rainwater dissolves carbon dioxide from the air as it passes through it
H2O + CO2 = H2 CO3
Rain water + Carbon dioxide = Mild carbonic acid
2) The mildly acidic rainwater reacts chemically with the calcium carbonate in the rock
H2 CO3 + CaCO3 = Ca (HCO3)2
Mildly acidic rainwater + calcium carbonate = Calcium bicarbonate
3) Calcium bicarbonate is soluble in water so it is easily washed away

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Common Limestone Features
Surfacefeatures
Clints and Grikes
A limestone pavement is a natural karst landform consisting of a flat, incised surface of
exposed limestone that resembles an artificial pavement. This pavement is filled with joints.
Carbonic acid will attack the joints and overtime a small depression called a grike will form.
Clints are upstanding features that are formed on the areas of the pavement that resisted erosion.
Sink holes or swallow holes: A sink hole (or swallow hole) is a natural depression or hole in the
Earth’s surface commonly found in limestone areas. They are formed when joints or fissures in
the rock are enlarged by carbonation or when the roof of an underground cavern collapses.
Sometimes a river or stream may “disappear” down a sink hole and continue flowing
underground.
Dolines

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Dolines are large depressions formed by the solution or collapse of limestone. Frequently they
are covered by other deposits. Depressions can range from small-scale hollows to large
depressions up to 30m in diameter
Cockpits (depression) and Cones (hills)
In many limestone areas, the chemical weathering processes of solution and carbonation have
produced distinctive landscapes known as karst landscapes. Karst landscapes are dominated by
features such as sinkholes, disappearing streams and caves. One well known karst landscape in
the Caribbean is the Cockpit Country in Jamaica.
The Cockpit country is a large area with many small hills and depressions. This karst landscape
was formed in an area with white limestone. In this area, the rock has many joints. These joints
cross each other as some run from east to west and others run generally from north to south.
Water collects in these joints. Therefore the rock near the joints is dissolved relatively quickly
forming depressions called COCKPITS. Small hills called CONES form in areas which are
further away from the joints as the rock in these areas is not dissolved as quickly. The photo
below shows a small part of the Cockpit Country.
Depressions
are cockpits
Hard rocks
stand up as
cones or hills

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Resurgent streams
Resurgent stream arise when the limestone is underlain by an impermeable rock, such as clay.
This force the water out into the open, forming a spring or resurgent stream. There are many
caves and sinkholes within the Cockpit Country. No rivers or streams can be found on the
surface in the area because they have all disappeared into swallow holes and flow underground
through caves.
Underground features
Caves: A cave is a naturally occurring underground hollow or passage, especially one with an
opening to the surface of the Earth. Caves often form in limestone areas when underground water
dissolves the rock, forming tunnels passages and even large caverns. There are several caves in
Barbados, the most famous of which is Harrison’s cave. Part of Harrison’s Cave can be seen in
the photo below.
Many interesting limestone features can be found within limestone caves. Some common ones
are stalactites, stalagmites and pillars.

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Stalactites: Water dripping from the ceiling of a cave contains dissolved calcium carbonate.
Some of this calcium carbonate may be deposited on the ceiling of the cave. Over time the
deposition of calcium carbonate will form an elongated feature which hangs down from the
ceiling. This feature is known as a stalactite. Several stalactites can be seen in the photo above.
Stalagmites: As water drips from the ceiling onto the floor of the cave, calcium carbonate may
be deposited on the floor of the cave. Over time this may form an elongated feature which rises
vertically from the cave floor. This feature is known as a stalagmite. Several stalagmites can be
seen in the photo above.
Note: A simple way to avoid confusion is to remember that stalactite has a “c” for “ceiling” and
stalagmite has a “g” for “ground”.
Pillars: A stalagmite may form directly below a stalactite as water drips from the ceiling of the
cave onto the floor. The stalactite may continue to grow downwards and the stalagmite may
continue to grow upwards until the two limestone features eventually meet. When this happens
they form a new feature known as a pillar or column which extends all the way from the ceiling
of the cave to the floor. A limestone pillar can be seen in the photograph below.

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COASTS
Waves are the most commonly formed by friction as the wind blows over the surface of the
sea.
What cause waves to break?
In deep water the surface waves form part of a circular movement of water. This explains why
there is actually horizontal movement of water in the oceans. However, look what happens as the
waves get nearer to the coast:
 As the sea near the shore is shallow the circular motion of the waves is interrupted by
friction with the seabed
 The water motion becomes more elliptical ( shaped like a rugby ball)
 The wave grows in height and begins to topple forward
 Eventually the waves break on the shore. Water moves up the beach as the swash and
then drains back down the beach as backwash.
When waves near the coast, the bottom of the wave is slowed by friction with the sea bed.
Because the top of the wave is experiencing less friction, it moves faster and eventually
topples over the bottom of the wave and breaks.

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The power of the waves when they reach the coast depends on three factors:
 The distance of open water over which the wind has blown. This is called the fetch.
The longer the fetch the more the powerful the waves
 Strength of the wind- The stronger the wind the more powerful the waves.
 Duration of the wind – If strong winds have blown over a long period of time, this will
result in powerful waves.
Wave Terminology
Crest: The top of the wave. Trough: The low area in between two waves. Wavelength: The
distance between two crests or two troughs. Wave height: The distance between the crest and
the trough.
Wave Frequency: The number of waves per minute. Velocity: The speed that a wave is
traveling. It is influenced by the wind, fetch and depth of water. Swash: The movement of water
up the beach. Backwash: The movement of water back down the beach after the swash is
completed
Constructive and Destructive waves
It is possible to identify two types of waves:
Constructive Waves:
Constructive waves are low but powerful waves that surge up the beach when they break. Their
swash is much more powerful than their backwash, much of which percolates through the
beach as the water flows back to the sea. Constructive waves are created by storms many miles
away from the coast and they travel fast across the ocean.
They are called ‘constructive’ waves because they transport beach material to the top of
the beach, thereby ‘constructing’ it. Because the swash is stronger than the backwash they tend
to deposit material and build beaches up

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Destructive waves
Destructive waves are essentially the opposite of constructive waves. They are taller and tend to
crash down onto a beach rather than surging up the beach. There is little swash but the backwash
is powerful. This leads to erosion of the lower beach, hence the term destructive. Destructive
waves are usually formed during local storms that are centered close to the coast.
Because the backwash is stronger than the swash, destructive waves erode and transport
material away from beaches.

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COASTAL EROSION AND EROSIONAL FEATURES
Abrasion/Corrasion:
When waves approach the coastline they carry material such as sand, shingle, pebbles and
boulders. Abrasion occurs when these materials are hurled against cliffs as waves hit them,
wearing the cliff away. It is the most effective method of erosion
Hydraulic action- This occurs when water is thrown against the land by breaking waves, thus
compressing the air which is contained in any cracks in the rocks. When the wave retreats the air
expands, thus exerting pressure on the rocks. When repeated over a long period of time, the
expansion of air in the cracks may cause the rocks to shatter.
Attrition
As in the case of material carried by rivers, the material carried by waves also becomes broken
into smaller fragments. This is the result of the pieces of material hitting against each other and
against the land. Beach material is knocked together in water reducing their size and increasing
their roundness & smoothness. THIS PROCESS IS NOT RESPONSIBLE FOR THE EROSION
OF CLIFFS.
Corrosion/solution:
Seawater contains carbonic acid, which is capable of dissolving limestone. The dissolving of
soluble rock, such as limestone.
Coastal Erosion is most effective when the waves are powerful and contain a lot of energy.
These waves need to break at or close to the foot of a cliff if they are to carry out erosion.
Features of Coastal Erosion
Headlands and Bays
Bays and headlands are formed in a very similar way to rapids (rivers topic). They are formed
when you get alternate layers of hard and soft rock. Alternating layers of hard and soft rock
allow the sea to erode the soft rock faster, forming a bay but leaving hard rock sticking out,
known as a headland. The altering rate of erosion of hard and soft rock is known as differential
erosion.

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Bay: An indented area of land normally found between two headlands. Bays are usually more
sheltered so there is less erosive power, meaning you often find beaches in bays.
Headland: A piece of land that sticks out into the sea. Waves refract around headlands so they
experience a lot of erosion forming features like arches and stacks
Wave cut notch and Wave cut platform
A wave cut notch is simply a small indent at the base of a cliff formed when a cliff is undercut
by the sea. When a wave breaks on a cliff, all of the wave’s energy is concentrated on one
specific point and this section of the cliff experiences more rapid erosion via corrosion and
hydraulic action. This eventually leads to the formation of a wave cut notch, when the cliff has
been undercut
Wave cut platforms are made in a similar ways to waterfalls and gorges (rivers topic). At high
tide the power of the sea attacks and erodes the bottom of the cliff. Over time this erosion creates
a wave cut notch (basically an eroded hole at the base of the cliff). As the wave cut notch gets
bigger, the weight of rock above the notch gets greater. Eventually the cliff cannot support its
own weight and it collapses. The process then starts again, with the erosion of the sea making a

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new wave cut notch. As the process continues the cliff starts to move backwards (retreat).
Because the cliff is moving backwards a wave cut platform (an expanse of bare rock) is created.
Caves, Arches, Stacks and Stumps
Waves are particularly good at exploiting any weakness in a rock, such as a joint.
Caves
Caves occur when waves force their way into cracks in the cliff face. An increase in erosional
activity can widen the crack over a period of time until it is large enough to form a cave. The
dominant processes of erosion are abrasion/corrasion.
Arch
An arch is formed when two caves develop on each side of a headland. With continuous erosion
they will grow in size and eventually meet giving rise to a feature known as an arch.
Stack
The roof of the arch has no support however and is highly susceptible to weathering via
exfoliation, salt crystallisation and biological weathering. As the weathering continues, the roof
of the arch will collapse leaving a stack, a tall, lone piece of land sticking out in the sea.

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Stump
This stack is exposed to the full force of the water and is weathered and eroded heavily. The base
of the stack receives a lot of erosion from hydraulic action and corrosion and, eventually, the
stack will collapse into the sea leaving behind a small piece of land called a stump.
Caves, arches, stacks and stumps are usually found on headlands, where wave refraction is
causing erosion on three sides. The waves always look for weaknesses in the headland (cracks
and joints). If they find a crack or a joint they will start attacking it. Hydraulic pressure will be
the main type of erosion. Overtime the crack may turn into a cave. Slowly the cave will get
bigger and cut all the way through the headland, making an arch. As the arch gets bigger the
weight of the arch roof gets too great and it collapses, leaving a stack. The stack is then eroded
by the sea leaving a stump.

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Blowhole: Cave sometimes develop in the face of cliffs. They develop when there is a weakness
in the rocks, which is enlarged by wave action. Waves pounding against cliffs can exert great
pressure, which causes the air in the cracks to be suddenly compressed. The alternate

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compression and expansion of the air in the cracks enlarges them, and in time they may develop
into caves.
If there are vertical joints leading from the roof of the cave to the top of the cliff, these also may
become enlarged. In time a passageway may be opened up, through which air and water can
escape to the surface at the top of the cliff. Such feature is known as a blowhole.
LONGSHORE DRIFT AND COASTAL DEPOSITON FEATURES
There are many different types of sediment at the coast including beautiful white coral sand, the
more common yellow sand, pebbles (shingle) and mud. Once rock fragments have been broken
off a cliff or brought to the coast by rivers, they enter the coastal (littoral) transport system.
Waves are very effective transporters of sediment. After a storm, beaches can look very
different.
Coastal transportation
Sediment is moved by the waves in a number of ways.
Traction: This is where heavier particles are slowly rolled along the sea bed.
Saltation: Some particles move in a bouncing manner as they are disturbed by other particles
knocking into them. They are too heavy to be carried by the water but light enough to ‘hop’
along the sea bed.

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Suspension: Lighter particles can be picked up and carried within the water. Sand is most
commonly transported in this way.
Solution: Dissolved chemicals will be transported in solution. Limestone (calcium carbonate) is
often transported in this way before precipitating out of solution to form new limestone deposits
on the sea bed. This is a very common process in the warm Caribbean seas.
Coastal deposition
One important factor affecting the movement of sediment and its deposition at the coast is the
angle at which the waves approach a shoreline. If the waves approach parallel to the coast,
sediment will simply be moved up and down the beach. There will be very little movement along
the coast. Under these conditions beaches will form in bays
If the waves approach a shoreline at an angle, sediment is transported along the coast in zig-zag
fashion. This process is called longshore drift and it results in a pile up of sediment at one day of
a bay.
Coastal deposition takes place in areas where the flow of water slows down. Sediment can no
longer be carried or rolled along and it has to be deposited. Coastal deposition most commonly
occurs in bays where the energy of the waves is reduced upon entering the bay. This explains the
presence of beaches in bays and accounts for the lack of beaches at headlands where wave
energy is much greater.
Longshore Drift
This is the process of waves moving (transporting) material (load) along a coastline. When
Waves approach the shore at an angle (usually in line with prevailing wind direction) the swash
moves material up the beach in this direction. Backwash pulls material straight down the beach.
The result is that material is transported in a zig-zag fashion.
Longshore drift only happens when the waves hit the beach at an angle. It is the process of the
swash transporting material up the beach at an angle and the backwash returning directly under
the force of gravity that causes material to be transported along the beach.

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Features of Coastal Deposition
Beaches
The best known feature resulting from deposition by waves is the beach. This may be made up of
mud, sand, shingle (small rounded stones) or boulders. Beaches are the result of longshore drift
produced by constructive waves. Beaches usually have a gently sloping surface. They are
generally formed between high and low tide marks. Sometimes, however, stormy conditions at
high tide may throw material up beyond the high tide mark, resulting in what is known as a
storm beach.
Most beaches in the Caribbean are made up of sand. Sandy beaches are most often found in
sheltered bays where they are called bayhead beaches. When waves enter these bays they are
forced to bend to mirror the shape of the coast. This is called wave refraction. It is caused by the
shallowing of the water as the waves enter the bay. Refraction spreads out and reduces wave
energy in a bay, which is why deposition occurs here.

CSEC GEOGRAPHY

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