This document looks at the Earth's internal forces. The main layers of the earth are described. The history surrounding plate tectonics is discussed. The different types of plate boundaries is also explained.

1. CSEC Geography
Internal Forces
Theory of plate tectonics
O.Johnson
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.

2. 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’.

3. 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.

4. 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

5. 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

6. 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.

7. 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.

8.  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

9. 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

10. 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.

11. 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

12. 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.
Extracted from Guinness, P., Rocke, J., Nagle, G., Oliphant, K., Rae, A., Ross, S., … Wyllie, A.
(2008). Geography for Csec. Nelson Thornes Ltd.