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Geography: Grade 7
Earthquakes, Volcanoes and People: Structure of the Earth
STRUCTURE OF THE EARTH
The Ancient Greeks knew that the Earth was about 12,750 kilometres in diameter, but it
was only in the 20th century that geophysical studies showed that the Earth has several
distinct layers, i.e. the crust, the mantle and the core, as shown in the diagram below.
Structure of the Earth
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The Earth’s Crust
This is the outermost layer of Earth and is made up of the continents and the ocean
basins. The crust varies in thickness from 35 to 75 km on the continents and 5 to 10 km
on the ocean basins.
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Earthquakes, Volcanoes and People: Structure of the Earth
Under certain mountain ranges, such as the Alps, the crust is as thick as 100 km. The
crust that forms the continents is made up of rock called granite and the crust under the
oceans is made mostly of a rock called basalt.
Basalt
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Granite
The Earth’s structure can be compared to a boiled egg, with the crust being the rigid and
thinnest layer, like the shell of the egg. Like the eggshell, the crust is brittle and the layers
can break.
Diagram of the Structure of the Earth
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The Mantle
The mantle is the 2nd layer and is a hot layer of semi-solid rock. It is made up of ferro
(iron) magnesium silicates. The mantle is about 2 900 km thick and is separated into the
upper and the lower mantle. Pressure increases as one travels into the mantle, towards the
centre of the Earth.
The mantle is where most of the internal heat of the Earth is located. Large convection
cells in the mantle circulate heat and may cause tectonic plates to move.
Comparing again to an egg, the mantle would be the egg white.
The Core
This is the last and inner layer and unlike the egg yolk, is divided into two parts, the
liquid outer core and the solid inner core. The outer core is 2 300 km thick and the inner
core 1 200 km thick.
The outer core is made up of a nickel-iron alloy and the inner core is mainly iron. The
Earth’s magnetic field is controlled by the liquid outer core, which spins as the Earth
rotates. The core has extreme temperatures and pressures, which keep the iron and metals
liquid and flowing. The core is twice as dense as the mantle.
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In addition to the layers of the Earth, based on its composition, the structure of the Earth
is also divided into layers based on its mechanical properties, as shown in the diagram
below:
Diagram of Asthenosphere and Lithosphere
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Lithosphere
This is the top layer which consists of the crust and the upper mantle. ‘Lithos’ in Greek
means ‘stone’. This layer has many plates which contain the Earth’s continents and
oceans and which move around due to tectonic forces. The upper part of the mantle is
cooler and more rigid than the deep mantle. The lithosphere is thinnest under oceans and
volcanically active areas.
Asthenosphere
This is a semi-liquid hot layer in the lower mantle, which is weaker than the lithosphere
and therefore allows the lithosphere to move around. ‘Asthenes’ in Greek means ‘weak’.
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The Tectonic Plates
The tectonic plates are huge masses of land on Earth which float across the surface of the
mantle. A plate is a large, rigid slab of solid rock and the word ‘tectonics’ comes from
the Greek ‘to build’. If you put these 2 words together, plate tectonics refers to the how
the surface of the Earth is built of plates.
Plate tectonics is the term that is used for the theory and study of the structure of the
tectonics plates and was developed in the 1960s. Plate tectonics helps us to understand
why earthquakes, volcanoes and tsunamis occur, as well as how great mountain ranges
like the Alps, Himalayas and the Andes were formed.
Alps Mountain Range
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Andes Mountain Range
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Himalayas Mountain Range
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Courtesy of NASA
The Earth’s lithosphere (outermost layer) is broken into the following 7 large plates:
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Eurasian Plate
Indo-Australian Plate
Pacific Plate
North American Plate
South American Plate
African Plate
Antarctic Plate
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There are also smaller plates, such as: Arabian Plate, Filipino Plate, Caribbean Plate,
Philippine Plate, Juan de Puca Plate, Nazca Plate, Cocos Plate and Scotia Plate.
The plates ‘float’ on the magma and move up and over each other. This movement of the
plates means that continents move. This is known as Continental Drift.
The plates move both horizontally (sideways), and vertically (up and down). These plates
are all moving in different directions and at different speeds, from 1 cm to 10 cm per
year.
Over long periods of time they also change in size. They are anything from 80 to 400
kilometres thick.
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Before plate tectonics, Continental Drift explained the break up of the supercontinent
‘Pangaea’ (meaning ‘all lands’), over 200 million years ago, eventually resulting in the
continents as we know them today.
Map of Pangaea Showing Modern Day Continents
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Go to the module to see an animation of the break up of Pangea:
http://mycyberwall.co.za/get-smart/geography/grade-7/structure-earth
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Plate Boundaries
At places on Earth where 2 or more plates meet, a plate boundary is formed. These are
called different names depending on the movement they make, i.e.:



Crashing/crunching – convergent boundary
Pulling apart – divergent boundary
Side sweeping – transform boundary
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Diagram Showing the Various Tectonic Plate Boundaries
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Convergent Boundary
Places where the plates crash or crunch together are called convergent boundaries.
These plates only move a few centimetres a year, so collisions are very slow and last
millions of years. Sometimes they result in huge mountain ranges or deep trenches.
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As the plate digs into the Earth’s hot interior, some of the rock in it melts. This rock rises,
causing earthquakes on the way up and volcanic eruptions when it reaches the surface.
When a thin, oceanic plate collides with a lighter and thicker continental plate, the
oceanic plate is forced under the continental plate. This is known as subduction.
When two land masses meet, neither will slide under each other. Instead, they will crush
together, crumple and fold and this results in folded mountain ranges.
An example of a convergent boundary is in South America, where the oceanic plate is
crashing into the South American continent. This is formed in the Andes mountains in
South America, the Himalayas in Asia and a deep trench in the Pacific Ocean near Japan
called the Mariannas Trench.
Aerial View of Andes Mountains
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Himalaya Mountains, Asia
Map of Mariannas Trench Near Japan
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Divergent Boundary
These boundaries are found in places where the plates are pulling apart or moving away
from each other. This creates earthquakes and volcanoes.
When the Earth’s surface area (lithosphere) is pulled apart, it breaks down parallel faults
that tilt slightly apart from each other. As the plates separate, the block between the fault
cracks and drops down into the soft interior, the asthenosphere. This causes a rift valley.
Magma (liquid rock) comes up to fill the cracks and forms a new crust. Earthquakes
occur along these boundaries and volcanoes form where the magma reaches the surface.
When this happens on land, large rift valleys, between 30 and 50 km wide are formed, for
example, the East African Rift Valley in Kenya and Ethiopia and the Rio Grande Rift in
New Mexico.
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The map below shows the location of historical volcanoes, as well as the Afar Triangle
(shaded at the center) Thich is where three plates are pulling away from one another; the
Arabian Plate and two parts of the African Plate (the Nubian and Somalian ), splitting
along the East African Rift Zone.
Map Showing Location of East African Rift Valley
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Rift Valley in Ecuador
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When two oceanic plates move away from each other, this is known as
seafloor spreading. The rift valley that is formed will be much
narrower than the one created by two land masses pulling apart,
usually about 1 km wide.
Examples of this are the Mid Atlantic Ridge and the East Pacific Rise.
Plate separation on divergent boundaries is slow. For example on the
Mid Atlantic Ridge, it is about 2 cm a year.
Map Showing Mid-Atlantic Ridge
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Mid-Atlantic Ridge through Iceland
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Transform Boundary
In this type of plate boundary, the plates slide together in a sideways movement. They
push against each other causing tension and then release this tension in a sudden, violent
jerk, which causes an earthquake.
These boundaries are marked in some places by features like stream beds that have been
split in half and the two halves have been moved in opposite directions.
The most famous transform boundary is the San Andreas
Fault in California, USA. To the west of the fault is the Pacific
Plate which is moving north west. To the east of the fault is the
South American Plate, which is moving south east.
Although transform boundaries do not have spectacular
features, their sliding motion causes lots of earthquakes. The
most famous earthquake on the San Andreas Fault was in 1906.
It destroyed much of San Francisco and killed more than 600
people.
Aerial View of San Andreas Fault
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View of Destruction Caused by 1906 Earthquake in San Francisco
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