Download Glossary Earth Forces completed

Earth Forces Revision
Structure of the Earth – from surface to centre there are three
main layers
 The crust – is the earth’s hard and rocky surface and is by
far the thinnest. Under the oceans it is about 6km thick,
while under the continents it is up to 90km thick.
 The mantle is the thickest layer (around 2900km) and is
made up of semi-molten rocks, rather like a soft plasticine.
Temperatures are very high, around 2000°C
 The core is at the very centre and is easily the hottest,
reaching temperatures of 5500°C. There is a molten outer
core and a solid inner core. The inner core is a solid ball,
mainly of iron. It does not melt because of the great
pressure from all the other layers, pressing down on it.
The World’s Plates
The earth’s crust is not a solid piece of rock but has a cracked
surface. Each piece of crust is called a plate. Where the plate
edges meet is called a plate boundary. It is at plate boundaries
that most earthquakes and volcanic eruptions take place and
where the highest mountains are formed.
These plates move very slowly, just a few mm each year. Plates
are moved along by convection currents in the mantle.
Pangaea
200 million years ago all continents were joined together in a
supercontinent called Pangaea. Two pieces of evidence that prove
this are:
 The north east coast of South America seems to fit into the
west coast of Africa – like a giant jigsaw
 Rocks and fossils of a similar age and type are found in
these two continents.
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Plate boundaries
Due to convection currents in the mantle, the earth’s plates are
moving very slowly. They do not move constantly, but, at plate
boundaries, they move in three ways.
Parting plates
Where two oceanic plates are moving away from each other the
plates are parting. This can be seen in the middle of the Atlantic
Ocean. At the boundary between the plates, red hot molten rock
from the magma is coming up and hardening to form new crust
when it touches the cold water of the ocean. Much of this new
land is building up as underwater mountain ranges e.g. the MidAtlantic Ridge. If the peaks rise above the ocean surface volcanic
islands are formed. An example is Surtsey, near Iceland.
Colliding plates
Some plates collide. This can be seen where oceanic plates collide
with the continental plates. The heavier oceanic plate is forced
downwards into the mantle where it melts into magma. When
plates collide two geological events can occur:
 The magma rises up to the surface, erupting to form a
volcano
 There is an enormous force as one plate rubs against the
other and pressure builds up over a few years. When the
plates pull apart energy is released, causing the ground to
shake – an earthquake.
Sliding plates
When plates slide past one another the plates can stick and later
slip apart. This causes an earthquake.
Volcanoes
A dormant volcano is one which is described as ‘sleeping’. This
means that although it hasn’t erupted in living memory it may
erupt again.
An active volcano is one which has erupted recently, in living
memory.
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An extinct volcano is described as being ‘dead’. It has not erupted
for a very long time.
Volcanoes are formed by molten material from the mantle. It is
called magma and is forced through an opening in the earth’s
crust called a vent. When the magma comes out of the ground it is
called lava.
One of the main types of volcanoes is called a composite volcano.
It has the following features: It is usually cone-shaped
 It is formed from layers of ash and lava
 At the summit of the main vent there is a bowl shaped pit
called a crater.
Volcanoes release sulphur, ash, dust, lava bombs and lava when
they erupt.
Volcano Case Study
Mt Saint Helens (a dormant volcano) erupted in May 1980. It was
triggered by a minor earthquake measuring 4.1 on the richter
scale. It killed 60 people and reduced the height of Mt Saint
Helens by 390 metres. Trees were flattened and destroyed and a a
massive mud slide wiped out houses, roads, bridges and farmland.
Earthquakes
Earthquakes are measured by instruments called seismographs. A
richter scale is used to demonstrate its strength.
The focus of an earthquake is the point where it happens under
the ground. The epicentre is the closest point to the focus
above ground.
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Earthquake Case Study
The Haiti earthquake occurred in January 2010. It measured 7.0
on the richter scale and the epicentre was 25km form the capital
city, Port au Prince. The damage done to Port au Prince was
devastating as a result of the poor infrastructure causing buildings
to collapse readily.
Tsunami
A Tsunami occurs when an earthquake happens in the crust under
the sea. This creates a large wave which can travel up to 600 km
per hour.
As a tsunami approaches land and the sea becomes shallower, the
speed of the leading wave slows down and this allows the faster
waves behind to catch up. The effect is a wall of water sweeping
onto the coastal area. The effect is devastating. People drown,
boats are washed in shore. Cars and houses can be picked up by
the force of the water and dumped somewhere else. Farmland is
destroyed, as are roads and railways.
Tsunami Case Study
The Japanese tsumami that happened in March 2011 killed over
11,000 people. It was caused by an earthquake off Japan’s east
coast, measuring 9.0 on the richter scale (the most powerful ever
to have hit Japan). The effects were devastating and the threat of
radiation poisoning was very high because of the destruction to
the Fukushima nuclear power plant.
Aid
Agencies such as Save the Children can help by providing
emergency (or short term aid) after a disaster happens. This
includes tents, bedding, food and medical supplies. Long term aid
is to help the area recover after a disaster e.g. building roads
which have been destroyed, new housing and hospitals etc.
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