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Disasters
Lithosphere/crust: the earth’s crust
which includes landforms, rocks and
soils. Mostly solid rock, rigid and
contains high strength. 5-7km
Mantle: Partially molten rock. 500 to
2000 degrees. It is 2900km thick.
The asthenosphere is the hot, soft,
semi-fluid upper part of the mantle.
Outer core: molten iron and nickel.
4000 to 6000 degrees. It is 2300km
thick.
Inner core: Mostly iron. Solid because
of pressure. 7000 degrees max. It is
1200km thick.
Hydrosphere: The water on or at the surface of earth. Eg- oceans, lakes, rivers
Atmosphere: Blanket of gas surrounding Earth. It is 145km above sea level, and nitrogen and
oxygen make up most of it.
Theory of Plate Tectonics:
The theory says that the Earth’s crust is divided into a number of plates, which move across
the layer of partially molten rock in the asthenosphere. Convection currents and
gravitational forces cause the plates to move. Heat from the core causes mantle rock to
rise and spread out. It then cools and falls due to gravity. The plate’s movement vary from
2-10 cm per year. There are 13 major plates. Movement of the crust can cause earthquakes
and most volcanoes.
Evidence:
1. Geometric fit of the continents: jigsaw can make super continent called Pangaea
2. Age of rocks and fossils: rocks and fossils are similar and of the same age on sides
of continental margins.
3. Earthquakes and volcanic distribution: these occur on location of plate boundaries
4. Mid-oceanic ridges: they show where new plates are formed, pushing plates apart.
5. Magnetic reversals
Sea-floor spreading: The idea of sea-floor spreading is that new crust is being made in the
mid-oceanic ridges. Half of the new crust goes to each side of the ridge. The growing ridge
pushes the continents apart. The rock to make crust is supplied in the mantle.
Mid-Oceanic Ridges: They are an underwater mountain range, and are usually made by
tectonic plates. Magma constantly emerges onto the ocean floor through a rift. Usually, the
magma builds up and forms new crust. The rocks that make up the sea floor are usually
younger near the centre of the ridge and the older ones are further away. It is evidence of
the emergence of new magma. As the crustal plates move about, the magma emerges.
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Divergent boundaries/Spreading zones eg-Mid Atlantic Ridge
At divergent boundaries, plates are moving apart from each other and the space that this
creates is filled with new crust due to the process of sea-floor spreading.
Convergent boundaries eg- Himalayas
At convergent boundaries, plates collide with each other. However, if one plate is pushed
under the other plate, it is called a subduction zone. They are usually indicated by chains of
volcanoes and ocean trenches. If a thin oceanic plate collides with the thick continental
plate, the oceanic plate is usually pushed down. The plate that is pushed downward into the
mantle melts and molten rock rises in a line of volcanoes. A deep-sea trench forms where 1
plate is subducted under the other. When two continental plates collide, they either
crumple and compress, or one plate burrows under or overrides the other. This creates
mountain ranges. When two oceanic plates converge, they form an island arc as oceanic plate
is subducted.
Transformational boundaries:
They occur when plates slide and grind along transform faults and usually causes
earthquakes. An example is the San Andreas Fault.
Volcanoes:
It is a vent or chimney, and its surrounding is
built up of volcanic material, which connects the
magma in the magma chamber with the surface.
The study of volcanoes is called vulcanology. A
person who studies volcanoes is called a
vulcanologist.
Volcanoes can be classified by their recent
history of activity:
1. active: any volcano with some form of
current activity eg- eruption release of
lava or seismic activity.
2. dormant: any volcano that hasn’t erupted
for a long time but could do so again
3. extinct: any volcano that will not erupt
again
The viscosity of the lava extruded from a volcano is the main force behind the eruption
type and that is the main factor controlling the shape of a volcano.
1.
Composite (stratovolcanoes): formed from viscous lava, made up of pyroclastic
(erupted rock fragments such as pumice) and lava, they have steep slopes eg-Mt.
Fuji. Formed from convergent boundaries.
2. Shield volcanoes: formed from less viscous lava which flows away from the vent and
over time forms low sloping volcanoes.
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3. Caldera: erupt so explosively that little material builds up near the vent. Eruptions
empty part or the entire underlying magma chamber, causing the region to sink
under its weight. This results in a basin shaped depression. It is roughly circular,
and can be several kms in diameter.
What comes out of a volcano?
Gases eg- carbon dioxide, nitrogen, carbon monoxide, hydrogen, sulphur dioxide
Aerosols: sulphur dioxide reacts with water to form liquid aerosol particles of sulphuric acid
Liquids: lava
Solids: lava, fine dust
A volcano erupts when the pressure of magma within the volcano gets so high that it moves
upwards through a weakness in the crust.
Earthquakes: A sudden violent shaking of the ground caused by movements within the crust.
Most earthquakes occur on or near plate boundaries. They usually occur when transform
fault plates grind pass each other. Rocks in these plates contain a lot of elastic potential
energy. When the rock reaches its breaking point, it breaks the rocks and releases that
energy, causing earthquakes. The energy travels as a wave motion. Waves that travel
through the earth are called seismic waves.
The epicentre is the place on earth above where the earthquake has occurred. The focus is
the exact point where an earthquake occurred. The waves that travel out, causes the earth
to shake.
The shaking is detected using a seismograph, which measures and records details of an
earthquake. The chart it records to is called a seismogram. The most commonly used scale
for measuring earthquakes is called the Richter scale, which measures the power of an
earthquake. Earthquakes can cause a lot of damage to buildings and other structures. They
can cause landslides.
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