Plate tectonics

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  • Created on: 10-02-14 13:45
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Plate Tectonics
Earth's Formation
The Earth is thought to have been formed about 4.6 billion years ago by collisions in the giant
disc-shaped cloud of material that also formed the Sun. Gravity slowly gathered this gas and dust
together into clumps that became asteroids and small early planets called planetesimals. These
objects collided (primordial heating) repeatedly and gradually got bigger, building up the planets in
the Solar system including the Earth. Scientific understanding of Earth's internal structure is based on
observations of topography and bathymetry, observations of rock in outcrop, samples brought to
the surface from greater depths by volcanic activity, analysis of the seismic waves that pass through
Earth, measurements of the gravity field of Earth, and experiments with crystalline solids at
pressures and temperatures characteristic of Earth's deep interior.
Earth's structure
At the centre of the Earth is the Core, the densest part of the earth, which is split into the Inner Core
and Outer Core.
Inner Core - solid - contains iron and nickel.
Outer Core - semi-molten - contains iron and nickel.
The temperature of the outer core ranges from 4400 °C in the outer
regions to 6100 °C near the inner core.
Around the Core is the Mantle which is largely composed of silicate
rocks - iron and magnesium.
Upper rigid part of Mantle falls into the Lithosphere.
Layer below this is called the Asthenosphere (semi-molten
Core high temperatures generate convection currents.
Earth's Surface
While the Earth appears to be a perfect sphere when seen from space,
it is in fact a geoid. This means that it bulges around the equator and is flatter at the poles. The cause
of this shape is centrifugal forces, generated by the earth's rotation, which flings the semi-molten
interior outwards, like in a roundabout.
The earth consists of 7 major plates, where the plate's meets are known as plate boundaries or
margins. The lithosphere is divided into lots of slabs called tectonic plates.
Outer layer of the Earth is called the Crust
Thinnest and least dense part of the Earth's structure.
Contains rocks rich in Silicon, Oxygen, Aluminium, Potassium and Sodium.
There are two types of crust
Continental crust: 30-70km thick and comprises of less denser rocks. Sedimentary rocks,
Igneous and Metamorphic. (SIM)
70km thick beneath the world's major mountain ranges i.e. Himalayas.
Oceanic crust: 6-10km thick and in relative to continental crust is denser. Basaltic in nature.
Together the crust and the rigid upper mantle are collectively known as the
Moho Discontinuity

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Moho ­ Zone that separates the Earth's crust from the asthenosphere.
Discovered in 1909 by Andrija Mohorovicic.
Tectonic plates are slabs of lithosphere that can move around because the part of the mantle
directly below is much weaker.
The word "discontinuity" is used for a surface at which seismic waves change velocity.
One of these surfaces exists at an average depth of 8 kilometres beneath the ocean basin
and at an average depth of about 32 kilometres beneath the continents.…read more

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As magma erupts from mid-ocean ridges, magnetic minerals in molten rock align
themselves with the direction of the Earth's magnetic field.
Magnetic minerals are aligned opposite to magnetic minerals deposited in reverse polarity.
Creates series of Alternating Magnetic stripes along the sea floor.
Crust is older the further away from the mid-ocean ridge ­ means that plates are moving
apart ­ new rocks are bring added equally on either side.
Continental drift theory developed further by scientists growing it into the theory of plate tectonics.…read more

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This uplifting of the ocean floor occurs when convection currents rise in the mantle
beneath the oceanic crust and create magma where two tectonic plates meet at a
divergent boundary.
o Hotter expanded crust forms a ridge.
o The central part of the ridge may feature a valley where a section of the crust has
subsided into the magma below.…read more

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This feature might widen still further and become submerged by the sea.
o To the north two rifts have widened in to the Red Sea and the Gulf of Arabia.
o Rifting has continued and new ocean floor is forming between Africa and Arabia.
East African Rift valley formed by three plates pulling away from one another the Arabian Plate and
two parts of the African Plate.…read more

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Continental are of lower density than the Asthenosphere
beneath them, their buoyant nature cause them not to
o They collide with each other and simply become uplifted
and buckle to form fold mountains i.e. Himalayas.
o Erosion and weathering reduce height of mountains, but
isotactic lift in some areas, produced by continuing plate
motion, means that mountain i.e. Everest which is
increasing in height of 2.5cm per year.…read more

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Another is a harmonic tremor, which is steady movement of magma, deep in the mantle.
There's a strong relationship between earthquakes, volcanic activity and plate tectonic theory. 95%
of the world's earthquakes and most volcanoes are located along plate margins
Vulcanicity Exceptions to plate margins
Hot spots create Volcanoes near the centre of a plate rather than margin. Most active volcanoes are
associated with convergent and divergent plate margins, not all conform to that patter i.e.…read more


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