Earth Structure
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- Created by: Ellie Rivers
- Created on: 03-01-14 13:07
The Structure of the Earth
- The core is Iron-nickel which creates the magnetic field
- The inner layer is solid and is 1,250km thick
- The outer layer is liquid and is 2,200km thick
- The mantle accounts for 80% of the earths volume
- It is semi-solid rock that is 2,900km deep
- The upper mantle is the asthenosphere, it has plastic properties that allow it to flow under pressure
- There is oceanic crust - 5km composed of dense basalt or continental crust - 30 to 100km composed of less dense granite
- The crust and the upper mantle form the lithosphere
- Pressure and temperature increase with depth reaching over 3000degrees in the core
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Plate Tectonic Theory
- Earths crust and Lithosphere is broken into 7 large peices and several smaller ones known as tectonic plates
- Plate boundaries are zones of violent tectonic activity
- Convection currents move tectonic plates across the asthenosphere constantly
- New crust is added by volcanic activity at mid-oceanic trenches at constructive plate margins and is moved by sea floor spreading
- It is then destroyed at subduction zones or destructive plate margins
- The outcome is tectonic activity along plate boundaries and the movement of the continents known as continental drift
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Evidence for Plate Tectonics
- Continental drift is the most powerful evidence
- Matching shapes of the continents on opposite sides of the ocean
- The same species of fossil plants and animals found in modern Africa and South America
- The crucial evidence is palaeomagnetism - iron particles in the sea floor show the earths polarity at the time they solidified
- The earth reverses polarity every 400,000 years the ocean floor shows mirror patterns of polarity either side of an ocean trench
- This proved new lithospeheric crust was being formed by volcanic activity and convection currents were puching old crust away
- This is called sea floor spreading - as ocean crust moves, the continents 'ride' the natural conveyor thus creating continental drift.
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Constructive Plate Boundaries
- New crust forms at Constructive plate boundaries where rising plumes of magma stretch the crust and lithosphere
- This results in intense volcanic activity, most often, on the ocean floor
- Volcanic eruptions build submarine volcanoes and mountain ranges and mid ocean ridges
- Parallel faults associated with tension in the crust form rift valleys that seperate the submarine mountain chains
- Mid ocean ridges are offset by large transverse faults
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Destructive Plate Boundaries
- Subduction zones where oceanic crust is destroyed when two tectonic plates converge
- The older, denser plate is subducted
- The plate including water and sea floor sediments descends into the upper mantle
- Magma, which is less dense than surrounding rocks, then rises towards the surface
- Lava, gases and ash erupt at the surface through volcanoes and fissures
- Oceanic-Oceanic subduction forms island arcs and Oceanic-Continental subduction forms fold mountains
- Destructive plate boundaries include volcanoes, earthquakes and ocean trenches
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Fold Mountains + Ocean Trenches
- Fold mountain ranges include the Himalayas and the Andes
- The Andes were formed when the Nasca oceanic plate converged with the South American plate
- Sedimetary rocks are squeezed against the South American plate and crumple to form the Andes - The Nasca plate is subducted and creates huge intrusions of magma benath the mountains
- The Himalayas were formed by the convergence of the Indo-Australian plate and the Eurasian plate - the crust is so thick there is no volcanic activity
- Ocean trenches are narrow trenches, hudreds of km long - the zone of subduction
- They occur parrallel to to island arcs and fold mountain ranges
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Conservative Plate Boundaries
- The two plates slide past each other
- This can be violent and results in powerful earthquakes
- Volcanism is absent
- The boudary between the Pacific and North American plate is a conservative margin known as the San Andreas Fault
- Earthquakes occur frequently and present major hazards to metropolitan areas such as San Francisco and Los Angeles
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