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Tectonics: 1.2 There are theoretical frameworks that attempt to explain…
Tectonics: 1.2 There are theoretical frameworks that attempt to explain plate movements
Explain the theory of plate tectonics
Earth’s tectonic plates move at a speed of 2-5 cm per year. There are seven very large major plates (African, Pacific), smaller minor plates (Nazca, Philippine sea) and dozens of small microplates. They all fit together into a constantly moving jigsaw of the rigid lithosphere (below this lies the asthenosphere). Each plate is about 100km thick. Its lower part consists of upper mantle material while its upper part is either oceanic or continental crust.
Lithosphere
Thin outer shell of the earth.
Asthenosphere
A layer of molten rock, this is melted and flows (like putty) under pressure.
The theory of plate tectonic has developed becuase of a number of key discoveries
The idea of Arthur Holmes in the 1930s that the earth's internal radioactive heat was the driving force of mantle convection that could move tectonic plates.
The discovery of the asthenosphere, a weak, deformable layer beneath the rigid lithosphere on which the lithosphere moves.
Alfred Wegner's continental drift hypothesis in 1912 that postulated that now-separate continents had once been joined.
Explain how the processes operate at different margins
Constructive Plate Boundary
How it happens
: two oceanic plates move apart, the new land is created in the gap between the new plate, magma rises in the gap to create a shield volcano.
Both plates are oceanic (has the ocean on top) and they are both heavy and dense.
Example: Mid-Atlantic Ridge - South American plate moves away from African plate and the North American plate moves away from the Eurasian Plate.
Collision Zone
Transform zones are where the two plates are together and it consists of transform faults. These faults ‘join up’ sections of constructive plate boundary as they traverse the earth’s surface in a zig-zag pattern.
It is on a conservative plate boundary
Earthquakes can occur often, e.g. San Francisco and the San Andreas fault
There is no volcanic activity as the plates slide past each other.
Transform Zone
An Example of a collision zone is the Himalaya mountains (the Indo-Australian and the Eurasian plate). This is when the plates are the same density, so subduction is not possible. Instead, the plates have ‘crumpled’, creating enormous tectonic uplift in the form of the Himalaya and the Tibetan Plateau.
A collision zone is where two continental plates are in a collision together
Neither of them subducts because they both have the same density.
Magma is being generated at depth but cools and solidifies beneath the surface so eruptions are very rare.
The plates are still slipping which means that earthquakes occur.
Destructive Plate Boundary
These have a 10 - 400 km focal depth.
It creates an ocean trench, fold mountain, composite volcano and earthquakes.
Convection currents pull the oceanic plate down.
The oceanic plate subducts under the continental plate as it is much denser and heavier.
Continental plate: lighter, less dense, older (over 150 million years old)
Oceanic plate: Heavier, denser, younger, has ocean on top
