The Earth is formed of four concentric layers that have very different physical and chemical properties: inner core, outer core, mantle and crust.
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- The centre or inner core is the hottest part of planet Earth. A solid mass of iron and nickel this centre has temperatures up to 5,500 degrees Celsius. With this extraordinary heat the inner core functions as engine for the planet:
- Surrounding the outer core is the outer core, a liquid layer of iron and nickel, extremely hot, this layer has similar temperatures to the inner core:
- The widest section of the Earth is the mantle with an approximate diameter of 2,900 km. The mantle is composed of semi-molten rock called magma. The upper parts of the mantle are hard rock but deeper within the rock is softer and beginning to melt:
- The outer layer of the planet is known as the crust. It is a solid rock layer which averages about 60 kilometers in thickness.
The crust and the upper mantle form a cold, strong layer known as the 'lithosphere' (from the Greek for 'rocky' sphere) floating on the inner mantle. The lithosphere is fragmented into a dozen of huge, irregularly shaped pieces, called tectonic plates, which are in constant motion and slide over, under and past each other on top of the partly molten inner layer. There are two kinds of plates: oceanic crust (i.e. the plates under the ocean) and continental crust.
There are 7 large and many small moving plates. The plates have a depth of 50 miles and on average they move only a few inches a year relative to one another. Coastlines and plate boundaries do not always align.
Although the upper mantle is solid rock, it slowly flows in a convection current because of heat dissipating from the core. Convection currents in the mantle cause the plates to move several centimeters a year in different directions.
If you look at the convection currents, you'll see that:
- as the hotter Mantle rises cooler material sinks;
- the material rises and spreads out while the crust splits and diverges;
- the plates converge and subduct as the material sinks.
Stress is exerted the plates as they move and those around them. They may collide, sink, or pull apart as the plates scrape boundaries creating stress which results in strain setting energy free. Subduction occurs when plates collide and one is drawn beneath another. This process can take thousands of years. The collision of plates creates mountains as rock layers are forced upward. As the plates diverge lava pushes through the mantle, cools and forms a new section of crust. Plates moving slowly along side each other create friction and intense heat as they slide resulting in volcanic activity if the rock melts. Earthquakes can occur if the plate slips sliding away or towards each other.
In some cases this is a gradual movement. Sometimes the plates lock together unable to release the energy accumulated which builds up in the rock.
When this energy elevates to the elastic limit of the rocks, they will break free causing the ground to shake. This usually occurs when two plates either ride over, or slide against each other, and the material at the edge of the tectonic plates deforms and ruptures at its weakest point. Thus the strain energy stored within the plate is released in the form of vibrations.