Global Distribution of Tectonic Hazards

Key Words
• Plate boundary - the boundary or margin between two tectonic plates.
• Tectonic hazard - a natural hazard caused by movement of tectonic plates (including volcanoes and earthquakes).
• Tectonic plate - a rigid segment of the Earth’s crust which can ‘float’ across the heavier, semi-molten rock below. Continental plates are less dense, but thicker than oceanic plates.

Plate Tectonics and the structure of the Earth

Plate Tectonics is a theory that tries to explain how the Earth is structured and what it is made up of. Below is an idealised diagram of the Earth's interior (middle bit). The Earth formed approximately 4.5 billion years ago following a huge explosion of a star. The materials that make up our earth slowly gathered together due to gravity, to create a ball of hot molten material. This material has slowly cooled over geological time, forming a crust at the Earth's surface of rocks. These rocks are fractured into huge segments called Tectonic plates.
These tectonic plates are moving about very slowly, pushed and shoved around from underneath by currents within the mantle called convection currents. Another theory suggests that this is more localised, with slabs of crust pushed apart at constructive plate magins (SLAB PUSH) and where the plates increase in density away from these margins get pulled down into the mantle (SLAB PULL) by gravity and local convection cells.
Beneath the crust temperatures start to rise as you descend into the second of the Earth's zones, the Mantle, a zone of molten Silicates and other minerals. The Earth does have a solid core of Iron and Nickel, which is solid despite temperature of 3700°C because of the intense pressure there.

The plates and plate margins
The tectonic plates are made up of different materials, and there are 2 broad types;
Continental crust is thicker, older and lighter, and is composed mainly of Granite. It is 22 mi (35 km) thick on average and less dense than oceanic crust. Continental crust is more complex than oceanic crust in its structure and origin and is formed primarily at subduction zones at destructive plate margins.
Oceanic crust is younger and heavier, and is mainly composed of basalt and Gabbro. It is mainly formed at constructive margins or spreading mid ocean ridges.

The Tectonic Plates vary in size and the Earth's surface can be likened to that of a boiled egg which has been cracked. The major plates include the Pacific, Eurasian, African, Antarctic, North American and South American, and the Indo-Australian. There are other smaller plates however, such as the Philippines and Cocos plates. The tectonic plates join at zones called plate margins, where most of the world’s volcanic and earthquake activity occurs. Remember that this is a theory proposed by Alfred Wegener as CONTINENTAL DRIFT in 1912, and is now supported by lots of evidence since.

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