More about the earth’s mantle

The earth’s core is so hot -- 4200 degrees Celsius at the centre -- that it is liquid.

The earth would have cooled and become completely solid millions of years ago without an energy source in addition to that of the sun. The source is radioactive decay occurring deep within the earth -- geothermal energy.

Idealized diagram of the earth’s deep structure.  (Source: South Pacific Geoscience Commission.)

In most areas, this heat reaches the surface in a very diffuse state. However, in some areas geothermal resources are relatively close to the surface.

The earth’s crust is broken into a number of “tectonic plates” that float on the molten layers underground -- like the cracked shell of a hard-boiled egg.

Tectonic plates move about very slowly -- usually at about 2 to 12 centimetres a year. Where they move apart or collide with one another, volcanoes may form above “hot spots” in the crust and mantle.

The largest tectonic plate is the Pacific Plate. The borders of the Pacific Plate and other major tectonic plates have so much volcanic activity (they contains about two-thirds of the world’s active volcanoes) that the rim of the Pacific Ocean is called the “Ring of Fire”.

The hot interior may also be closer to the surface at the edges of tectonic plates, and that is the case below some parts of Fiji. Although Fiji does not have volcanoes, it has several locations where geothermal heat is near the surface and able to be accessed by drilling.


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