WHERE IS THE HEAT COMING FROM?
The Earth is composed of a number of layers which are known from the centre outwards as the inner core, outer core, mantle and crust (see fig1). Temperatures in the Earth’s core reach over 4,500°C and in the mantle it varies from more than 4,000°C to 1,100°C in the outer part. In the Earth’s crust the temperature increases with depth. The average increase in temperature with depth (=geothermal gradient) is approximately 25°C – 30°C per 1,000m. Most of this heat (>75%) comes from the mantle but some (± 20%) comes from the radiogenic decay of uranium (U), thorium (Th) and potassium (K) isotopes.
The planet Earth is a dynamic thermal engine with most of the heat originating from radioactive decay from deep within. Without this phenomenon, the Earth would have ‘frozen’ into a solid, devoid of any life. Instead, the Earth is in a state of continuous change and renewal, creating new crust whilst at the same time consuming existing crust. This fundamental geologic process is known as plate tectonics. |
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| The crust (± 35km thick) and the outermost mantle are known as the lithosphere, and together these layers behave as a rigid shell. This shell is broken into 12 large and several smaller plates by slow moving convection currents in the underlying mantle. Where the plates move apart, mid-oceanic ridges are formed (long chains of sub-marine volcanic mountains which sometimes come to the surface, e.g. Iceland, azores). Where plates collide, one is commonly thrust beneath the other, forming deep oceanic trenches and causing powerful earthquakes. The down going plate (known as sub-duction zones) will eventually melt in the mantle, resulting in magma bodies which, because they are lighter than the surrounding rocks, rise to the crust, often giving rise to volcanoes, forming island arcs such as New Zealand, Indonesia, etc. Where plates collide or move apart, the geothermal gradient is often much higher than the average of the Earth crust (25-30°C) and may reach 200°C/km or more. |
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