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Chapter 3: Volcanoes
The earth is believed to have formed when gravity pulled the dust of space into a planet. During this process an incredible amount of heat was released.
The heat contained in the early earth could only be lost in three ways: by conduction, by radiation, or by convection. The early earth did not have a crust to prevent heat being lost to space, and so most heat was probably lost by conduction and radiation, and new heat was probably brought
to the surface by convection. But with time the surface cooled, and a crust formed.
Rocks are not good conductors of heat, so
the heat produced inside the earth simply could no longer be lost through the earth’s crust. As a result,
very little heat could be lost by radiation or conduction. Thus, as the crust developed, the rate at which heat
was lost must have slowed down. The core of the earth, however, remained molten and very much hotter than the crust, providing the conditions for convection to continue.
Convection: the source of
volcanoes
Just as the water in a saucepan churns when it is heated from below, so the heat stored within the earth can cause a slow circulation to occur in the mantle and bring hot rock close to the surface. Earth scientists think that the rock in the outer mantle may be hot enough to move slowly and even may be molten in places.
Comparing the upper mantle with the way in
which heated water moves in a saucepan helps
us understand the way in which heated rock may
move just below the crust. In a saucepan hot water currents tend to rise in columns before sinking again. This circulatatory movement makes up a cell. Many cells may form in a heated saucepan of water; if convection inside the earth happens in a similar way, we would expect the pattern of convection also to produce cells, with molten
The early earth probably lost heat by radiation, conduction and convection.
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(Above) As the earth’s surface cooled, a crust formed, preventing conduction and radiation.