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rock rising in some places and sinking down in others. Of course, the pattern of convection in a spherical earth would be more complicated than in a saucepan. Furthermore, the material in the mantle would move extremely slowly.
Convection and
magma
If convection can work in the outer
zones of the earth, this has immense
implications for the way in which the crust moves.
The earth’s crust is very thin compared with the thickness of the mantle in which the convection currents may work. So, any convection currents operating in this region would be able to apply an immense force to the underside of the tectonic plate. Convection could therefore be expected to be powerful enough to make the crust move.
Where the material rises up, the crust may well
be split apart, allowing molten material (magma) to flow to the surface and form volcanoes. Where material sinks again, it may well draw the crust together before trying to pull it back into the mantle. The crust is, however, much less dense than the mantle and so would not readily be drawn down. Rather, the edges of the tectonic plates would be likely to collide and crush the crust to form mountain ranges. If, however, one piece of plate were to be dragged back down into the mantle, as it descended, it would be likely to melt, releasing low density material that could then slowly rise back through the crust and erupt as volcanoes.
There is much evidence to support this idea,
and it does help explain many features of volcanic activity. Volcanoes, for example, are common along the boundaries of most tectonic plates. Furthermore, the world has essentially two types of volcano, one kind (with runny lava and little explosive activity) forming where the crust appears to be splitting apart, and the other kind
An earth plate contains both continent and ocean floor.This is the African Plate, the part of it above the sea we know as Africa.The African Plate
is also part of the Atlantic Ocean floor.
(Above) With the crust formed on the surface, convection breaks up the crust, allowing new magma to well up from the mantle along some boundaries.
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