What are cratons and greenstone belts?
As mentioned elsewhere, through the process of differentiation the earth is sorting itself by density: the heavy stuff is sinking towards the core and the lighter stuff is accumulating at the surface, kind of like scum.
A craton is a continent-sized "raft" of low density felsic rock (commonly granite) which has been purified by the earth and accumulated at the surface. A good example is the North American craton, which occupies most of what we call the United States, Canada, and Mexico east of the Rocky Mountains.
But what about west of Denver? Cratons grow as plate motions move them about the surface of the earth and they run into other crustal plates. The North American craton is currently adding material along its western margin, where it is in contact with the Pacific plate along a long, linear zone of faulting called a subduction zone. Most of what is added is granitic, which is thought to form in the cores of volcanic arcs associated with the subduction process. However, lots of other stuff gets added in along with the granite. It is not uncommon for sections of oceanic crust (usually basalt) to get caught up in the accretion process and become wedged in between sections of granite. We call these inclusions of mafic crust "greenstone belts," and it is clear that this has happened many times in the past as the North American craton has developed into what we now see. The term "greenstone" comes from the color of the rock: the mafic minerals which make basalt black are commonly changed into a green mineral called chlorite as they are exposed to surface weathering processes. The result is a green stone which geologists cleverly call a greenstone. (It's brilliance such as this which justifies all the big bucks geologists are paid.)
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