Sial or SiAl is the name for the upper layer of the Earth's
crust, which is also known as the
continental crust because it is absent in the wide oceanic basins. This layer is made of rocks rich in
silicates and
aluminium minerals. When the sial comes to the surface it is typically
granitic, so sometimes this layer is called the 'granitic layer' of the crust. Geologists often refer to the rocks in this layer as
felsic. On the continental
plates the sial runs between 5 km and 70 km deep. Because in a real sense the sial floats on the
sima, mountains extend down as well as up, much like
icebergs on the ocean, hence the great variation in depth. See
Isostasy.
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