I'm addressing your suggestion that schist and granite were formed following the great flood as a result of compression and heat due to the overlying sediments. In OE, that's certainly possible, though you'd have to add a whole lot more overlying sediment and tectonic stress. But large amounts of granite and schist are found exposed on the surface of the earth today, mostly in mountain ranges. Sometimes, very large mountain ranges.
How did these rocks form without the overlying sediments to bury, compress, and allow them to cook deep in the earth?
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Granite is a plutonic rock which forms from the cooling and crystallization of magma in the subsurface, generally, several kilometers deep. Granite has a medium-grained texture, meaning you can see the individual minerals forming the rock.
Volcanic rocks cool and crystallize in the surface or near-surface environment, and tend to be finer-grained. Rhyolite is the surface/near-surface equivalent of granite.