Rock Cycle?

Question

How does the rock cycle explain the curious fact that sedimentary rocks account for about 75% of all surface rock outcrops, while they only represent about 5% of the crust by volume? Put another way, why are sedimentary rocks not evenly distributed throughout the crust?

Answer 1

Remember how each rock type is formed:  igneous rock is cooled magma (externally or internally) and is probably more prevalent in lower levels of the crust.  Metamorphic rock is rock that was once something else and morphed from the heat and pressure put upon it.  Again, this is something that needs to happen deeper down in the crust in order for there to be enough heat and pressure.  That leaves sedimentary rock-- rocked formed by pressed together layers of sediment.  The sediment comes from the surface erosion of rock and soil that is then deposited at low points under bodies of water.  Relatively speaking, it is the rock that is nearest the surface because of how it is formed, so it would make sense that it is the type of rock most often exposed in surface rock outcrops.  So, my thought on why it is not evenly distributed throughout the crust is that as the plates shift and sedimentary rock is pushed downward, it becomes metamorphic rock.  The only way to stay in the sedimentary category in our wonderful rock cycle is to keep near the surface and never change!