Suppose we find a coarse-grained igneous rock at the surface of the Earth (the entire Sierra Nevada mountain range consists mostly of coarse-grained igneous rock).  Think of the stories it tells!  How did it come to be at the Earth's surface since we can see it must have cooled deeper in the Earth's crust?  Suppose you find an igneous rock that is made mostly of crystals too small to see (it looks massive and uniform), but there are a few big, sparkly crystals in it.  What story does it tell?
 

The rock of the Sierra Nevada must have cooled at depth in the Earth's crust, then been exhumed to the surface in some way.  The process which we can observe happening today that might exhume something from depth is erosion.  Thus, we infer that erosion of formerly overlying rock has raised the coarse grained igneous rock to the surface.

   Porphyritic Trachyte (2 crystal sizes)
Two different sizes of crystals in an igneous rock imply two different rates of cooling.  There was an initial cooling stage at depth in the crust when the magma cooled slowly, growing large crystals.  Then, when the magma was a mush of large crystals and still-molten material, it erupted to the surface and cooled quickly, leaving the big crystals imbedded in a matrix of small crystals.

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