Why are granite and basalt more common than rhyolite and gabbro?
There are several interconnected reasons.
To set some ground rules: All of these are igneous rocks. Granite and rhyolite are considered felsic, while basalt and gabbro are mafic (click here for more information on mafic and felsic). Felsic rocks, in general, form the bulk of the continental plates, while mafic basalt forms the seafloor. Since the surface of the earth is covered by oceanic and continental crustal materials, granite and basalt are very common.
So far so good, but what about the rhyolite and gabbro?
Let's think about the differences between intrusive and extrusive igneous rocks for a bit. Basalt is extrusive and comes out of volcanoes, but the lava had to come from somewhere. This is where the concept of a "magma chamber" comes in - the subsurface source of liquid rock to feed an extrusive volcanic event. In simple terms, gabbro is just the cooled magma chamber for the basaltic flows that erupt at the surface. In the same respect, granite is the cooled (and exposed) magma chamber for extrusive rhyolite.
So why do we see the slowly-cooled intrusive version on the continents, but the rapidly-cooled extrusive basalt in the oceans? Well, again there are several contributing factors. One relates to the heat involved. Mafic magma is much hotter than felsic magma. Because of this, it is easier for the basaltic lavas to reach the surface while still in the liquid phase. Felsic magma, starting much cooler, generally crystallizes before reaching the surface. Therefore, more basalt than gabbro, and more granite than rhyolite.
Another reason is the internal crystalline structure of the silicate minerals. Mafic magmas crystallize to form relatively simple atomic structures (isolated tetrahedra and single chains), and therefore flow easily (the higher mafic temperatures also contribute to this ease of movement). Felsic magma on the other hand crystallizes to form more complex atomic structures (double chains, rings, and sheets) which impedes the flow- think of a log jam in a river for an appropriate analogy.
Hope this helps. This is surely oversimplified on several levels, but is (hopefully) a reasonable summary.
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