The High Sierra

So the Farallon Plate shoved under the North American Plate and dove into the mantle, where the leading edge melted, five to ten miles down. If that melted rock had risen to the surface while still melted, volcanoes would have resulted, and the emergent lava would have quickly cooled to become various kinds of volcanic rock. The Cascades are a volcanic range like that. It happens quite often. In the Sierras, the Alps, and the Himalayas, the melted rock never made it to the surface. It pushed up against the rock over it, but that roof held, so the melted rock slowly cooled down there. That deep slow cooling of melted crustal rock creates granite; and a big mass of granite is called a batholith. The Sierra Nevada batholith emerged into the light of day only after the vertical miles of Earth over it eroded away. Now it extends from a bit north of Lake Tahoe to Tehachapi Pass in the south. If you could see the whole batholith free and clear in the air, it would be shaped something like a backpacker's air mattress: 450 miles long, 60 miles wide, and, because it still extends about six miles deep into the earth, while standing two to three miles above the land surrounding it, about eight or nine miles thick. None of this was a uniform process. The batholith never looked exactly like an air mattress down there; it was maybe more like a band of cumulus clouds, with individual blobs of melted rock mashed together at their edges. Those individual blobs are called plutons. They're lumped together like party balloons, or the bumpy top of a parasail. That's the range as we see it: the slightly rounded tops of about 20 oval blobs, all pressed together. They're often oval rather than circular, averaging around 10 miles by 20 in size.