Plutons and Batholiths

If you come across an outcrop (exposure) of coarse-grained igneous rock, chances are you are standing on a pluton or batholith that crystallized several km below the Earth's surface. It may represent the magma chamber of an extinct volcano or a magma body that never produced any eruptions. A pluton is a relatively small intrusive body (a few to tens of km across) that seems to represent one fossilized magma chamber. A batholith is much larger (up to hundreds of km long and 100 km across) and consists of many plutons that are similar in composition and appearance. Batholiths indicate a long period of repeated igneous intrusions over a large area, such as might be expected along a subduction zone.

Because plutons are so large, it is tough to get a good shot showing their relation to the surrounding rocks. This photo shows a pluton (white granite) poking up through the surrounding gray (volcanic) rocks. Spirit Mountain, Nevada Photo by CE Jones
Like many desert granites, the granites of Spirit Mountain weather into all sorts of interesting shapes. Note the troglydite for scale. Photo by CE Jones
The Spirit Mountain granite displays an obvious coarse-grained texture despite being fairly weathered. Many natural rock outcrops are weathered: their original appearance has been altered due to the dissolution of some minerals and conversion of others to clay minerals.

Next page: Sierra Nevada Batholith (Yosemite, etc.)

After that, interesting close-ups of the Sierra Nevada Batholith

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