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Classification of Igneous Rocks

Chart showing the types and components of igneous rocks. Graphic includes a list of extrusive and intrusive (plutonic) rock names and a chart showing the percent of different minerals present in each rock type by volume.

The igneous rocks in Point Reyes National Seashore are primarily granodiorite, with lesser amounts of granite and tonalite. The rocks cooled slowly, deep underground (intrusive) within a massive plutonic body called a batholith, and are therefore coarse-grained. They are of felsic composition (containing primarily light colored minerals). Tonalite is similar to granite except that feldspar is more abundant as plagioclase than as alkali.

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USGS graphic by J. Johnson

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Point Reyes National Seashore, Code: PORE

Point Reyes National Seashore, Marin County, California
Latitude: 38.0550003051758, Longitude: -122.879997253418

NPS.gov Collection

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