The San Gabriel Mountains Anorthosite is an intrusive igneous body that cooled 1.2 billion years ago. It intrudes 1.7 billion-year-old rock that is the oldest rock exposed in California (Carter, 1980). The anorthosite formed from a magmatic body that was some 15 km (10 miles) wide. Geologists call a body of magma this large a pluton. The name anorthosite comes from the fact that the rock is composed almost entirely of the mineral anorthosite, which is a type of plagioclase feldspar. This mineral is white-colored and that is the reason the slopes above the stop location appear so light-colored. Plagioclase feldspar is a type of mineral with a high concentration of silica. The anorthosite rock thus has a high concentration of silica and falls under the category of being in the class of igneous intrusive rocks called granitic. Although the rock solidified far beneath the Earth’s surface, it is now exposed at the surface because the overlying rock was removed by erosion as the San Gabriel Mountains have been uplifted.
In addition to silica, anorthosite is rich in the element aluminum. The anorthosite rock could be mined for its aluminum, but it is easier and less costly to extract aluminum from the mineral bauxite. Someday in the future, if bauxite becomes nearly mined out, the anorthosite might serve as a source for aluminum.