While volcanic activity has long since ceased in Southern California, it has left its mark. From cinder cones in the Mojave Desert, geothermal vents in the Imperial Valley, and extensive basalt in the Santa Monica Mountains, volcanoes have played a big role in the geology of Southern California.
The most recent volcanic activity was relatively recent in geologic time in the Imperial Valley to the east of San Diego. Some of the volcanism is as recent as about 2-2.5 kya. Activity continues even today in the form of fumaroles and mud pots. Another area of more recent activity is in the Mojave Desert. There, two large cinder cones were formed near Amboy on old US 66. Amboy Crater and Pisgah Crater were both active around 100 kya years ago. The basalt of the Santa Monica Mountains are much older. They date to the Miocene Epoch and are about 20 million years old. In the Agua Dulce area, the base of the Vasquez Formation has basalt dating back to the Oligocene Epoch (about 30 mya). Most of the now extinct volcanism took place during a time when the San Andreas Fault was forming.
These pages will feature some of the more interesting aspects of these rocks and formations.