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 geologic history of Southern California. Some of these areas will see volcanic activity in the future, though probably not for a few thousands years or more.

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. Other areas of more geologically recent volcanism can be found in other areas of the Mojave as well. There is a small volcanic field with numerous cinder cones, basalt flows, and even a lava tube in an area called Cima Volcanic Field southeast of Baker, California. These erupted between 7.6 mya and about 10 kya.

The basalt of the Santa Monica Mountains, known as the Conejo Volcanics, are much older. They date to the Miocene Epoch and are from about 17 mya. In the Agua Dulce area, the base of the Vasquez Formation has basalts dating back to the Oligocene Epoch (about 25 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.