Earthquake swarms are common in the Coso area, often producing hundreds of tremors over periods as short as a few days. This brisk and robust seismic activity is common in volcanic areas, such as Long Valley Caldera near Mammoth Lakes, and Yellowstone Caldera at Yellowstone. The Coso Volcanic Field shows examples of volcanic activity, probably last active 30,000 to 40,000 years ago, but ash emission and small cone building episodes may be Holocene (>10,000 years) in age.