California is generally considered to have six potentially active volcanoes, Medicine Lake, Mount Shasta, Lassen Peak, Clear Lake, the Long Valley Caldera and Cosco Peak. The Ubehebe Crater in Death Valley has always been assumed to be an ancient feature, the result of a phreatomagmatic explosion; an explosion that occurred when rising magma encountered subterranean water, causing the water to vaporize instantly and leading to an explosion that blew rocks over the floor of the valley. There are several smaller craters nearby that appear to have similar origins. Since Death Valley is thought to be a very dry place, it was always assumed that the explosions took place long in the past, when the valley had a wetter climate.
How a phreatomagmatic explosion created the Ubehebe Crater. From Jake McDonald's Dessert Southwest Capstone 2007 Trip Page.
On 18 January 2012 a paper published in Geophysical Research Letters by a team from the Department of Earth and Environmental Sciences at Columbia University led by Peri Sasnett, describes a new study of the Ubehebe Crater, which suggests that it may have been formed much more recently, and that further activity at the site could happen again at any time.
The team used a method developed to date moonrocks, which relies on detecting isotopes created by cosmic rays hitting rocks at the surface to determine how long had passed since the rocks were exposed. To their surprise they discovered the most recent explosion must have occurred about 800 years ago, at a time when Death Valley was even hotter and drier than it is now.
This has two implications. Firstly, it is clearly not to dry in Death Valley for phreatomagmatic explosions. Actually this is not too much of a surprise. There is still groundwater deep beneath the surface of Death Valley, and the idea condition for phreatomagmatic explosions is quite dry; if there is too much water it will tend to swamp the explosion, reducing the size of the eruption. Secondly, magma has been rising towards the surface of Death Valley much more recently than had been realized, and therefore this could happen again.
Authorities at Death Valley National Park are relaxed about the study; it is unlikely that any explosion would happen without warning, so they do not have to worry about accidentally blowing up tourists, and none of the prior eruptions have been big enough to cause damage beyond their immediate vicinity. They have far more to worry about from car accidents and tourists getting heat exhaustion.
Volcanism on the West Coast of North America are fed by the subduction of the Juan de Fuca Plate beneath the North American Plate. As the subducting plate sinks into the Earth's interior it is heated by the heat of the planet's interior. Some of the melted material then rises up through the overlying North American Plate, forming volcanoes at the surface.
Diagram showing how the subduction of the Juan de Fuca Plate causes volcanism on the West Coast of North America. From the United States Geological Survey.