In light of the motion picture Volcano (a 1997 fictional story depicting a volcano emerging under Los Angeles), we posed the above question to U.S. Geological Survey geologists. We gratefully received this answer:
The molten rock (magma) that feeds volcanoes comes from much closer to the surface than the core, which is about 2,900 kilometers (about 1,750 miles) deep. Volcanoes are located where there is a source of magma. Lots of times this is at plate boundaries and that's also where there are lots of faults and earthquakes. The San Andreas Fault is a place where two plates are sliding PAST one another, so there are lots of faults and earthquakes. One of the main places where rock is melted is where one plate slides UNDER another. That happens further north in the Cascades of Washington, Oregon, and northern California and that's why they have some active volcanoes (like Mount St. Helens) there. Los Angeles and southern California may have a lot of potential for earthquakes, but are probably safe from volcanoes for a while.
Ronald R. Charpentier, Geologist
U.S. Geological Survey
Geologist Charpentier adds the disclaimer that this is his opinion and is not necessarily the official opinion of the U.S. Geological Survey or the U.S. Government.
Thanks to Ron Charpentier and Michael Marlow of the U.S. Geological Survey
The closest volcanic area to Los Angeles is the Coso Volcanic Field that lies just north of Ridgecrest, California, about 181 miles north of Los Angeles.