What volcano in California is acting like it might soon create a caldera?
There are only two significant calderas in California at the present time, Long Valley caldera near Mammoth Mountain and Medicine Lake caldera east of Mount Shasta (first link). Of these, only Long Valley shows signs of current activity. The Clear Lake volcanic field north of San Francisco is active and is underlain by a large volume of viscous, silica-rich magma (second link). Although there is no caldera at the present time, increased activity could result in an explosive-type eruption and possible caldera formation.
