This is the "Soda Dam" near the Jemez Hot Springs. This calcium carbonate structure is the result of super-heated water disolving calcium carbonate from a limestone formation deep in the mountain. This is also evidence of a remaining magma chamber about 3 miles deep. The chamber is currently not moving and we are hopeful that it will stay that way. As such chambers go it is fairly cool (only 400 C or 750 F, give or take 100 degrees). When I was a kid the Jemez was considered an extinct volcano. Sometime around 1970 it was upgraded to dormaint. Today because of the magma chamber and the dozen or so hot springs related to this volcano it is now classified as active. Two other super large volcanoes are also active in the US: Yellowstone in Wyoming and Long Valley in California.