That's actually a "small" one in the area of supervolcanos, but it is worrisome because of the location, and slightly puzzling because it does not appear to be located above a fixed hotspot that the crust moves over like Yellowstone and because it is located in an area that should already be "dead" when it comes to volcanos.
From what we understand about how volcanos are formed, the majority are inland of an active subduction zone. That is why the volcanos north of San Francisco up to Canada are all still active, and those south of there are all dead. The mountains of the Sierras north of SF are active, with various forms of stratovolcanoes resting above pools of magma in various states.
And to the south, increasingly what is seen are "ghost volcanos". The volcano is no longer there, it has been eroded away and all that is left is a giant chunk of granite sitting where the magma chamber used to be.
That is why Long Valley is a bit of a puzzle, it does not match what we understand of such features. The subduction of the Farallon Plate to the west ended over 30 million years ago, so there should no longer be a slab down there melting to create more volcanos. Yet there it is, still active. Only a single eruption that they have discovered, but a resurgent magma dome and continuing activity in the magma chamber.
In a way, the reverse of a puzzle over the Yellowstone Hotspot. That has been throwing out multiple highly explosive eruptions, even when it was sitting over an area that should have been throwing out much more docile eruptions and creating stratovolcanoes.
So is Long Valley in the process of dying, or is something still fueling it and it can erupt again? They simply do not know, so are keeping a close eye on it.
The Valles Caldera in New Mexico is much the same as the Long Valley. At least two eruptions between 1.2 and 1.6 mya, and once again sitting in an area that it should not exist. Where off-shore subduction had ended over 45 mya, and there should be nothing that we know of that is driving it. And it also is dormant but still active.
www.nj.com
en.wikipedia.org
earthobservatory.nasa.gov
