California Supervolcano Has Split Personality

By Larry O'Hanlon | Tue Mar 2, 2010 07:00 AM ET

There's some good news and bad news regarding a sleeping supervolcano in California.

The good news is that the Long Valley caldera, like her big sister Yellowstone in Wyoming, is not getting ready to super-erupt and again cover half of North America in ash. The bad news is that when Long Valley does someday get hot and bothered again, it may prove to be surprisingly bi-polar.

A new study that uses argon isotopes in the lava rocks to refine the timing of different eruptions over the last 160,000 years reveals that when Long Valley has spewed forth in the past, it's been in two very different styles at nearly the same time, which makes it a bit of a volcanic oddity.

One eruption style is a gloppy, not very explosive lava called basalt that poses little blast danger unless it contacts groundwater or snow.

The other sort of eruption involves more glass-rich or "silicic" magma that appears at Long Valley to hitch a ride with the basalt to the surface. When it gets there, it tends to come out of the ground in the more explosive, ashy style of Mount St. Helens.

http://news.discovery.com/earth/california-supervolcano-eruption.html