9 June 2015
Oh, man. Summer is a terrible time for keeping up with blog posts, but I’ve had a good reason to be absent – I was off in Denver on business and slightly wilder parts of California with my alma mater’s summer field course. I mean, what geologist could pass up the chance to tag along on a trip to Long Valley and Yosemite?
During the Long Valley and Mono Lake portion of the trip, I actually did do a little work, serving as the trip’s volcanology expert and talking about lava domes as much as anyone would let me. Because Long Valley may be a beautiful caldera and the site of one of the world’s largest eruptions, but it also has domes. Boy, does it ever have domes.
We spent a lot of our time at Panum Crater, the youngest of the Mono Craters chain of domes. The majority of the chain was emplaced from ~20,000 to 1,300 years ago (with some eruptions possibly as old as 50,000 years) along a structural boundary of some sort (there’s been argument as to whether it’s the edge of a pull-apart basin or the edge of a failed caldera). Panum was erupted about 660 years ago, so in geologic terms it’s just a baby. Compared to the other domes in the Mono Craters, which can be as much as 0.5 cubic km in volume, Panum dome’s volume of ~0.007 cubic km is pretty small.
Of course, it doesn’t look like that when you’re up on the dome itself – compared to humans, domes are imposing no matter what.
As part of their exercises for the day, I gave the students a brief primer on the different eruption styles that happened at the dome and then sent them off to interpret deposits and come back with a progression of events. The Panum Dome eruption started with phreatic explosions when magma came into contact with subsurface water (either a local water table or other confined body). These explosions threw out chunks of old, cold country rock (mostly granite), which was pretty evident when compared with the newer volcanic deposits, and built up a “tuff ring” around the vent.
When the water supply was finally exhausted, the magma made it the rest of the way to the surface and began erupting juvenile material as pyroclastic density currents – flows and surges, with intermingled ash falls. (Ash is the first thing to vanish after an eruption, since it’s so easy to wash or blow away in erosion processes. The top of the tuff ring is pretty ashy, but it’s a sand-sized ash rather than really fine particles.)
Eventually the eruption ran out of steam (quite literally – water vapor is a major gas component in magma) and transitioned to pushing out an initial lava dome. There are several stages of dome growth evident at Panum (an early dome that collapsed during the building of the ejecta or tuff ring, and the later dome that we see today). The remaining dome is a fantastic amalgam of obsidian and pumice, showing that not all of the magma was degassed at this point, and that both components were both high in silica and cooled fairly quickly.
The collapse of the early dome formed block-and-ash-flows, which are fairly common on growing (and not growing) domes. We didn’t see these deposits at the dome, but there are some local washes and roadcuts that can be dug out to expose them. What we did see was a lot of tephra from the following Strombolian eruptions, including some beautiful breadcrust bombs. These form when a glob of lava is thrown out of a vent and the outside cools quickly, but the inside is still hot and expanding and cracks the outer crust. The ones we saw were quite pumice-y:
The bombs were pretty cool, but my favorite bit was climbing up onto the dome and exploring the intermingled obsidian and pumice. All of it is rhyolitic, meaning it’s pretty high-silica (70% or more), and it makes for a very glassy dome no matter what part you’re looking at.
There were even spines to explore, which don’t always survive dome-building. A spine occurs when lava is extruded from the vent and sticks together instead of crumbling or falling over or forming a shear lobe (basically a short-ish lava flow). They’re usually quite fragile and tend to fall over.
Close-up, the dome rock had some really interesting features, like these cracks in a pumice block:
And these bits that oozed out through cracks in a cooling block:
After a bit of discussion, the students correctly interpreted the series of events (phreatic eruptions –> pyroclastic density currents –> Strombolian eruptions –> dome-building), and then had a little while to explore the surface of the dome. The W&M crowd usually doesn’t have a chance to see volcanic rocks this young (in Virginia, the youngest ones are in Triassic rift basins and they’re still 230 million years old), so this was a great stop for them. And for me, since I haven’t yet had the chance to explore a drive-up lava dome – the Santiaguito domes that I worked on in Guatemala were much less accessible. I’m definitely going to have to go back and check out the Mono-Inyo Domes, which I only saw from a distance on this trip.
Sieh, Kerry, and Bursik, Marcus, 1986, Most Recent Eruption of the Mono Craters, Eastern Central California, Journal of Geophysical Research, Vol. 91, No. B12, p. 12,539–12,571.