28 June 2015

Domes galore: Mono Craters, Long Valley

Posted by Jessica Ball

Being the guest lecturer on a geology trip is great, especially if you get to talk about your favorite geologic features. But what does one do when volcanology day is over and it’s time for the students to do a moraine mapping project?

My rock of choice. It was quite the prime spot for all purposes, including impromptu patty-cake games.

My rock of choice. It was quite the prime spot for all purposes, including impromptu patty-cake games.

Naturally, sit on a convenient rock and observe some lovely lava domes. (Okay, I spent some time helping students out with basic questions about geology and mapping, but really, while I appreciate glaciers, moraines still pretty much look the same to me. I vaguely remember what I learned about cosmogenic radionucleide sampling methods in college but it’s not something I do regularly.)

Moraines and domes, a delightful combination (if you're not the one doing the mapping project)

Moraines and domes, a delightful combination (if you’re not the one doing the mapping project)

From the Bloody Canyon moraine complex on the south side of the Long Valley Caldera, there’s a fantastic view of most of the Mono Craters chain of lava domes (minus the Obsidian and Glass Creek domes, which I’ll get to next time). The farther up you go on the moraines, the better the view – so really, hauling myself up a diamicton was worth it. And it was great practice for my drawing skills, which I don’t get to exercise much lately. Here’s the result of a solid half-hour of sketching:

Yes, I know there's no scale. You'll just have to assume that the trees are all pines and...well, tree-sized.

Yes, I know there’s no scale. You’ll just have to assume that the trees are all pines and…well, tree-sized. Also, trees are hard to draw when they’re really tiny.

Unfortunately field notebooks aren’t good for panoramas. The domes are also not labeled because I didn’t have the right paper with me, but here’s an annotated panorama (thanks to this awesome oblique map from 1983):

Mono Craters (hopefully correctly) labeled.

Mono Craters (hopefully correctly) labeled – click to see the whole picture!

There are actually more than 30 different eruptive units there, all extruded between 20 ka and the present (with the southernmost domes erupted in the last 5,000 years). Like Panum, all of them are rhyolitic (with a good deal of obsidian in outcrop). Unlike Panum, these domes are BIG. I mean seriously big. You can think of a lava dome as a sort of very short stubby lava flow in some senses, but it really doesn’t do them justice, especially when you get right up next to them.

Domes from Mono Lake

Domes from Mono Lake

The North and South Coulees are ~ 0.5 cubic km just in themselves, and there are a lot of lost and buried pyroclastic fall and flow deposits that you don’t see anymore making up possibly a full cubic km of material. (Some of these deposits have been documented and dated to figure out the eruptive sequence in the area, which may have begun as far back as 55 ka with vents that are buried beneath the current domes.) The fact that they’re in a clear line oriented roughly north-south is an indicator that there’s a structure controlling the eruptive vents (lava domes not directly on volcano summits tend to be associated with dikes, especially in this area), but there’s disagreement as to what exactly that structure is. At the very least it’s some sort of fault zone that created space to allow magma to the surface.

That’s not all for the domes in Long Valley, though – in the next post I’ll show you Obsidian dome, which is further to the south and very aptly named.

Further reading:

Bursik, M., Sieh, K., 1989, Range front faulting and volcanism in the Mono Basin, eastern California. Journal of Geophysical Research v. 94, no. B11,
p. 15587–15609.

Hildreth 2004, Volcanological perspectives on Long Valley, Mammoth Mountain and Mono Craters. Journal of Volcanology and Geothermal Research, v. 136, no. 3-4, p. 169-198

Marcaida et al. 2014, Geochemical fingerprinting of Wilson Creek formation tephra layers (Mono Basin, California) using titanomagnetite compositions. Journal of Volcanology and Geothermal Research, v. 273, p. 1-14