Mono Lake Black Point (#0471)

We walked out to the Black Point fissures several years back. I have a few pix toward the end of this album:
www.ipernity.com/doc/289859/album/443103

Interesting the slots are not black at all. Does that mean that the lava is only surface covering over a pre-existing point?

From my old notes: The sediments making up Black Point are tephra, debris thrown out by an explosive volcanic eruption, the explosions being driven by magma at depth encountering groundwater. On land such an eruption yields a classic crater, a "maar," with the tephra forming a ring around a central depression. Most of the tephra is just broken-up country rock, but bits of lava get thrown out as well, and I suspect the dark debris on top of Black Point is a mantle of basalt fragments. Actual flowing lava never breaches the surface in a maar eruption. The Black Point deposits don't have the classic maar shape because they were erupted under Pleistocene Lake Russell, which filled the Mono basin at the time. The fissures are expansion cracks due to later uplift of the deposits, probably by more magma intruded below.

Btw, classic maars look a lot like impact craters and were at one point taken seriously as a model for lunar craters.

Gee, you'd think nature would do things more simply! (I'm teasing). Thanks for this. It would be interesting to do a tour of the Owens Valley with you just to get a sense of what the visual (surface) evidence of volcanic activity tells us. Of course, that would mean having to understand the Great Basin as well, since I had been (incorrectly) assuming that the Great Basin had at once been a small number of very large lakes.

Those of us in the natural sciences have been known to get irritated with physicists, mathematicians, and such--the Real World is usually a great deal more complicated than their models! ;) (There's an old joke about a physicist-turned-milkman that begins, "Consider a spherical cow of mass m--")

In the so-called "pluvial" (i.e., the Ice Age, in the Pleistocene), essentially all of the closed basins in the Great Basin held lakes. If the levels got high enough the lakes would merge over the lowest sills between them. The big lakes, Lahontan and Bonneville, were on the west and east sides, respectively, of the Great Basin. Overall the east-west profile across the G.B. is convex in the middle, which is what localized to big lakes to either side. They still consisted of multiple sub-basins, however.

What happened in the Pleistocene was evidently that the ice cap covering the northern part of the continent drove all the storm tracks farther south, so that the Sierra got an enormous boost in snowfall (hence the growth of the huge mountain glaciers), and the mountains in the Great Basin got more precipitation as well. You even grew substantial glaciers in places like the Ruby Mountains, and smaller ones elsewhere as well. There's a classic old paper by Eliot Blackwelder, from the early 30s I think, on glacial evidence in the G.B. and surrounding areas.

You know 'milk' is a social construct, it doesn't really exist....

I realize that the peak at Great Basin NP would have been above the lakes, but somehow I thought most of ranges across central Nevada (e.g., south of Elko) were either underwater or didn't exist and were carved out by retreating glaciers.

When the conversation turns to such vast changes in climate, the effects of human seem comparatively miniscule :)

The topography across Nevada in the late Pleistocene was pretty much as it is today. A few tens of thousands of years, the age of the youngest glacial event, is a moment ago, geologically. The mountains across Nevada are geologically young, but considerably older than that--the faulting and crustal extension that formed them started a few million years ago or so. We used to have students calculate how long it would take to raise the Sierra scarp there by Lee Vining, taking the modern earthquake recurrance rates and the fault offsets seen on the Lundy Canyon moraines as typical. You get numbers around 6 million years. The Sierra Nevada block defines the western side of what's called the Basin and Range, defined by this style of faulting and extension.

>When the conversation turns to such vast changes in climate, the effects of human seem comparatively miniscule :)

You're developing a geologic perspective! ;)