Another geology-related diary. This location was visited on the afternoon July 31st, 2013, the same day as the visit to Devils Postpile and the tufa formations at Mono Lake.
Panum Crater is the remains of a rhyolitic volcano that erupted 600-700 years ago. It is located to the south of Mono Lake and represents one of the more recent eruptions of the Mono-Inyo crater sequence. (The uplifting of Paoha Island in Mono Lake and some other features around the lake are additional and more recent activity.)
Panum Crater is considered a textbook rhyolitic volcano in part due to the interior lava dome(s) that formed and built up never overran the surrounding tephra ring of ejected material. Therefore the dome and ring are clearly separate. (USGS link to page on Panum Crater with a nice explanation on crater formation and how the magma characteristics drive the volcano’s type and formation. And a link to the Wikipedia page on the Mono-Inyo Craters which provide some nice satellite and aerial photos of the formation along with further background.)
Sign at the entrance to the trail up the cone explaining the crater and formation sequence
Trail running the edge of the cone — mainly small pumice and sand. The cone is on the left and the rhyolite dome on the right.
Not all the pumice ejected by the volcano was small in size
The rhyolite dome showing some of the “spire” formations
A large block of rhyolitic crumble breccia on the side of the dome.
Looking southeast from a high part of the dome at the Northwest Coulee and some of the older Mono-Inyo crater sequence.
Looking southwest from the dome up the valley. Very hazy and the bordering mountains can only barely be made out
Looking north from the dome at the cone and Mono Lake beyond. Not a lot growing up here — and practically no shade in the afternoon sun
Crumbling mix of rhyolite and obsidian. Chemically very similar with a high silica content, the mineral formed depends on cooling time and gas content of the magma. If it cools very quickly without much chance to form a crystalline structure you get obsidian, which is a glass.
A rhyolite “spire”. The dome essentially is a series of these pushing up and then falling over. Then more magma pushing up and displacing the cooled top
Close-up showing the rhyolite mixed in with the obsidian
Here there is an interesting banding effect
The top of the dome is a mix of rock sizes — large boulders down to small gravel sized pieces
Some blocks are almost solid obsidian. The local indians used this for tools and weapons since it could be knapped to form a very sharp edge
Obsidian in the sun has a sort of “milky” appearance sometimes due to its translucence
A set of boulders with bands of obsidian
And a close-up
No real nature observations to add. It was afternoon and very little was stirring in terms of insects, birds, etc. And there are few plants growing on the dome and cone; some brush and a few pine trees that I guess have found pockets of soil.
A few closing comments regarding the hike itself. First, the outer ring is mainly loose pumice, sand, and small stone. It’s like walking on a beach, except in this case it’s a hilly beach. A bit tiring in the hot sun and altitude. The inner dome is rocky, and often uneven and thus you need to keep track of where you are putting your feet. And this area gets very hot in the sun and there is little to no shade anywhere. Make sure you have water, sun block, and the appropriate clothing on. (Though I expect anyone doing hikes out this way is already taking proper precautions.)