Hiking Into Panum Crater at Mono Lake

One of the most interesting features near Lee Vining and Mono Lake is Parnum Crater.  Panum Crater is a volcanic cone that is part of the Mono-Inyo Craters, a chain of recent volcanic cones stretching south from Mono Lake to near Mammoth Mountain in the Sierra Nevada. Panum Crater is a rhyolitic lava dome that formed about 650 years ago.  Products of rhyolitic eruption are pumice and obsidian, the volcanic glass that Native Americans used to make arrow points and scrapers.

When we heard of Panum Crater, we knew we wanted to hike it, so finally today we drove over to do it before the snow and rain sets in for the next 2-3 days.

Here we are in our classic trailhead photo.  It was pretty cold (either side of 50F) and increasingly windy as the day went on, which is why we were bundled up:

Panum Crater formed in a sequence of events. The first was a large, violent eruption that threw old lakebed sediments into a large crater formation.  Once this debris was blown out, a fountain of cinders, ash and pumice began to fall back towards the earth, forming a pumice ring, or cinder cone, around the original vent. This cinder cone is still visible today.  In the third stage, the remainder of the thick magma slowly rose to the surface in a series of domes. Each dome began with an outpouring of the viscous, rhyolitic lava which hardened and formed a cap over the vent. As magma continued to push up, the cap (or dome) shattered and fell to the outside of the newly formed dome. This happened so many times that a new mountain was created out of these broken pieces, called crumble breccia. The mountain continued to build in this manner until the force within the volcano weakened and no more new domes formed.  Fourth, as the final dome hardened, a period of spire building began. Thick lava pushed up through cracks of the hardening dome and formed castle-like spires.

The interesting thing about Panum Crater is that it has two hiking trails:  one around the rhyolite/pumice rim, which was formed from spewing ash; and one up onto and down into the "plug" or inner cone, which was formed of blasted rock and lots of obsidian and pumice.  We decided to hike the ash rim first.  Here is one of our early views as we hiked around the rim, north toward Mono Lake:

As we circled around the rim, we came back into view of the Sierra Nevadas, to the west:

On the northeastern point of the rim, we were at the highest level the ash rim reached, and we had this spectacular 360 degree view of the Mono Basin from the rim of Panum Crater.

Here is a view back to the south, with the Mono Craters (600-700 year old volcanoes - the youngest mountain range in the United States) to the left, and the Sierra, including Mammoth Mountain, to the center-right:

On the western rim, we approached another high point, and this photo shows pretty dramatically the difference between the ash rim, which is behind Kathy in the photo, and the plug, or inner cone, to the left in the photo:

This photo looks back to the north the opposite direction of the prior photo.  Here, the plug, or inner cone, is to the right, and the ash rim to the left.  Some lone trees have figured out how to make a home in the no-man's land between:

Having hiked all the way around the ash rim, we descended down into the canyon between the rim and the plug, and then started climbing the wall of the plug, or central lava dome.

The central lava dome is made up of pumice and obsidian, which are essentially the same material, but the difference between the two has to do with gas escaping as the magma cooled. The pumice was ejected with gas bubbles in it, making it extremely light in weight.  The magma that remained pressurized while it cooled quickly or that had already lost its gas, formed the obsidian.  Another common texture, called breadcrust, can also be seen in the dome. Breadcrust textures form when the inside of a cooling rock is still hot with gas escaping from it while the outside surface has already cooled. As the gas expands from the inside, the outside surface cracks to allow the gas to escape, forming rock surfaces that resemble the crust on baked bread.

Kathy immediately spotted some huge obsidian boulders, and she wanted this photo snapped of her with them.  Obsidian is interesting in at least two ways.  First, after it is formed, over a relatively short time, water can intrude into it and transform its glassy, shiny surfaces into duller, more "normal" volcanic rock --

Second, since obsidian and pumice are essentially the same rock with gas bubbles (pumice) or without (obsidian), you can find boulders that are obsidian on the outside, which presumably degassed quickly enough to let the obsidian form a shell without bubbles, and pumice on the inside, where gas bubbles were still trapped as the rock re-solidified.  In the photo below, you can even see sections of obsidian where the underlying pumice released gas and created a bubble under the obsidian, which later may have partly shattered due to the air underneath its fragile surface:

From the inner cone, we had 360-degree views, just as we had on the ash rim:

We eventually followed the trail down into the ancient cone.  Here, large, sharp boulders were strewn all over the internal walls like boulder-fall.  Some trees found a foothold near the top of the crater and in this photo Kathy is looking up and wondering why we even decided to hike down into the crater:

We emerged from the crater, retraced our steps around the original rim of the central cone or plug, and then back out to the ash rim before descending back to our Jeep at the trailhead.  From the rim, we could make out South Tufa, the location of calcium carbonate tufa formations on the south shore of Mono Lake, shown in the photo below, which is where we were going to head as soon as we finished our lunch:

More (a lot more!) on the tufa in our next blog entry.