A Hike Up Glass Mountain

It was Earth Science Week, and a relatively small group of Nevada geologists went on a two-day field trip to a couple places that included Medicine Lake volcano. Here we are beginning a half mile hike to an overlook point on Glass Mountain, a rhyolite to dacite flow-dome complex that lies at the east edge of the Medicine Lake caldera (the caldera forms a small part of the greater volcano or volcanic area, AKA Medicine Lake Highlands or Medicine Lake volcanic highlands).

I took the photo of geos walking up the bulldozer road while looking straight into the sun. The almost blinding light from the sun peeking through clouds and gleaming off the numerous glassy shards and fragments of rhyolite lying on both sides of the road generated numerous lens flare spots—partly because I need a new camera! Although the effect doesn't look too bad, it was difficult to get useful pics in this direction (almost due south).

The location I've given at the end of the post is the start point of our hike (also see the embedded Google Map just above that, which shows the entire hike route). We had driven west from S.R. 139 on a paved road that is variably called County Road 97 or USFS service road 97, then we turned right onto USFS 44N01. After about a mile and a half, we turned left onto the Glass Mountain Pumice haul road, which was inactive at the time: the pumice mine doesn't operate year round. I don't know if this road is really open to the public, but signs on the road said, "Watch for Trucks," not "Keep Out."

Looking eastward from the bulldozer road at a roundish mass in the blocky rock of the northernmost dacite flow lobes, .

The road we walked up follows and climbs the steep face of the largest of the rhyolite flow lobes, where the rhyolite has partly buried a slightly older, darker dacite flow. Possibly a little breadcrust texture can be seen in the brownish-weathering mound near the foreground of the photo (above). Breadcrust texture, defined here for breadcrust bombs, results during cooling. Its presence can be taken as in indication of an original, fairly non-disturbed flow surface; the texture can also be used to identify hot (v. cool or cold) volcanic fragments in something like a lahar. Ron Schott has one example of a breadcrust-textured rhyolite from the Black Rock Desert volcanic field (UT), and Erik Klemetti has another rhyolite example from Panum Crater (CA).

Here at Glass Mountain, many of of the blocky pieces and fragments at the surface of the flows have broken during flow formation or during later physical weathering. Breadcrust texture, if present on this rounded mass, for example, would indicate that this particular surface might be an original flow surface: the surface was molten hot, then it cooled enough to form a crust, then it cracked because the interior was still hot and expanding. I'm not really sure, however, if we're  really looking at breadcrust texture.

The blocky surface of the northernmost dacite flow at Glass Mountain, with what appear to be pressure ridges or ogives.

Google Earth image of the northernmost part of the rhyolite-dacite contact area. Magenta marks the contact; a few flow lines or ridges are in cyan.

The lighter, more viscous, younger rhyolite is on the left; the darker, less viscous, older dacite is on the right.

Chunk of black obsidian with reddish oxidized (and partly devitrified?) bands.

Vesiculated obsidian (pumice or "puffed obsidian", AKA pumiceous lava flow rock, part of the rhyolite flow rather than tephra) at the end of our hike (Google Maps location).

Here's the hike, from "unnammed road" in the upper center, white, to "unnamed road" in the lower left, red. Despite the 0.7 mile one-way distance given by Google Maps, I calculated the distance on Google Earth as 0.95 miles round trip, and about 330 feet up. It took me 28 minutes to hike to the overlook; others took less time and some took more. YMMV.