Hiking Down Glass Mountain

A few weeks ago, we were about to hike back down Glass Mountain after our short hike part way up the glassy rhyolite flow dome. We had taken some overview photos and had stopped at the old working in the foreground (above) to collect some scouring stones. (And then we went off on a float-testing tangent.)

I had vowed to myself, and to MOH, to keep my eye out for some reddish, oxidized glass on the way down...

...and indeed, I found some.

The pile of obsidian and pumice rubble is eroding from a part of the rhyolite flow that includes dense, black obsidian, white to tan-colored pumiceous rhyolite, and reddish brown to reddish orange oxidized rhyolite glass.

I've zoomed in a little here. The black glass looks a little bluish because it's reflecting the sky (what little of it was showing through the clouds).

The oxidized and probably partly devitrified (?) obsidian always seemed to be perched on the treacherously glassy scree slopes above the road we were hiking on. I didn't find any good examples of the orangey rock along the road, and I wasn't about to go climbing over the glassy talus, without sturdy boots and leather gloves.

This rock from in an earlier post shows thin, oxidized bands and streaks, and I'll have to let it stand for a closeup example, although the reddish orange color is not as intense as in some smaller pieces or in the outcrops above the road.

Looking out over the northern dacite flow, and beyond to Timber Mountain.

The photo really captures the blocky and jagged nature of the surface of the dacite flow. It wouldn't be easy to walk across that terrain; one should have very sturdy boots and equally sturdy leather or work gloves. And please don't even consider shorts!

The northern dacite flow is the darker flow area in the upper part of this Google Earth image. The rhyolite flow is the long, almost cylindrical-shaped area emanating from Glass Mountain itself, the small, grayish, round area west and a little south of center.

Read more about the geology in this post.

An old tree grew around a large obsidian boulder (and is therefore younger than the flow). Trees are fairly sparse. 

A geo-type walks down the trail.

Pine trees grow on the angle-of-repose slope, their trunks bent from downhill creep of the colluvium.

I've zoomed in here to take a closer look at a mass of oxidized, reddish brown obsidian near the base of a large, conchoidally fractured obsidian outcrop.

Finally! Down the trail just in time for lunch.

An unreadable metal sign in front of the dacite flow. Maybe it said "Keep Out," once upon a time.

Related Posts:
A Hike Up Glass Mountain
Views from Glass Mountain (and some geology)
Which of These Rocks, Mostly from Glass Mountain, CA, Will Float
The Conclusion of Our Float Test
Hiking Down Glass Mountain (this post)
Links: Pumice and Pumicite

2015 Top Ten Posts at LFD...

...and—as for 2014, 2013, and 2012—a few more. My rules, as modified from last year:

Because of the vagaries of stat reporting (in this case, by Google Analytics), I'm listing both the top ten posts for the year (in large font, with respective Top Ten number), and the top post for month's without a top-ten posting (non-Ten-rated posts are in a smaller font).

And so, here are the top 10 posts, plus 2 extra, along with a few pretty pictures:

January:

• Cliffs of the Ruby Mountains: Some Pretty Pictures #4

February:

• Things You Find in the Field: Old Sedan #6

March:

• Geo-beers: Volcanic Double IPA

April:

• Dust Storm at the Needles Rocks, Pyramid Lake #3

May:

• Siberian Haze in North-Central Nevada #5

June:

• The Beginning of an El Niño Trip: North-Central and Western Nevada #8

July: No posts in July!

August:

• Too Much Smoke — And Pelicans!

September:

• Way Too Much Smoke: Volcanoes (and Rivers and Ridges) in the Pacific Northwest #1

• Finding a Thesis: Klondyke District #10

October:

• A Hike Up Glass Mountain #7

• Finding a Thesis: Into the Palmetto Mountains #9

November:

• Views from Glass Mountain #2

December:

• Lida Summit Roadcut

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.