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.

A Bit More about Toulon Peak

It was a hazy winter day when I pulled off I-80 at the Toulon exit, thinking that I'd find a dirt road and check out some tufa mounds and domes along the same Lahontan shoreline where this mound occurs, but about nine miles away (shoreline miles).

If you pull off at Toulon, take the Ragged Top dirt road, which heads NW then WNW toward a low pass in the Trinity Range south of Toulon Peak. Don't, in this case, head toward Toulon, the moth-eaten old mill seen just right of the sign.

I drove the road up the alluvial fan, passing by the distant tufa mounds, not finding a road that would take me directly to them. I finally turned onto a side road about 1.5 miles beyond the I-80 underpass, but realized I was quite a ways past the shoreline of interest.

I decided to, once again, test blog a photo from my new mobile device, using the Blogger App for Android, which is getting quite variable reviews. A major problem for me is that the app has no option to control placement of the photo in the post or the size of the photo (without going to the web version of my Blogger Dashboard, which works quite well).

I shot this photo with my phone, looking west. Toulon Peak is the high peak.

I shot this photo with my camera. At this point, neither photo has been modified by adding contrast or color, nor has either been sharpened.

My biggest disappointment was something I noticed over the holidays while in Alaska: the Blogger App will not give very many choices for location. In Alaska, I was given the choice of a very precise (and accurate) location that would have pinpointed my mom's house within a mile or less (more precision than she would have liked), or some nearby locations such as a local park and several commercial establishments. I wanted to just use "Anchorage, AK" as the location, but the app doesn't allow entering anything not on its commercially oriented list.

Near Toulon, within two miles of I-80, the Blogger App could only give me the location of "The Americas", not nearly close enough for my purposes. I was within range of the Verizon 3G network, had no problem uploading the post and publishing it, but the app didn't offer me a way of indicating I was anywhere within the states, within Nevada, or close to Lovelock, let alone near Toulon. I'm pretty sure that the #NSA would have been able to locate me, and no doubt Google Maps knew where I was, although I didn't think to check it out -- I was in a bit of a hurry, trying to outrun the setting sun.

Here's a version of the phone photo, cropped to more closely match the camera shot, and with added contrast and slightly adjusted color.

And here's my enhanced camera shot.

The hills in the foreground, all the way back to the white hill capped by orange-brown and dark brown rock, are composed of relatively young volcanic rocks, which are generally listed on state maps as being within the 6 to 17 Ma range. The white section of rocks, called Tts on the Pershing County map, probably correlates with the Esmeralda Formation to the south and the Elko Formation way to the northeast. These are the early to more recent basin-filling sediments, often tuffaceous, sometimes diatomaceous, that began filling basins just after the start of basin-and-range faulting and basin opening. These are typically capped by young ash-flow tuffs, young andesites or basaltic andesites, and basalts. It's unclear to me what the capping dark brown and orange-brown rock is from this distance.

Toulon Peak appears to be underlain mostly by young volcanic rocks, also shown as being within the 6 to 17 Ma range, although Jurassic-Triassic sediments and metasediments of the Auld Lang Syne Group and some Cretaceous granodiorite are shown on the county and state geologic maps, mostly south (left) of Toulon Peak.

A Google Earth view of the area, as seen from the same location I stood at when taking my photos. The white section shows better in this view.

Here, I've cropped the Google Earth view to more closely match my photos. We can see more detail in this view, because Google Earth isn't plagued by inversion haze and smog.

The geology is still somewhat obscure, but it is a little clearer (especially upon examination of Google Earth) that the units in the foreground are probably the same as the units back into the middle ground, where a dark brown volcanic layer overlies a white unit, which overlies an orange-brown unit, which in turns overlies the lower white section. In some lighting, and on Google Earth, at least two dark bands show up -- these may be basaltic sills (similar to some down in the Gilbert mining district near Tonopah), or they may be vitrophyres within young ash-flow tuffs.

The volcanic section described above, as seen in Google Earth.

Of course, this is mostly speculation until field checked!

As it turns out, this is the location of the Ragged Top caldera, about which I could find very little online, but which I was just clued to by a commenter who mapped in the area in 1987. I'm listing a few references about the area below. What I found was that the caldera is about 12 million years old (Masterson, 1981). The same source lists a 23 Ma age date for an obsidian nodule on the west side of the range, indicating that some older volcanic rocks (in the 17 to 34 Ma range) occur in the area. Age dating may be complicated by hydrothermal alteration in the region.

It appears from the county map and Google Earth that the caldera is located mostly south of my photos, beginning just south (left) of the white rocks described above, in what is mapped as Twt (welded tuff) on the county map. The caldera, however, could be located in the younger Tr around Toulon Peak.

A Few References:
Harpel, G., 1980, Geology and tungsten mineralization of a portion of the Ragged Top Mining District, Pershing County, Nevada [citation only]: University of Nevada, Reno, M.S. thesis.

Johnson, M.G., 1977, Geology and mineral deposits of Pershing County, Nevada [front pages only]: NBMG Bull. 89.

Masterson, W.D., IV, 1981, Epithermal gold mineralization in the Velvet District, Pershing County, Nevada [entire pdf]: University of Texas at Austin, M.A. thesis.

Thole, J.T., 1991, Ragged Top Caldera; geology and geochemistry of a Miocene volcanic center, Pershing County, Nevada [citation only]: Washington State University, M.S. thesis.