Links: Pumice and Pumicite

In keeping with my recent posts about Glass Mountain, Siskiyou County, CA, I've put together a few links relating mostly to pumice:

Chesterman, C.W., 1956, Pumice, pumicite, and volcanic cinders in California and Technology of pumice, pumicite, and volcanic cinders [pdf; text]: CDMG Bulletin 174, 119 p.

Presley, G.C., 2006, Pumice, pumicite, and volcanic cinder, in Kogel, J.E., Trivedi, N.C., Barker, J.M., and Krukowski, S.T., eds., Industrial rocks and minerals (7th ed.): Littleton, CO, Society for Mining, Metallurgy, and Exploration, Inc., p. 743–754.

Pumice and Pumicite Statistics and Information - USGS links and reports.

What is the fate of the pumice rafts? (2012) - at Eruptions.

Pumice flows (down the page), a type of pyroclastic flow - at SDSU.

Pumice - at MEC.

Pumice - at Minerals Zone.

Pumice - at Sandatlas.

Pumice - at Volcano Discovery.

The Conclusion of Our Float Test

We're about to test the rocks I posted about last week.

We have six float candidates (description here).

I voted for rocks #2 and #3 (as counted from the left in the photo shown above), with a "might" for #1 and "probably not" for #5. I did have the distinct advantage of being able to hold the specimens, thereby testing the heft

In comments, I got votes for:
1) Howard: none of the above
2) Lockwood: #3, with maybe but "I'd guess not" for #1 and #2
3) Ivar the Old: #3 (after thinking about #2 and then throwing it out)
4) Mathias: #3 and #2 (a "maybe" on the latter)

Rock #1. The larger of our two scrubbing stone rejects.

I wasn't sure if this rock would float: It has a lightweight heft for it's size, but I'm more used to feeling the heft of hand sized rocks, testing to see if a rock is limestone/dolomite v. barite, or limestone/dolomite v. fine-grained diopside skarn. I placed it carefully into the water, with one flat side down rather than end on into the water.

It's floating!

It's bubbling like crazy, and so I suspect the smallish vesicles will fill up with water, and when it gets completely waterlogged, it will likely sink (not a part of this test).

Rock #2: The smaller of our scrubbing stone rejects (remember, you can read more about these scrubbing stone rejects and see more detailed pics here).

I voted that this rock would float, although truth be told, the vesicle size is very similar to that of Rock #1 (so why didn't I vote for both of them?).

It's floating!

Possibly a higher percentage of the upper surface, compared to the upper surface of Rock #1, is sticking up above the water, but it's hard to judge the relative percentage of the entire rock in comparison with #1. And once again, I think it would probably become waterlogged eventually, and then it would sink.

I was sure that Rock #3 would float. It's a large piece of classic pumice.

Yep, there it is, floating. Most of the rock is sticking out above the water, almost the reverse of the previous two examples.

The pumice has small to very large vesicles.

Close-up of the largest of the air pockets or vesicles.

Overall, the pumice is quite frothy looking.

Rock #4, a piece of banded rhyolite glass.

I knew this rock would sink but included it in the test because it's composed of glass of about the same chemical composition as the glass in the previous three rocks.

It's on the bottom, no surprise.

Rock #5, vesicular rhyolite, also from Glass Mountain.

This is a large rock, lightweight for it's size, but it felt denser to me than the scrubbing stone rejects (Rocks #1 and #2) and doesn't look as vesicular. Consequently, I expected it to sink.

I placed it into the water with one of the flat sides down, and let go of it slowly.

It's definitely on the bottom.

Rock #6, a flattish piece of slaty phyllite.

I knew this rock (which is not a single large piece of mica) would sink, but I included it in order to a test of a rock with a different makeup than the other five. I also chose it for it's flat shape, to test whether the shape might make it float.

Nope. It's gone.

Three of five rhyolite samples from Glass Mountain floated and could therefore be called pumice. Their vesicularity, in order from most vesicular to least vesicular, is as follows: Rock #3, Rock #2, Rock #1, followed by Rock #5, which didn't float, and followed a long ways by Rock #4, which also didn't float. The difference between Rock #5, an example of vesicular rhyolite flow rock, and Rocks #1 and #2, pumiceous rhyolite flow rocks (the scrubbing stone rejects) is not great and might not be apparent when doing field work. Rock #4, which consists of dense, glassy banded rhyolite, is not vesicular at all. Rock #6, not a volcanic rock and not from Glass Mountain, also didn't float. It probably has a density similar to or slightly lower than the density of the dense rhyolite glass of Rock #4.

And that concludes our float test.

Which of These Rocks, Mostly from Glass Mountain, CA, Will Float

I've gathered up a collection of rocks to see which ones will float and which ones will sink. All but one of these are glassy volcanic rocks from Glass Mountain in northeastern California. The ones from Glass Mountain are all rhyolitic.

This test is a follow on to a short discussion in the comment section of a recent post, resulting in two questions we can ask about the six rocks I've set on an outdoor table in the backyard: Which of these rocks will float? And can we call any of these rocks pumice?

Rock #1: A scouring block of vesicular rhyolite (flow rock) from Glass Mountain. Rock collected by MOH.

Rock #2: A second scouring block from the same locality as Rock #1. Rock collected by myself.

Rock #3: I wouldn't hesitate to call this pumice. It's from the pumice pit at Glass Mountain (Google Maps location). Rock collected by MOH.

Rock #4: Banded rhyolite obsidian from Glass Mountain.
Rock collected by MOH.

Rock #5: Vesicular rhyolitic flow rock from Glass Mountain, with flow lineations on the upper surface. Rock collected by MOH.

Rock #6: Crenulated slaty phyllite from eastern Nevada.
Rock collected by myself.

I threw the last rock into the mix just to test something smallish and flat. Also, it's shaped somewhat like the state of Nevada, one of my favorite states.

Okay! Here we go! Any guesses?

Vote for your selection(s) in the comments.

Before We Hike Down Glass Mountain...Some Scouring Stones

Before we go ahead and hike back down from the rhyolite flow on Glass Mountain, take a look, once again, at this view to the south. In the shadows on the right, you can see some beams and timbers from 1930s to 1940s mining operations that produced scouring stones, blocks, or bricks. The trail down took us right past the remains of the old operation (or, possibly, a particular part of the operation: roads snake all across the rhyolite flow). We stopped to look, realizing that anything we might find would likely be rejects, not finished stones that didn't make it into haulage trucks. MOH and I duly grabbed a couple examples.

Scouring stone #1.

The scouring blocks were produced from pumice and vesiculated rhyolite near the top of the rhyolite flow. An old report (CDMG, 1957, p. 2) says:

This pumice occurs as the crust of an obsidian flow and forms jumbled masses of angular blocks associated with black obsidian and dark gray coarsely vesiculated obsidian. Choice pumice blocks are medium gray in color, commonly range in maximum dimension from 1 foot to 3 feet and are relatively free from hard, dense bands. The pumice is loaded into trucks by tractor elevators and hauled to a processing plant near Tulelake where they are cut into "Grillmaster" scouring bricks.

Here's an example of a scouring brick you can buy today.

And here's what scouring block #1 looks like from a couple different angles:

Flow foliation shows well on one of the smoothed surfaces.

As we'll see in the next two photos, we're actually looking at a combination of foliation and lineation on the side shown above.

On the top, here, and down the sides, we can see good flow lineation.

And looking end-on, we see a kind of pock-marked surface: We're looking straight down the flow-lineation tube-like structures. The lineation can be seen in some of the pockety holes.

Scouring stone #2.

A pockety end surface with lineation plus foliation on the other surfaces.

On this smoothed surface, some dark gray or black spots show what the rock might have looked like prior to strong vesiculation.

I'm wondering: Would both of these rocks qualify as pumice by the float-test method?

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.