Saturday, March 15, 2014

Saturd80's: Miscellaneous Reminders Edition

I was reminded of several 80's favorites in a series of unrelated instances this week, so in no particular order, Laurie Anderson, The Big Top (The video portion of this pretty and amusing bit of music is sort of painful for me to watch; I'd recommend scrolling past and just follow along with the lyrics below):

When Buckminster Fuller came to Canada, he kept asking the same question:
“Have you ever really considered how much your buildings actually weigh?”
The Canadians took this very seriously. "We never thought of it that way!"
He showed them plans for domed cities, cities with no basements, no foundations.
Cities that could be moved in a minute.
Portable cities.
Portable towns.

He said:
Think of it as camping out.
He said:
Think of it as one big tent.
He said:
Think of it as
The Big Top ...
spinning ...
lightweight ...

He said: Think of it as
The Big Top.
Spinning ...
Lightweight ...
Flyaway ...

He said:
Think of it as the
Big Top.

The Stranglers, Vladamir and the Beast (very odd and VERY funny ending!):

Yaz, In My Room (Alison Moyet has the most incredible voice, especially haunting in this song):

Geo 730: March 15, Day 439: Sedimentary Obsidian

Yes, yes, I know, obsidian is volcanic. But this obsidian is also sedimentary. The road gravel here is full of rounded clasts of this volcanic glass. I don't know where it crops out, or if the original erosion surface even exists anymore, but clearly, there was at one point an obsidian dome or flow that was exposed to enough running water that cobbles and gravel were transported some distance to round them- which wouldn't necessarily be very far.

It's fairly easy to get some very, very pretty examples of conchoidal fracture from these. The "trick" with glassy rocks like this is to *gently* tap the rock. If you whack on these as you would with a more typical stone, you'll end up with shattered splinters, and more likely than not, blood. The other part of the trick is to "pluck" rather than strike. It's probably best to show this in a crude picture:
Striking the obsidian cobble, again, *gently,* as shown will cause it to fracture approximately as illustrated by the thin dotted line. I've spent many hours "breaking glass" to make arrowheads, and have no hesitation doing this with my bare hands (as Dana can attest), but do be careful. The edges are pretty much as sharp as it's possible for any material to be- much sharper than a razor blade. I don't do this with kids around- I don't want them to see how easy it is before they've developed a healthy respect for the stuff and its edges. Another trick is to immediately take a decently large (fist-sized), fairly soft rock of another type, and "brush down" the edges to dull them. Fun! Remember, every time you break open a rock, you're seeing something no one in history has ever seen before.

Note: If you decide to visit this spot, please see Tuesday's comments on safety issues.

Photo unmodified. August 19, 2011. FlashEarth Location.

Friday, March 14, 2014

Geo 730: March 14, Day 438: Spherulites

A nice example of spherulites, devitrification structures formed when glass crystallizes into minerals like feldspar and quartz. I have the sense that they form quite quickly after an eruption. Vapor phase minerals must, by their nature, form while the lava is still quite hot, and I've seen cases where those minerals coat spherulites- meaning the spherulites formed first.

Note: If you decide to visit this spot, please see Tuesday's comments on safety issues.

Photo unmodified. August 19, 2011. FlashEarth Location.

Thursday, March 13, 2014

Geo 730: March 13, Day 437: Opal and Chalcedony

Dana commented repeatedly, about this stop, that on approach and drive-by, it just looks like a pile of uninteresting, dull gray rock. And it's true: not until you get your nose up against the outcrop do you see the brilliant splashes of color. Most of the wild coloring is in opal; the chalcedony/agate tends to be either white or colorless and translucent. Opal is amorphous (non-crystalline) hydrated silica, while chalcedony is microcrystalline quartz, most often with a fibrous habit that may or may not be visible to the naked eye, but can be made out clearly in a microscope. Under the scope, you can't resolve the individual crystals, but you can see their orientations en mass. Opal tends to have a vitreous luster, while chalcedony has a more waxy luster on broken surfaces. And "agate" is a term that is more useful to collectors of pretty rocks, but to geologists simply means color-banded chalcedony.

Under the chisel tip of the hammer, another lithophysa is visible, with an inner rim of vapor-phase silica, and above the head is a patch of white agate, with a brilliant patch of reddish-orange opal to the left. None of the opal here is gemmy- that is, it doesn't have the shimmering play of color that one would see in jewelery opal. There is gem-quality opal in the area, but sites where it occurs are mostly claimed (there are quite a few pay-to-dig sites in Virgin Valley), and the gemmy material is mostly opaline replacement of wood. Finally, spherulites are abundant throughout the mass.

Note: If you decide to visit this spot, please see Tuesday's comments on safety issues.

Photo run through Paint.Net's autolevel routine for contrast and saturation. August 19, 2011. FlashEarth Location.

Wednesday, March 12, 2014

Geo 730: March 12, Day 436: Lithophysal Rhyolite

Rhyolite is a high-silica lava, typically 70% (plus or minus a little) SiO2 by mass. In molten form the silica tends to stick together in chains, like a polymer, so it's "sticky." It tends to be quite viscous, or resistant to flow, and ions have a difficult time migrating into stable crystal nucleation and growth sites. Thus, when they cool fairly quickly- and especially if they have low water content (Water acts in an opposite manner to silica: it makes molten rock less viscous and increases ion mobility)- lavas like rhyolite tend to be glassy. And as I mentioned yesterday, this outcrop is quite glassy. I was warned by Sharky on my first visit to this spot that "They're glassy, so be careful." Being the bright student I was (and dressed in a tee-shirt and shorts, but thankfully, also thick glasses), I walked up to a solid exposure and gave it a sharp thwack with my hammer. Then spent several minutes picking out splinters of glass from my arms, legs and face. Ah, the joys of field geology...

Glass is not "stable" in a geologic sense- that is, it would "rather" be in mineral form, such as quartz and feldspars. Glassy rocks are referred to as "vitreous," and processes and textural forms associated with that glass turning into mineral crystals are referred to as "devitrification (structures)." This photo is crammed full of such structures. The most obvious to me are the vuggy vesicles partially filled with agate. The agate is clearly layered from the bottom up, and is a good geopetal indicator- that is, the layering is horizontal, and the flat, open, surface is the direction of "up." (This is the solution to the problem rock I sent along to Cannibal Panda about a year ago, and another sample from this outcrop is pictured several times in that post.) Vuggy vesicles with secondary infilling of this nature are referred to as "lithophysae" (singular, "lithophysa"), meaning "rock flower," and at times, the resemblance to a flower or bud can be striking.

But wait! There's more! If you look carefully at the full size image, you can see that the inside of the lithophysae are lined with a thin (~2-3 mm) ragged coating of silica that was deposited before the agate. This is likely vapor phase silica, possibly tridymite, but one would have to use x-ray diffraction to be certain- they're too fine-grained to distinguish with a petrographic scope. In fact, the only thing one can distinguish in a thin section of this rock is abundant sanidine- a discovery that disappointed, but shouldn't have surprised, me.

Wikipedia just cleared up a ponderment I've had since igneous petrology, lo these many years past... what is the actual distinction between lithophysae and spherulites? Apparently, lithophysae are simply one division of the larger group of devitrification structures known as spherulites. The former are just larger, cavernous (that is, open, with void space) examples of the latter. With another glance at the photo, you should be able to quickly see that the rock is choked with roughly spherical, pea-sized forms; these are spherulites, where a devitrification front radiated outward from a central nucleation point, probably halting when the rock cooled to the point that crystallization was no longer possible.

If that's not enough to flabber your gast, this may represent a rheomorphic flow (See Magma Cum Laude for a good discussion), but I don't know enough to say how likely that possibility is. There is definitely some deformation in this flow. The question is, did the flow erupt as a viscous mass, and flow into place in its current form, or did it erupt as an ignimbrite, settle to the ground, weld into a sticky mass, and subsequently undergo some plastic deformation? The latter would make it a rheomorphic flow. If I knew where the lower contact of this flow was, I might be able to tell- for a simple flow, I'd expect a blocky, brecciated contact, for a rheomorphic flow, the contact would be pretty sharp, and likely start off ashy, then get more vitreous upwards. However, I don't know where that contact is, and don't particularly feel like jack-rabbiting off through the sage in an attempt to find it. As is often the case, I'm content just to ponder the puzzle.

Safety note: If you choose to track this spot down, please see yesterday's safety comments.

Photo run through Paint.Net's autolevel routine for contrast and saturation. August 19, 2011. FlashEarth Location.

Tuesday, March 11, 2014

Geo 730: March 11, Day 435: Setting the Stage

This is the scene at the best rhyolite outcrop I know of, at least in terms of "pretty." It was getting later in the day, and I was getting tired. When that happens, I'm less focused on getting photos, more focused on staying coherent, and this outcrop is plenty confusing. As a result, I only have a few pictures here. But the primary purpose of this shot is to help establish the location with some notable landmarks. The big hill on the right is composed of horizontal layers of ashy tuff; you can tell in other, accessible, locations that it has mostly been reworked by water, though there are some spots where it looks as if it's more likely direct airfall. The hill looks to be capped with one or more basalt flows. Also in this photo, the farther slope looks deceptively dark due to it being in the shadow of a cloud. The road cut on the left is our target.

The best spot to park is about a hundred yards up the hill (westward) on the north side of the road. There are some real safety issues here, mostly due to traffic. It tends to be very fast in remote areas like this, and as you can tell from this shot, the visibility into (and out from) the cut is basically nil. The best exposures are on the outside of the turn, unfortunately, but the berm is good and wide. The upside of being remote is that you can hear isolated vehicles approaching well before they arrive. Move away from the road as best you can, and turn to face the oncoming vehicle- as with crossing an intersection, everyone is more comfortable when everyone knows that the other one is aware of and paying attention to them. The other issue is the nature of the rock itself: even though it doesn't really look it, it's very glassy. Use hammers carefully, eye protection is a must, and leather gloves wouldn't be a bad idea (though honestly, I've never used them here- but I almost always shed a minor amount of blood at this site). All that said, it was still a spot I stopped at with properly prepared and informed high school students.

And speaking of setting the stage, the valley in the distance is Virgin Valley, one of the few places in the US where one can find gem-quality opals, the site of a CCC camp, and home to a very pleasant warm spring, which was our ultimate goal on this day. Among numerous other features, it's also the headquarters of the Charles Sheldon Antelope [Pronghorn] Refuge. Virgin Valley is very high on my list of "most awesome places you've never even heard of."

Photo unmodified. August 19, 2011. FlashEarth Location.

Monday, March 10, 2014

Geo 730: March 10, Day 434: Vesicles

Geology has lots of words for things that aren't there- that is, holes. In this case, the holes represent gas coming out of solution in molten rock; you can think of it as effervescent lava. When the lava cooled and solidified, the bubbles remained. We call these vesicles, and the texture, vesicular. It's extremely common in lava flows. This was a loose cobble that had rolled down the slope, so I didn't see the actual exposure, but often, the vesicles are more abundant toward the top of the flow- just as with bubbles in water, they tend to float to the top (though lava is much more viscous than water, so the rate of rise is slower). The concentration of bubbles to the top of a flow can give an observer a good sense of what direction was up at the time. In this area, though, I have no reason to believe there has been significant folding or deformation to make the question of stratigraphic "up" an issue of concern.

Photo unmodified. August 19, 2011. FlashEarth Location.