Some Humphreys geology questions

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I was fascinated by the nature and distribution of obsidian bits on the San Francisco Peaks. The only place I've seen wild obsidian before, it was a massive plug in what was obviously the main vent of a cinder cone. Here, it seemed to occur in dense patches of small pebbles. A few appeared to have what might be called "original" surfaces, but most were fractured on all sides. At first I thought of tool-making debris, but there wasn't enough similarity between nearby fragments for that to make sense.

Were these created by explosion of a larger mass, like a plug?

Is their distribution more uniform than it seemed to me? The distribution in small regions seemed uniform, but the populated regions appeared separated by areas with no obsidian on the surface. My sampling and observation could be impaired for many reasons, such soil/vegetation cover and wandering attention.

[hippiepunkpirate]

I don't really know much about the obsidian of the area, though I did find an impressive hunk up on O'Leary last year. I can ask around the NAU geology building to get the scoop. Making an educated guess, I would say that seeing even distribution over small regions is due to the many complex lava flows that form the mountain. Obsidian is often thought of as a "pyroclastic", but is more simply the result of molten lava cooling at an incredibly rapid rate. I think that much of the obsidian you encountered was the result of certain individual lava flows cooling quickly. They were probably pretty small flows that have been subsequently eroded, thus are now viewed in isolated, dense patches of small pebbles. But that's not fact, just my interpretation, I'll try to ask around.

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What character would such erosion take? I wouldn't have expected so many unworn fracture surfaces.

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I have one other unresolved question. This was on Fremont Saddle, which makes no sense to me. http://hikearizona.com/photo.php?ZIP=153910. It's greenish, many-layered, has a kind of greasy feeling, but hard. It is filled with shiny specks that look like tiny crystals. Some of them look almost metallic, but I don't think they are. Is this a piece of a vitrophyre? If not, what is it?

[hippiepunkpirate]

Not sure about the original fracture surfaces stuff, haven't learned about that in any geology class, only breaking open rocks to get a "clean surface" to analyze mineral composition. As far as Fremont Saddle goes, check out this article: http://www.swxrflab.net/sfvolfld.htm Their field site on the Peaks for obsidian is Fremont Saddle. Really would like a PageRob opinion on the original fracture!

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@hippiepunkpirate
Thanks for the article. This part was interesting:

The material will not allow controlled fractures and is generally useless for biface manufacture, although flakes can be removed on the more vitreous materials. This source was not apparent in any of the sites in this study. There appears to be some prehistoric reduction here, but the density is light and many 'flakes' could have been produced by freeze-thaw processes. Published references include Jack (1971), Moore et al. 1960, Robinson (1913), and Schreiber and Breed (1971).

Please let me know if you have any ideas about that other photo.

[PaleoRob]

I've seen original obsidian in nature along the southern Cascades in Northern CA. As it is glass, basically, it is very easy to fracture - so natural "original" fracture surfaces are going to be concoidal. It can be really difficult to differentiate between natural fracturing of obsidian and purposeful flaking (especially regarding flakes and cores) unless you know what you're looking for.
As to the green rock - I have no idea. Was it hard? Did you try to scratch it?

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As to the green rock - I have no idea. Was it hard? Did you try to scratch it?

Harder than obsidian, but carbide scratched it. Besides the tiny square crystals (maybe pyrite?), there were some black thread-like crystals on the other side. They were very fine; about half the thickness of a hair.

[hippiepunkpirate]

Talked to my pal Pete Kohler today (NAU geology professor), he hooked me up with a geologic map of the San Francisco Peaks. For Fremont Saddle, it lists the rock types lava flows of pink rhyolite and black obsidian. Pete said the small fragments are definitely weathered from larger boulders.

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@hippiepunkpirate Thanks for checking. That seems consistent with the article you cited.

[azbackpackr]

I'm sort of envious of all this knowledge of geology you guys have. Only thing I have to add to the conversation is that my younger son is a pretty good flintknapper, and has gone somewhere up in the Flag area and brought back large chunks of obsidian. He says the place he goes is favored by all the flintknappers. (I have no idea how many serious flintknappers there are in this state...)

[fotogirl53]

There is an area not far north of Parks, AZ called Obsidian Tank. There are all sizes of obsidian rocks laying around the tank and up the drainage. I've walked at least 1/2 mile up and found big ones--like 6 inches in diameter. They just look like grey rocks 'til you hit 'em with a hammer to break 'em open! My kids loved that "field trip"!

[azbackpackr]

Ok, yeah, I think that is where those folks go.