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Identity HelpPillow lava?

25th Mar 2019 23:32 UTCThomas Farley

01857450016028282754504.jpg
Any ideas as to what these are? Just five or six reddish boulders a mile off of Tecopa Road (Old Spanish Trail) near Emigrant Pass, Inyo County, California. This is the Nopah Range Wilderness Area. Boulders are smooth, nearly polished to look at. Ancient volcanoes near here as well as an ancient seabed. Trilobites in shale just a few hundred feet below this spot along with outcroppings of chert. On this ridge, lots of quartz and quartzite. These are the Google friendly coordinates: 35.903833, -116.059361


I am thinking this may be pillow lava. The other picture was one I took a few years ago at the Geology/Earth Sciences Building at UC Davis, CA. They have a rock garden. With really big rocks. I know it's not the same color but color is, of course, not always diagnostic. This monster came from Lake County, CA. Pillow lava is said to, according to the sign that went with it, "diagnostic of lavas erupted underwater." Am I on the right track? Thanks in advance, Thomas

09849330015652601723124.jpg

26th Mar 2019 00:12 UTCThomas Farley

-- moved topic --

26th Mar 2019 01:10 UTCPaul Brandes 🌟 Manager

If I could take a look at this in person, that would help. I don't believe this is a pillow lava as I don't see the "bulbous" head or the tail/root pillows typically have. Also, I don't see the skin that commonly forms on the surface as they're being erupted.


As to what it is, again I'd have to see it in person.

26th Mar 2019 01:50 UTCLouis Zulli

The first photo looks like a meta-sedimentary rock rich in hematite. Maybe take a look at the literature related to the Kingston Peak Formation.

26th Mar 2019 10:07 UTCThomas Farley

Paul Brandes: Thanks. The UC Davis boulder exhibits the characteristics you describe, what I picture does not.

26th Mar 2019 10:15 UTCThomas Farley

Louis Zulli: Thanks. In discussing the Kingston Peak formation, this paper mentions pillow lava in the Panamint Range, on the east of Death Valley at least 20 miles away, but not the Nopah Range. "in the Panamint Range, pillowed basalt interbedded with diamictite demonstrates synchronous subaqueous volcanism." https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/96/12/1537/186896/glacial-and-syntectonic-sedimentation-the-upper?redirectedFrom=fulltext

Thanks again. Tom

26th Mar 2019 10:26 UTCLouis Zulli

Tom,


You're welcome. You might want to look at this article. It mentions bedded iron deposits, which is what I thought of when I saw your first image. I don't know the area, so the region discussed in the linked article might not be quite right.


Louis

26th Mar 2019 12:01 UTCHarold Moritz 🌟 Expert

Neither are pillow lavas. Your first photo is some kind of rock, probably a chunk of resistant rock maybe quartzite, showing a typical "desert varnish". This phenomenon happens as a rock sits out in the open in a desert climate for millenia, becomes smooth from weathering, and gets coated with a glossy mix of very common Mn and Fe hydroxide minerals if there are traces of Mn and Fe-bearing minerals in the rock. The thickness of this varnish can be used to tell how long it has been exposed. Not sure what the second photo is of, may a glacially smoothed boulder of pegmatite? Lava pilows are black, not pale green. I see a label, but may be for the rock next to it?

Pillow lavas form in whole giant flows of rock near an active volcano and would be obvious en mass, dont occur as an isolated rock. The surface would be rough and glassy. There are great videos you can watch from the USGS Hawaii volcano observatory web site.

26th Mar 2019 12:47 UTCEd Clopton 🌟 Expert

Harold Moritz Wrote:

-------------------------------------------------------

> Pillow lavas form in whole giant flows of rock

> near an active volcano and would be obvious en

> mass, dont occur as an isolated rock.


That's true for rocks found where they formed, but pieces may have been transported from elsewhere. I have found chunks of pillow basalt, presumably from the Lake Superior area, in glacial/alluvial deposits near the Mississippi River in eastern Iowa.


I agree that the specimen in the first photo doesn't look like pillow basalt and that the boulder in the second photo may be a weathered pillow lava of some kind but isn't a particularly good example.

26th Mar 2019 13:27 UTCPaul Brandes 🌟 Manager

The whole rock isn't one pillow, but is made up of several individual pillows. I see at least four pillows on the side shown, and could probably find more if I saw it in person. Concerning colour, fresh pillows are black, but once exposed to earth's atmosphere, typically get a greenish cast to them. Many of the pillows around the Marquette, MI area have that green colour to them, especially on their outside surfaces.

26th Mar 2019 14:30 UTCNathalie Brandes 🌟 Manager

Pillow basalt can be green, especially if they are mildly metamorphosed. Low grade metamorphism of basalt creates what geologists call "greenstone". The colour comes from the minerals epidote and chorite that are formed during metamorphism.


Nat Brandes

Professor of Geology

27th Mar 2019 00:18 UTCThomas Farley

07341670016028282765336.jpg
Thanks everyone for the comments, I appreciate the effort to try to help out. I am reading up. I'm attaching a picture of the sign that goes along with the pillow lava boulder at UC Davis. Their Geology/Earth Science Department took years to source and plan their rock garden. Again, thanks. Thomas

28th Mar 2019 05:44 UTCV. Stingl

Quartz veins in basaltic pillow lava? I think this is not possible.

28th Mar 2019 18:55 UTCDoug Daniels

I agree that quartz veins in a basaltic pillow lava sounds implausible. Then again, how do those amethyst vugs form in basalts in South

America? (I know two different environments) But, I would assume the geologists at UC Davis that identified the things were rather competent at identifying things.

28th Mar 2019 19:21 UTCEd Clopton 🌟 Expert

The text on the panel probably means that it was the fractures that formed during cooling, and they were later filled with quartz. It's challenging to make subtleties like that clear when trying to be very concise.

28th Mar 2019 19:27 UTCLouis Zulli

The sign suggests that the pillows were erupted 160 Ma in the past, and that cooling fractures in the pillows were (much?) later filled with quartz.

28th Mar 2019 19:27 UTCNathalie Brandes 🌟 Manager

Just curious, why would quartz veins be implausible in basaltic pillow lava? I've seen quartz veins in many basalts all over the world.


The pillow basalt pictured in this message looks like it comes from the Franciscan Complex. An article that discusses the quartz veins (and green colour) of this rock is: Swanson, S.E. and Schiffman, P., 1979, Textural evolution and metamorphism of pillow basalts from the Franciscan Complex, western Marin County, California: Contributions to Mineralogy and Petrology, v. 69, p. 291–299.


Nat Brandes

Professor of Geology

28th Mar 2019 19:49 UTCFrank K. Mazdab 🌟 Manager

Yes, the quartz in these examples are secondary, not the results of igneous crystallization.


Nonetheless, in any case, many basalts are actually quartz-normative, which means that while you won't see quartz phenocrysts in the rock (that would be "modal quartz"), the last crystallizing (or even quenched) melt can be quite silica-rich ("quartz-normative" means that the magma composition is sufficiently silica-rich to calculate the presence some amount of quartz, if you took a chemical analysis of the rock and hypothetically assigned all of the measured elements to the typical Bowen's reaction series minerals that constitute common igneous rocks).


Now consider that when the hot lava comes in contact with the water, the rock immediately begins to undergo some degree of alteration along the pillow margins, and some of that could include attacking and dissolving some of this late reactive glassy silica-rich material. Since the solubility of silica in cold water is fairly low, this dissolved silica will precipitate in fractures, vugs, etc. as the local trapped hot silica-rich fluids cool. While the UC-Davis example suggests the silica veining is contemporaneous with the basalt cooling (EDIT: the fractures formed during cooling... the timing of the quartz addition is not implied. Sorry... the mention of quartz on the sign is an unfortunate non-sequitur, and not helped by the confusing inclusion of cooling, which has timing implications)... silica (and other components) can also be introduced by hydrothermal fluids much later, as I suspect is more likely the case with the Parana amethysts.
 
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