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Identity HelpPillow lava?
25th Mar 2019 23:32 UTCThomas Farley
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
26th Mar 2019 00:12 UTCThomas Farley
26th Mar 2019 01:10 UTCPaul Brandes 🌟 Manager
As to what it is, again I'd have to see it in person.
26th Mar 2019 01:50 UTCLouis Zulli
26th Mar 2019 10:07 UTCThomas Farley
26th Mar 2019 10:15 UTCThomas Farley
Thanks again. Tom
26th Mar 2019 10:26 UTCLouis Zulli
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
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
-------------------------------------------------------
> 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
26th Mar 2019 14:30 UTCNathalie Brandes 🌟 Manager
Nat Brandes
Professor of Geology
27th Mar 2019 00:18 UTCThomas Farley
28th Mar 2019 05:44 UTCV. Stingl
28th Mar 2019 18:55 UTCDoug Daniels
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
28th Mar 2019 19:27 UTCLouis Zulli
28th Mar 2019 19:27 UTCNathalie Brandes 🌟 Manager
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
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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