May 29, 2011
Mystery Rock #2
Posted by Evelyn Mervine
Earlier this month I posted pictures of a mystery rock that were sent to me by one of my blog readers. On Friday I received an email from someone who had been directed to my blog from the Skeptics Guide to the Universe Forum. A friend of this person had found an interesting rock in a rock wall in El Paso, Texas and wanted to know if anyone could identify this rock. Below are some pictures of beautiful Mystery Rock #2:
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| Mystery Rock #2, Photo 1. |
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| Mystery Rock #2, Photo 2. |
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| Mystery Rock #2, Photo 3. |
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| Mystery Rock #2, Photo 4. |
This rock is a little bit easier to identify than the first mystery rock. Again, though, geologists do find it more challenging to identify rocks from pictures (rather than in person) and from a rock wall (rather than an outcrop).
Let’s go through our identification questions:
-Is the rock man-made or natural?
This is a natural rock.
-Is the rock igneous, sedimentary, or metamorphic?
This is an igneous rock.
-What type of igneous rock is this?
This appears to be a porphyritic igneous rock with phenocrysts of feldspar in a very fine-grained or possibly glassy matrix.
Let me explain some of the terms in the rock description for those of you who are not geologists. Igneous rock literally means “fire rock” and is a rock that solidified from a partially or completely molten state. Igneous rocks either slowly crystallize deep in the Earth or rapidly crystallize on Earth’s surface from molten rock erupted by volcanoes.
Porphyritic is a term used to describe igneous rocks that have a distinct size difference in crystals. In the case of the rock above, there are phenocrysts (large crystals) in a matrix of much smaller crystals. It is a little difficult to tell from the photos, but the matrix seems to be very fine-grained, possibly even glassy. The matrix either consists of tiny crystals that are too small to see in the photos or is glassy, meaning that there are no defined crystals but rather amorphous, unordered solidification.
How to porphyritic igneous rocks form? Well, they generally form when magma that has been slowly cooling for a long time, possibly in a magma chamber, is suddenly erupted to Earth’s surface. Deeper in the Earth where magmas can cool more slowly, large crystals have time to form and grow. Those phenocrysts are quite large (they are so large I’d even venture calling them “megacrysts”), so they had to form through slow growth over a very long time in a magma chamber or similar environment deep in the Earth. Magma takes while to crystallize completely, so sometimes partially-crystallized magmas are brought to Earth’s surface and erupted as lavas. When these partially-crystallized lavas are erupted, the rest of the molten rock cools quickly, and there is no time for large crystals to form.
Minerals crystallize out of magma in a certain order, following something called Bowen’s Reaction Series. In certain magmas, feldspar is one of the first minerals to crystallize out of a magma. My guess is that the large crystals in Mystery Rock #2 are feldspar. It’s always difficult to identify minerals from pictures, but the crystals look like feldspar to me– they are the proper shape, and they exhibit characteristic feldspar twinning. So, my guess is that Mystery Rock #2 formed when magma first spent some time deeper in the Earth and crystallized large feldspar crystals. Then, that magma containing the feldspars was brought to Earth’s surface and erupted. The rest of the molten rock cooled so quickly that there was no time for large crystals to form. Instead, the lava rapidly solidified into very tiny crystals or perhaps even glass, which forms if the cooling is very, very rapid. If the rest of molten rock had had time to slowly cool deeper in the Earth, perhaps the rock would have been something like a granite rather than a porphyritic igneous rock with feldspar phenocrysts.
The person who found this rock also mentioned that the large crystals have a very pretty green hue to them. Feldspar is generally white, brown, gray, or pink, so perhaps I’m missing something, but I believe the green hue is just from alteration of the feldspar crystals.
Other geologists– what do you think? Please weigh in on the identification of Mystery Rock #2.
Finally, to the owner of this rock– this is a truly gorgeous rock. If you ever decide you want to rid yourself of this rock, please give it to a geologist or other rock-lover.
That is a wonderful rock. I'm a bit rusty, but I agree that it looks like feldspar. I imagine the green coloring is from weathering. I bet if it was chipped a bit, it wouldn't be green.
Based on photo 3, and to a lesser extent #1, I would call that texture glomeroporphyritic, meaning that the plagioclase feldspar crystals have aggregated in clusters- I always thought of it as "glommed" together to help me remember the term. That is indeed a gorgeous sample! http://www.britannica.com/EBchecked/topic/235540/glomeroporphyritic-texture
I concur with your diagnosis, Evelyn, and second Lockwood's ID of a glomeroporphyriric nature for some of those phenocryst clusters. On the subject of the green tint in the plagioclase phenocrysts, I'd suggest it's probably a bit of secondary alteration (chlorite or epidote, most likely), thought I can't rule out a biologic origin.I don't know where one would quarry such nice large glomeroporphyroblasts, but I'd sure like to. I've collected smaller ones from the Keweenawan lava flows on the east shore of Lake Superior, but I doubt this sample comes from there.
"Glomeroporphyroblast" is one of my favorite geology words. That totally has to be my Geology Word of the Week for G :). For the non-geologists, the word just means that the big crystals are blobbing together in clusters. Also, in case anyone is confused, feldspar is actually a family of minerals. Plagioclase is a specific type of feldspar mineral. See more here:http://en.wikipedia.org/wiki/Plagioclasehttp://en.wikipedia.org/wiki/Feldspar
The best glomeroporphyritic textures I've seen in the wild are from the east side of the Steens Mt. (SE Oregon), as cobbles in the Alvord Creek drainage.Isn't the "-blast" suffix reserved for metamorphic textures and features?
Lockwood- I think you're right about the "-blast" suffix. So many geo words :).
Hello all ! I am the owner of the rock and I do have a theory on how it got here. I live with-in one block of the University of Texas @ El Paso (School of the mines). I think a long time ago a geology student or professor lived in the area and some how over the years the rock was walled in with the cut side out. I had to rescue it and just pulled it loose. :)If you need any more information I would be happy to answer any questions you have.
And Ive just named the rock Glomer! lolMy grandfather name was Homer and he built the house where I found the rock,the wall was added later.
Elise: I think that's a good theory, especially since the rock was cut in half to display those beautiful feldspars. Actually, I'm about to move myself next week, and I'm abandoning some rocks in the garden of the house where I live. I wonder if some future geologists will wonder how these rocks from all over the world came to be in a garden in Massachusetts. I hope this post is helpful and fun for you, Elise!
It was a blast for our forum members and for me to hunt this down and would really like to thank you all for the help. I will take good care of Glomer and its sitting right next to a nice piece of coprolite and a grand snail fossil I found while living in Austin.Thanks again everyone!
Sounds like Glomer has a good home, Elise. Thanks for letting me share the pictures on my blog!
[…] Mystery Rock #2 […]
maybe twenty years ago I got a present from a priest in Umbria (central Italy) he gave me a a piece of a stone thats also used in ancient floor mosaics in his church. I’ll send you a picutre if you are interesed – my piece is exactely like yours and comes from an old italian church – wired hugh…
That looks to me like a shoshonite.
That comment about living close to the school of mines pretty much sealed it. Looks like a geology prof or student picked it up. I’m pretty sure that this came from a dike that crossed the path of I-80 east of Auburn, CA. It was mostly taken out during construction of the interstate. It’s an andesite porphyry, sometimes called “Chinese picture-writing rock.” I bought a chunk years ago at the Quartzsite, AZ geology QIA trade fair held in January every year. My chunk was also cut on one side and was a great teaching example.
Years later I was shown a remnant of the dike in a service road parallel to the interstate. It was a field trip stop for geology students at Sierra College in Rocklin. I drove around looking for any exposures, but much of the property there is posted as “protected by Smith and Wesson.” Finally I spotted a piece in a wall.
I asked the homeowner where they had gotten it. It came with the house, as the wall was there when we bought it, was the reply. They had a small piece under a water faucet, which they gave to me.
Undeterred, I went back to the original outcrop, climbed to the top of the road cut, and luckily found a roughly 3′ x 2′ x 1 foot thick piece of this, buried in pine needles. With a little help and a bunch of effort, we got this piece out of the ground, over to the edge of the road cut and pushed. Thud. It was a greater effort getting it into the trunk, and it rode around for a week until we could figure out how to get it back out.
I’ve had it for years now, and every so often reduce its size with a sledge hammer to get pieces for a classroom or two. It’s a pretty rock.
I can’t find the paper that described it as an andesite porphyry, but I know what I read, at least. Nice to see that it has migrated as far as Texas.