May 29, 2011
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:
|Mystery Rock #2, Photo 1.
|Mystery Rock #2, Photo 2.|
|Mystery Rock #2, Photo 3.|
|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.