Wednesday, October 10, 2012

Rock Types

Rocks can be generally classified into one of three general rock classes. The first two of the three classes are easy to recognize:
1. Sedimentary rock, formed in layers by the accumulation of weathered rock fragments and/or chemical precipitates, usually under, or under the influence of water (and sometimes wind),
2. Igneous rock, which includes volcanic lava, as well as related (coarse-grained) intrusive rocks such as granite, diorite, and gabbro.
3. Metamorphic rock. This is generally more difficult to characterize and understand because it is modified – a derivative of - one of the other two.

About 90% of the queries we receive on Ask-a-Geologist, typically accompanied by a photo and asking "what is this rock", we cannot usually answer. In general, the photos have no scale and are blurry, and the light doesn't show the finer structures well. A geologist would want to turn the rock sample over in sunlight with a hand-lens, looking for mineral grains and their distinctive crystalline form or "habit", and perhaps scratch visible crystals with a knife-blade to check their hardness to aid her identification effort.

The following is a rare case of a query accompanied by a good quality photo (it even had a coin to provide scale!) and enough additional context information to allow me to identify the rock.

Last weekend I climbed Mount Mansfield in Vermont. The higher we got, the more silvery the rocks looked. Attached is a picture, but it doesn't really do justice to the silvery tone. My friend wanted to know what caused the rocks to look so silvery.  I said I'd ask the expert.
--Valerie W
Photo: Yannick Guenet

That's a schist. As in, that's a Gneiss Pile of Schist.  ;=)

Bear with me here – there is a point to this.

The silvery-ness that you see is caused by metamorphism, that is, a change to the character of the rock and its inclusive minerals. The word derives from metamorphosis – literally, change of form. Metamorphism usually is caused by the original igneous or sedimentary rocks being buried by tectonic forces at some time in their ancient past, but it could also be caused by hot fluids from a nearby heat-source (like an intruding granite body). Old-time miners would say that metamorphic rocks had been “stewed and cooked” – which is remarkably prescient.

The deeper a rock is buried, the greater the consequent increase in pressure - and also temperature – that it will experience. The photograph shows a complex rock that under great pressure has been deformed plastically – in this case, it is a schist. According to several sources (here's one: ) this means depth of burial at one time reached 15 - 30 km before it was uplifted by tectonic processes and then exposed by weathering. The result of this high pressure/high temperature transformation process is that the original minerals are converted into several new minerals in a schist, commonly including glaucophane and muscovite - the latter is usually called white mica. Typically, there is plastic flowage going on, which leads to the alignment of the glaucophane and muscovite in the flow direction - and you can see this in your photo. The muscovite in these rocks, however, is usually very fine-grained - sometimes not easily visible even in a hand-lens. The net effect is to give the rock an over-all glossy look, and if you ran your hands over it, a slightly greasy feel sometimes. By the way, there are blueschist (blue-gray in color) and greenschist varieties of this rock. The latter are prominently greenish in color, because this kind of schist is loaded with chlorite and epidote: green, chlorine-rich minerals both derived from original black minerals such as pyroxene, and dispersed throughout the resulting rock as a whole by the metamorphic (“stewing and cooking”) process.

You describe the rock becoming more silvery with increasing elevation. That could be because the entire mountain is upside down from its original emplacement, and as you rise in elevation you are in fact walking deeper in time and burial depth. While this sometimes happens during tectonic processes, it is not very common. Though it seems more deformed, I suspect that in fact you are seeing a gradational change from an even more strongly metamorphosed rock, called a gneiss, found at the lower elevations of your hike. This kind of rock doesn’t have the muscovite “sheen”, but instead is typically devoid of chlorite and platy minerals. Gneiss commonly has much larger crystals - this is because the rock was so hot, and maintained for such a long time a great depth of burial, that everything re-crystallized. The longer a hot, fluid mush is held in place, the larger the crystals can grow. Gneiss is typically formed at 15-50 kilometer depths. I suspect that the original rock mass is still upright, and that you were in fact climbing up from deeply metamorphosed gneiss to less-metamorphosed schist above it.


Mount Lemmon is a spectacular uplifted mountain range located far to the south and west of Mount Mansfield - it lies just north of Tucson, Arizona. From the city, the face of the mountain has the broad texture and layering of the original sedimentary rock that it was made out of. However, up close it is coarsely crystalline and very much NOT (any longer) a sedimentary rock. As you move farther north in that complex you are moving ever deeper in original burial depth, and the gneiss turns gradually to granite – back to an original plutonic form. The original sediment probably was weathered out of a nearby, much more ancient granite complex. There is a famous story of a PhD student (Dr. Ed McCullough, who eventually became the geology department head at the University of Arizona) finding a blastoid (head) of a Paleozoic crinoid in the metamorphic rock while mapping the complex with other faculty. This story – and Mount Lemmon - neatly tie all three rock types together in one package.