Where can you see some rocks? It’s winter and everything’s covered in snow –or you’re visiting family in some place where there’s virtually no bedrock exposed anywhere –or you’re simply stranded far from any good rocks in the center of a big city.
Take yourself on a field trip to a granite countertop store! You might not see very much real granite, but you will see some other types: folded gneiss, pegmatite, amphibolite, quartzite, maybe even some granite… and a lot of amazing metamorphic and igneous features and faults –and they’re all polished and none are covered by vegetation.
I needed a rock fix the other day while visiting my mother in SW Florida –so I drove to a granite countertop store. And wow— I saw all sorts of great stuff, a lot of which related to faulting and fracturing, and a lot of it could go right into a geology textbook. In Florida!
But first of all, the term “granite”. Countertop places call just about everything made of silicate minerals to be granite –and the other day, I didn’t see a bit of granite. Being an intrusive igneous rock, granite is generally pretty homogeneous in appearance, unlike metamorphic or sedimentary rocks, which tend to be banded or layered. (see this primer on rock types) Granite also has a specific chemistry, which translates to a pretty specific set of minerals: mostly orthoclase and plagioclase feldspars and quartz, with some white or black mica and maybe some amphibole thrown into the mix. As a result, granitic rocks tend to be pretty light-colored so there’s no such thing as a black granite –or a charcoal granite.
The first rock I saw on my Florida field trip was this gently folded quartzite (notice how the two opposing slabs make it look more folded than it is). It’s chock full of dark zig-zaggy things called stylolites roughly parallel to bedding, many of which are offset along small fault zones. Stylolites form because some rocks partially dissolve when they’re under compressive stress; the actual stylolite consists of insoluble residue left behind after the dissolution occurs. You mostly see them in limestone because limestone’s pretty soluble. They’re actually pretty unusual in quartzites.
But what I found so instructive with this rock was the faulting. Look! With the 100% exposure, you can see how the apparent offset along the fault just below the penny diminishes as you go to the left. That’s an important feature about faults: their slip tends to die out towards their tips.
Then there’s the positively swirly biotite schist to the left with white blobs made of the rock pegmatite. It looks to me like the pegmatite once cut through the darker rock as a dike, and then got pulled apart and folded during some later time. Biotite-rich rocks tend to deform very easily whereas pegmatite tends to be pretty stiff, so the pegmatite retained some of its shape as it broke up while the rest of the rock flowed around it.
And serpentinite! Serpentinite forms by metamorphism of rocks from Earth’s mantle, so they tend to be comparatively poor in silica and rich in iron and magnesium. This serpentinite is highly altered to a pretty brown color (which makes this a prized countertop rock) –and you can see that alteration’s taken place along fractures –right where you’d expect high temperature fluids to circulate. And if you’re the type of geologist who really gets into fractures… well here you go: 100% exposure!
For me, this last example of a ductile shear zone might be the most helpful. I always have a difficult time describing these features to students because ductile shear zones are conceptually difficult; they’re basically faults without any breakage. As you can see in the photo, material got displaced in a sense roughly parallel to the arrows –but nothing got broken. The metamorphic layers simply bend into the zone and thin out and don’t really break. THAT is a ductile shear zone!
Of course, you could also wander into the downtown of a big city and see amazing facing stones too –or you could look at the awesome countertop in somebody’s kitchen –or see incredible polished rocks in bathroom sinks or as floor tiles –and those places are great too. But countertop “granite” places have many many more samples to ogle –the they also have a refuse bin. You might be able to take home some samples—and some of those might even be granite!
on an earlier trip to a “granite” store, I found an amazing metaconglomerate –and blogged about the metaconglomerate and geologic time. Please take a look!
Also, most of these pics are available for free download from the search function at my website: geologypics.com.