geologictimepics

Geology and Geologic Time through Photographs

Archive for the tag “polished rock”

Countertop Geology: Desperate for rocks? Visit a “granite” countertop store!

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.IP18-0957c

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!

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Red garnet along with quartz and feldspar in gneiss -a metamorphic rock.

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.

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Some real granite –with quartz (Q), Orthoclase feldspar (O), Plagioclase feldspar (P), and the black mica Biotite (B).

 

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.IP18-0975

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.

Stylolites offset by faults in quartzite

Stylolites and bedding offset by faults in quartzite

In this next photo of a granitic gneiss, you can see an igneous-filled fault offsetting the rock near the top, but it dies out completely as you go downward.
Migmatite gneiss

 

Dismembered pegmatitte in gneiss

Biotite schist with pegmatite

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.

 

Alteration along fractures in serpentinite

Alteration along fractures in serpentinite

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!

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Ductile shear zone in gneiss (annotated on right)

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!

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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.

Rockin’ countertops–geologic time in our kitchens and bathrooms!

I stopped by a “granite” supplier yesterday –the kind of place that sells “granite” and “marble” slabs for countertops.  Besides the fact that almost none of the slabs were actually granite or marble, they were spectacular rocks that showed wonderful wonderful detail. I nearly gushed at the idea of taking a geology field trip there.  It’s local, and you seldom find exposures like this anywhere else!

slabs of polished rock at a "granite" warehouse --not sure if any of this is actually granite, but it all reflects geologic time.

slabs of polished rock at a “granite” warehouse –most of it’s not actually granite, but it all reflects geologic time.

Generally speaking, “granite” in countertop language means “igneous” or “metamorphic” –crystalline rocks that form miles beneath Earth’s surface and so require great lengths of time to reach the surface where they can be quarried.  When I first started this blog, geologic time with respect to igneous and metamorphic rocks were some of the first things I wrote about –it’s such pervasive and important stuff.

So the main point is that your friend’s kitchen with “granite” countertops surrounds you with geologic time every time you walk in there!

But check out that green polka-dotted rock on the right side of the photo.  Full of rounded cobbles –it’s a conglomerate, originating by sedimentary processes on Earth’s surface. Does it indicate great lengths of geologic time? A Young Earth Creationist might say it were a deposit of “the Flood” and end-of-story.

Here’s a closer look:

Polished conglomerate --individual cobbles are metamorphic rocks. The green color comes from the mineral chlorite.

Polished conglomerate –individual cobbles are metamorphic rocks. The green color of the background material comes from the mineral chlorite. That’s a penny (on the left) for scale.

The conglomerate is made of beautifully rounded cobbles and small boulders that are almost entirely metamorphic in origin.  Most of them are gneisses, which form at especially high grades of metamorphism, typical of depths greater than 8 or 10 miles!  After a (long) period of uplift and erosion, the rock was exposed to erosion, gradually breaking into fragments, which eventually became these rounded cobbles, and ended up in the bottom of a big stream channel or on a gravel bar somewhere.

But that’s not the end of the story, because this deposit of rounded cobbles itself became metamorphosed –so it had to get buried again. We know that because the rock is pervaded by the mineral chlorite, which gives the rock its green color.  Chlorite requires metamorphism to form.  Granted, the rock isn’t highly metamorphosed –there’s no metamorphic layering and chlorite forms at low metamorphic temperatures– but it’s metamorphic nonetheless, typical of depths of a few miles beneath the surface.

And if you look even closer, you can see some of the effects of the reburial pressures: the edges of some of the cobbles poke into some of the other ones. This impingement is a result of the stress concentrations that naturally occur along points of contact.  The high stress causes the less soluble rocks to slowly dissolve into the other, more soluble rock.

cobbles, impinging into each other. Stars on right photo show locations.

cobbles, impinging into each other. Stars on right photo show locations.

I’m already jealous of the person who’s going to buy this slab of rock. It tells a story that begins with 1) metamorphic rock forming deep in the crust, then 2) a long period of uplift and erosion to expose the rocks, then 3) erosion, rounding, and deposition of the metamorphic cobbles, 4) reburial to the somewhat shallow depths of a mile or two–maybe more, 5) more uplift and erosion to expose the meta-sedimentary deposit, 6) Erosion by human beings.

And me? Personally, I’d like to make a shower stall or a bathtub out of this rock –can you imagine???


Some links you might like:
a blog I like that’s about science and creationism
another blog about an ancient Earth and deep time
my original song “Don’t take it for Granite“. (adds some levity?)
Geology photos for free download.

 

 

 

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