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!
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:
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.
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.