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 –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 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.
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.
geologictimepics 
Reblogged this on Eat Your Brains Out; Exploring Science, Exposing Creationism and commented:
Compare this with my description of a pebble at http://wp.me/p21T1L-oc . In these illustrations, the rock under high local stress is that little bit more soluble as a result, The beauty of rock structures is something tht continues to amaze and delight me.
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I renamed the piece “The amazing geology of a “granite” kitchen countertop” while giving the original title in my introduction
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Paul Braterman, your description of this weakly metamorphosed sedimentary conglomerate of the cobbles of older meta-igneous gneiss is spot on. I have a modest collection of irregular scraps of graphic granite, porphyry and gneiss slabs, lining our back yard. A favorite of mine looks the same as your featured conglomerate, with the chlorite alteration. Perhaps you know the region or country of origin.
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I’m glad you liked it Jeffrey! And Paul, thanks again for re-posting!
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Thanks, Jeffrey Spence. The pebbles from the beach at Benalmadena, a pivot of comrtession and extension between the inner Betics and the Alboran Sea, with 30 km buried schist and neogene marine deposits within walking distance of each other, so no shortage of tectonic driving forces. More puzzles from the same location at https://paulbraterman.wordpress.com/2016/06/19/rocks-within-rocks-and-rocks-within-rocks-within-rocks/
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Thanks Paul! It IS amazing what you can see in a single stream-worn cobble, isn’t it? I love those things! Too bad the ones in my example require multiple cobbles to see what’s happening –but those rounded shapes sure make them fun to see.
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So if it’s not granite or marble, what is it?
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it can be a variety of things–many are metamorphic rocks –typically gneisses, like the one in my first photo on the right. Others are igneous –but only the lighter-colored ones of those are granite. The countertop places though call all igneous and metamorphic slabs “granite”. The “marble” may be actual marble, or more often travertine. Sometimes quartzite is called “marble”.
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Marli,
The new, stone countertop in the kitchen has so many colors and patterns it’s not likely I’ll ever “see” them all in my life time. Sometimes a “new” pattern catches the eye and I forget why I even went in the kitchen. But now you’ve posed a challenge, so I’ll take a clue from your delightful song and “look real close” to see if it’s even granite!
As always, Marli, thank you . . .
Larry
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Wow… thanks for such nice words Larry –and for watching my song!
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I am trying to buy new cabinets and was googling my “granite” countertops to see what looks good with them and saw your blog about this! I have an entire kitchen done with this slab but I don’t know how to put a picture here so here’s a link to FB. I had no idea there was this much of a story to it!!
https://www.facebook.com/stephanie.m.lewis.739?fref=photo
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I’m gradually catching up on your older blog posts, so you might see my comments popping up any odd place or time! The table tops – or at least 7 of the tables – at St Honore Boulangerie on NW Thurman in Portland are made of polished conglomerate. I don’t know how to insert the photo/s I took of them into this comment, so I’ll leave a link to them here:
https://drive.google.com/open?id=0B6ZxUCbosc_aRnRoRFRoT0k4SHM
https://drive.google.com/open?id=0B6ZxUCbosc_aZ1NRWmtSVWI1VUk
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Thanks-those are very cool! I’ll have to check that place out next time I’m in Portland!
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hi,
i know this post is old, but i have a black and white piece of gneiss I found in lower upstate NY. Is this normal coloration of gneiss? and is that what everyone is calling marble?
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Hi Kendr–yes, most gneiss is black and white –but countertop places usually call it “granite” rather than marble. What they call marble is usually actually marble!
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thank you! 🙂
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I know it’s 3 years later, but I swear I saw this same conglomerate slab at a Portland area counter top store, when I was shopping for a nice piece to use in my son’s new coffee shop. I was amazed at it’s beauty.
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