Colorado Voices
“A tiramisu of earth history”: New research reveals opal in Castle Rock
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Geologists discovered presence of opal cementing Castle Rock’s composite rock
Researchers at the Denver Museum of Nature and Science found that Castle Rock’s durability is due to microscopic amounts of the colorful gemstone, opal.
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Colorado Voices is a local public television program presented by RMPBS
Colorado Voices
“A tiramisu of earth history”: New research reveals opal in Castle Rock
Clip | 4m 47sVideo has Closed Captions
Researchers at the Denver Museum of Nature and Science found that Castle Rock’s durability is due to microscopic amounts of the colorful gemstone, opal.
Problems playing video? | Closed Captioning Feedback
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Learn Moreabout PBS online sponsorshipColorado is one of the coolest places to be a scientist, especially a geologist.
And that's because we've got the Rocky Mountains lifting up the tiramisu of Earth history right up next to us here.
And as those rocks weather away and get exposed, we get information about that tapestry of time.
And so Colorado's a special place to be a geologist because we have much of the last billion years of Earth history recorded and available to us right at our footsteps.
Right now, you're about fifty-five feet underground in the bowels of the museum where we store millions of objects of earth and human history.
This is a sample here.
This is from Australia.
That is this type of stuff that's gluing together our own Castle Rock.
So this project started like many projects in science: by serendipity.
My colleague Mark Longman was down in Castlewood Canyon State Park, and he was marveling at why the bluffs, which are covered by this weird type of rock known as a conglomerate, were holding up the cliffs not only in that park but in Castle Rock, Colorado, and in a lot of the mesas that are South of Denver.
A conglomerate is a type of rock, but because that one is found all over the area south of Denver, it's actually named after Castle Rock itself.
The Castle Rock Conglomerate, for which the town is named.
Typically, that kind of rock gets broken down by wind, by rain.
And so one of the first things you do as a geologist is you take a sample and you want to look inside that sample to see what it tells you.
But when you cut open the rock, you can start to see not only the particles, but you also see what's holding them together, what geologists call the matrix or the cement.
But we need to look at the microscopic textures in there.
And so to do that, we make what's called a “thin section, ” a wafer thin piece of rock that's glued to a slide like this one here By shining light through this under the microscope, and maybe changing the different types of light and orientations of the waves of light through here, we can actually tell what kind of minerals are in this rock and how they came to be.
One of the things that geologists will often do with a rock is we lick it.
And why do we do that?
Take a look at this now, and now take a look at it.
It gives us a window into the rock.
It's just like polishing the surface.
And that is a large picture of what we do when we make a thin section of a rock.
Turns out that the reason that this rock is as hard as most concrete is because it's glued together with a very special cement.
That cement is a mineral you probably call “opal, ” geologists might call that “chalcedony.
” And so the reason those mesas occur south of Denver and indeed Castle Rock itself is because those rocks are so hard that the rocks in between them were eroded, but they weren't quite able to be eroded themselves.
In a way, it's like having a geologic shield on the top of a hill.
And that geologic shield is an opal cemented conglomerate.
Why is that?
Why aren't all the conglomerates in Colorado glued together with the equivalent of Opal cement?
Well, it turns out that this rock has particles in it that came from a huge volcanic event.
And that volcanic rock is rich in the element silica.
And that silica, when rain percolates down through the rocks, gets dissolved, and then it migrates with the groundwater into adjacent spaces, and particularly rocks with a lot of holes in them, like conglomerates.
And so as that groundwater works its way through the particles in that rock, you can see filling all of the rocks, kind of like arteries or veins are very thin veneers of opal.
Great, so that happened in Colorado.
Why should we care?
Well one, it tells a story of how these rocks that are iconic parts of our landscape were formed.
But they also give geologists clues as to what might be happening on other parts of our planet.
Colorado probably has other rocks just like this that are cemented with microscopic opal.
So when you're out hiking and looking at one of these conglomerates, one of the things to remember is the opal in them is actually really microscopic.
So youre not going to be able to go out there and chip off a piece, and then run home and make a piece of jewelry.
So if you're out there and you're on public lands, before you collect something, please please check with someone who works there or us at the museum, because most places you're actually required to get permission to collect samples.
One of the great things about Colorado is that there's a million other rocks on our hillsides and there's a bunch of them that we haven't looked at under the microscope.
So there's a lot of opportunities for discoveries.
Maybe they'll be made by us, maybe they'll be made by you or someone else in our community.
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Colorado Voices is a local public television program presented by RMPBS