Study Sheds Light On The Mystery Of Grand Canyon’s Missing Rocks

Sep 2, 2021

A new study tackles the mystery of the missing rocks at the bottom of the Grand Canyon. Scientists call this feature the Great Unconformity. It’s a billion-year-chunk of history that somehow eroded away, and now, scientists think they know why. KNAU’s Melissa Sevigny spoke with geologist Barra Peak at the University of Colorado-Boulder about her findings.

Barra Peak pointing out the different layers at the Great Unconformity
Credit University of Colorado-Boulder

This feature we’re talking about, you can see it if you go down to the bottom of the Grand Canyon?

Yeah, you can.

Have you done that?

I have, yes…. It’s very obvious, it has these lighter colored rocks above it which are all horizontal, just one layer on top of another, like if you stacked a bunch of blankets on top of each other, or a cake. And then below… there’s a lot of rock, it tends to be much darker in color, it looks almost beat up, like it’s been pummeled and moved around and just had a lot of stuff happened to it…. In some places you can see they’re almost vertical compared to the horizontal layers on top, so there’s very clear distinction and orientation.

Going into this study, what was the question you wanted to answer?

We were really interested in trying to figure out when exactly this feature, the Great Unconformity, formed. So what that means, getting into the details of it, is it’s an erosional feature, so we know something happened to remove material from the surface of the earth, and when exactly that happened could have been anywhere in over a billion year time window. We were really trying to pinpoint within that window, when did the erosion occur, with the hope that by knowing when the erosion happened, we could relate this to a cause for the erosion.

Tell me more about that. What kind of events would have made all of this rock just disappear?

There have been a couple of hypotheses. The Great Unconformity is very obvious in Grand Canyon, that’s where it was first noticed in North America, but it’s also observed all over the world. Because of that, there have been these ideas that maybe there was a global cause. That could take several different forms. It could have been a tectonic event, related to the supercontinent cycle… It’s also been hypothesized that maybe this is related to a climate event that created conditions for erosion.

What did you find out?  

So what we found here is it seems that all of the erosion we see, even though it does not just correspond to just one specific time period, it’s probably related to the breakup for the supercontinent Rodina… so rather than being  a climate driven process, it’s a tectonic process. The way tectonics creates conditions for erosion, is that it’s exposing new rock material to the surface, what geologists call fresh material, and fresh material is very susceptible to chemical weathering, so interacting with the atmosphere breaks down that rock surface, and once it’s broken down it can be removed and taken away.

So you feel that by being able to pinpoint the dates more closely, you’ve been able to narrow down how this might have happened.

Yes. In this location, I should say…. It depends quite a lot on the specific location, what was going on. Which would disprove the hypothesis of there being a single global cause for this feature, rather it’s many different local conditions… and they all just happen to coincide in in time.

Thanks so much for speaking with me.

Thanks for having me.