Cratons and Rift Valleys: the Bobbing Cork of the Colorado Plateau

Last week, I wrote about how little I actually know about geology. But, still, I am resolved to understand something about where my beloved Colorado Plateau came from.

Here is what I have puzzled out.

The Colorado Plateau is a big chunk of craton (a great word I originally learned from John McPhee...whom I will get to in due time). Craton is defined as crust that hasn't been affected by mountain building for at least a billion years—and you thought your home town had nothing going on. Cratons can either be shields, where old metamorphic and igneous rocks are on the surface (the Canadian Shield, for example), and cratonic platforms, where there are sedimentary rocks piled up on top of the thick ancient rock.

The Colorado Platform is a roughly circular chunk of cratonic platform, surrounded by mountains. Its thick basement rock is covered with many layers of sedimentary rock. And it has rotated a bit clockwise, which has opened up a rift valley to its east, the Rio Grande Rift, through which the river of the same name now flows.

About 17 million years ago, the Colorado Plateau kind of bobbed up, causing a big increase in erosion. Snow fell on its higher elevations, increasing the amount of water flowing in a dry environment.

 Yeah, I should show you a map of where it is—try to ignore the fact that the mapmaker didn’t know how to spell “plateau”

Yeah, I should show you a map of where it is—try to ignore the fact that the mapmaker didn’t know how to spell “plateau”

A lot of other stuff happened over that time, of course, from hot spots that punched mountains through the cratonic crust to salt valleys. And what caused the plateau to rise up like that is a subject of some debate. In the future I'll get to a popular possible cause, involving a disappearing tectonic plate, which resulted in, among other things, the anomalous position of the Rocky Mountains (no, I did not realize their position was anomalous either).

Different languages, different approaches

None of the geology books I own about the specific geology of the Colorado Plateau ever use the word "craton", or ever bother to explain, in basic terms, what makes the Colorado Plateau the Colorado Plateau. They mostly give a quick overview of rock types, geologic processes, and geologic time, and then launch into insanely detailed descriptions of the origins of Navaho Sandstone and the Ali Baba Member of the Moenkopi Formation. That's fine, I guess, but it never got to what I wanted to know, which was what was different about this place.

If they'd just said "it's kind of like a raft of ancient rock, separate from the Basin and Range and mountain areas around it, a raft covered with layers and layers of sedimentary rock, which got pushed up and started to erode much more quickly", I'd have had an image to hold on to, a way of organizing my thinking.

But don't rely on me

But I'm not expert. And, as I said before, geologists either seem to write about the Colorado Plateau, or everything but the Colorado Plateau. John McPhee, in his many books about geology over the decades, collected a couple of decades ago into the omnibus Annals of the Former World, mentions it only once, while discussing something else. A lovely book called Rough-Hewn Land, by Keith Heyer Meldahl, is subtitled "A Geologic Journey from California to the Rocky Mountains" and taught me a lot, but deftly dodges around the Colorado Plateau in its eastward journey in an almost perverse way.

By that point I was taking this kind of personally. Sometimes it would appear on a map, but not be mentioned in the text. Sometimes it would lurk just off the edge of the map. There was almost a feeling to trauma to the whole thing, as if something so horrific had happened there that no geologist could manage to even remember having learned about the place.

My Essentials of Geology by Stephen Marshak, a book I have found quite useful, actually does mention the Plateau, and points out its cratonic nature in a map of the cratons of North America. But it too does not spend much time on it.

So a lot of what I have here I've kind of pieced together. The Colorado Plateau is a raft, or a cork (I like that because it's roughly circular), a block of rock of long standing, which has been a single unit for a really long time, has gotten covered with sedimentary rock and partly eroded quite a few times, but only in recent eras has floated up so high that it has eroded in a way that makes it so cool to visit.

I'll get into more detail on that when I get a chance.

So what gives?

What happened among those geologists? Was there really some terrible disaster that left most geologists denying that there even is such a place, while sunburned, dust-spitting characters in worn boots stumble out of the dry wilderness with improbable tales of the De Chelly erg and the Dewey Bridge Member while never really getting clear about where they have been? Do these two groups talk, or do they carefully never acknowledge each others' existences?

There is a story here, of course. If necessary, I'll make it up.