Time Beyond Imagining: A Scrambled Landscape in Unaweep Canyon

In our last exploration of the Cenozoic story of the Colorado Plateau, we were looking at a sort of mystery canyon, where a deep gorge had been cut into very tough Proterozoic rocks, but where no river exists today. The name of this beautiful canyon is Unaweep, which in the Ute language means "Canyon with two mouths". That's another view of the canyon in the top picture. To help explain what went on here, I have included a picture of another incredible place in the same region, Black Canyon of the Gunnison National Park. I wrote about the rocks of Black Canyon last year in this post, but had little to say about the location of the canyon itself. Like Unaweep, it doesn't make a whole lot of sense at first.

Notice how similar the two canyons are in certain respects. The main gorge is cut through the hard crystalline metamorphic rocks which date back to around 1.7 billion years ago, and there are softer sedimentary rocks along the canyon rims which have eroded back into a more muted topography. And then observe the big difference: Black Canyon is 'V' shaped, and Unaweep has a flat floor giving it more of a 'U' shape. A 'U' shape should cause even a basic geology student to think "glaciers", and glacial action would make a certain amount of sense. The ice ages ended only a few thousand years ago, and glaciers could have accounted for the shape of the valley and the fact that no obvious erosional agent exists in the canyon today. The region is elevated enough, and glacial action has indeed been suggested by researchers in recent times. I am unconvinced, though, because of the general lack of glacial features beyond and above the rim of the canyon. But it is a reasonable hypothesis.

On the other hand, the similarity of the canyons cannot be dismissed. If you removed the Gunnison River, and let debris fall off the cliffs into the valley, it would start to fill in. And what if the river were dammed in some fashion? Again, debris would fill the resulting lake, forming a flat valley floor. But how can a river disappear?

When canyons erode, they don't just get deeper. They get wider as a result of mass wasting of the canyon walls, but they also get longer as a result of headward erosion. The steepest part of a canyon system is often at the headwaters of the stream, and erosion tends to eat into the slopes of the mountains in an upstream direction. If a canyon is vigorously eroding into a mountain, it may actually intercept and divert drainages that were flowing in other directions. In other words, a river can steal the water from another river, an act of stream piracy. With the addition of more water from the stolen stream, the original river may erode even more rapidly, causing extensive changes to a landscape in a short period of time.

You can probably see where this is going: what about the beheaded stream? Bereft of water, the canyon downstream of the diversion no longer has a river of any consequence and erosion ceases. Mass wasting of the canyon slopes will start to fill it in, and one is left with a mysterious gorge with no river inside.

This is almost surely what happened at Unaweep Canyon. There is still a great deal of debate about the origin of the gorge; geologists are not actually sure which river once flowed here. Was it the Gunnison, the Uncompahgre, or was it actually an ancient path of the Colorado River? It is a neat little problem, and is well worth a visit if you are ever traveling in the country between Black Canyon and Grand Junction, Colorado. Follow Highway 141 south from the village of Whitewater to Gateway.

The location of the Gunnison River as it flows through Black Canyon is another mystery altogether! More on this soon...

The July Accretionary Wedge: Why I'm Teaching Geology

The latest Accretionary Wedge deadline has come and gone, and as usual I am a day late. A big thank you to Volcanista, our host this month, who has suggested the topic of our inspiration to enter the geosciences. I have talked about my inspirations a couple of times in previous posts, but I don't think I have told the whole story yet in one place.

Volcanista is certainly right about the idea that few people enter college thinking to major in geology or other earth sciences, and the reason is obvious: earth science does not receive the appropriate respect or emphasis as a science in the elementary and secondary school systems. In my ancient days earth science was simply not offered at all. Today in California, earth science is usually taught in ninth grade as kind of a bonehead science course. I am thankful to know a number of teachers who labor in the trenches, making the science come alive for their students. But by the time any of their students make it through their senior year, they have forgotten anything they loved about the science.

And so it was that I found my inspiration elsewhere. I, like any kid, was fascinated with dinosaurs and fossils, but as far as we knew in the 1960's, no one made a career out of paleontology. The dinosaurs were extinct, and so were the people who found them. They were historical figures from the late 1800's or the 1920's. It was in those days before Jurassic Park and paleontology rock stars.

When I was an eight-year-old there was a guy, Mr. Ritchie (I guess it makes me old, but we called big people Mr. or Mrs. in the day) down the street who had a really nice mineral collection. He would let the neighborhood kids look at his displays and I was fascinated by their symmetry and color. I spent hours digging in my backyard for crystals, and I especially loved a box of minerals left behind by a previous owner at our house. Even today I know there were sulfur and galena specimens in there because I recall the odor every time I work on the specimens in my lab.

Later in life (two years later), I saw the Grand Canyon for the first time, and as recounted in my blogpost a few weeks ago, I found my first fossils. I was hooked, but didn't know it yet. I still didn't realize that there was a science called geology that went along with astronomy and chemistry and those other cool fields.

High school was a fog and a near total loss as far as science was concerned (I did dissect a grasshopper and a pregnant perch, and probably am still adversely affected by the formaldehyde). I was in the scouts though, and learned to love topographic maps and orienteering. I also spent every available weekend hiking all over the mountains of Southern California and the eastern Sierra Nevada. I still didn't know I would be a geologist. It worked something like natural selection: I had the interest (crystals and fossils), and I had some of the basic skills (maps and compasses), I just hadn't found the field in which the skills and interests would be useful. Once I entered the correct situation, those pre-adaptations allowed me to survive! It happened when I entered the community college system in California.

My first quarter at Chaffey College included a class in Earth Processes with Rod Parcel (who passed away a number of years ago). I realized that maybe I was onto something. The next quarter I took a class in Earth Materials with Marlin Dickey, and at the same time I took a course on the geology of the Grand Canyon that included a week-long field trip into the incredible gorge. Some of my adventures are chronicled here and here. From the day I walked out of the canyon, I was hooked for good.

Given the tragic economic situation in California right now, our community college system is in a world of hurt, and I want to give a voice of support for how they support our state economy, and at the same time provide incredible opportunities for our citizens. They are not costly, and they provide an academic path to success that many people would otherwise miss if the system didn't exist. And so it was for me.

Time Beyond Imagining: A Scrambled Landscape

It's kind of strange, what's happened to the Colorado Plateau. For more than a billion years, the region was one of the most stable areas on the planet, lying submerged under shallow seas, or lying exposed as desert dunes or river floodplains. Something like two or three miles of sediments accumulated, providing us today with one of the more complete records of the earth's history for the entire existence of complex life forms. Then, all of the sudden (in geological time), it got scrambled up. The Laramide Orogeny, starting a mere 70 million years ago, upended the sediments into a series of domes, basins and monoclines. The entire region was lifted around two or three miles, and the inevitable, relentless forces of erosion began to strip away the sedimentary cover. Though we have mentioned the early volcanism and the deposition of lake sediments at Bryce Canyon and Cedar Breaks in recent posts, the real story of the Cenozoic Era in the plateau region is one of subtraction of sedimentary rocks on a massive scale.

The reasons for the disruption of the crust can be traced to my home state of California, along with Washington and Oregon. A subduction zone had been present here for tens of millions of years, but aside from developing a chain of volcanoes that dropped ash all over the region, the effect inland had been muted. During the Laramide Orogeny, it is thought that a huge segment of subducted oceanic crust did not sink into the underlying mantle as would be normal, but instead got caught scraping along the base of the continent, leading to the deformation and uplift that characterized the orogeny. As the oceanic crust finally did sink, it allowed the burst of volcanism to sweep across the region, as mentioned in the last few posts. It left behind a strange erosional landscape in the American West that includes mountains completely buried in their own debris, whole swaths of deep crust forming the summits of mountain ranges, older rocks pushed over younger rocks, rivers that run nowhere, other rivers that cross mountain ranges in illogical places, and canyons whose locations make no sense at all. And there is that ultimate expression of river erosion for the world, the Grand Canyon of the Colorado River.

This is the kind of thing that brings home the concept that we are living in one of the best times to actually be able to comprehend the history of our world: the story of a billion years of geological history pushed upward and exposed for our study and understanding.

This all brings us to a sort of puzzle for the day. The photo above is a beautiful canyon in western Colorado. It has been carved through a massive uplift, and the walls along the bottom of the canyon expose Early Proterozoic gneiss and granite; it is one of the most accessible places for observing these rocks, in comparison say to Grand Canyon where one has to hike 4,000 feet downhill to see the rocks. We obviously drove to the site. The canyon is well over 2,000 feet deep, including more than 1,000 feet cut through the very resistant metamorphic rocks. The name of the canyon in the Ute language is "Canyon with Two Mouths". And that's the problem of the day. The canyon does have two mouths, but it's missing something in-between: a river. There are two pathetically small creeks flowing out of the canyon in two different directions, and in no universe could those little rivulets have carved this gorge. So, where did it come from? What carved it?

Fire Down Below III - a Geological History of the Colorado Plateau

Another couple of perspectives of the volcanism that effected the Colorado Plateau between 20 and 30 million years ago.

A very accessible view of a classic volcanic neck can be had from Highway 163 a few miles south of the Utah-Arizona border near the town of Kayenta, Arizona. Agathla Peak (sometimes called El Capitan) is almost 1,500 feet high, and as can be seen in the photo on top is not composed solely of dark volcanic rock. Instead, much of the prominence is composed of large fragments of the surrounding sedimentary rock, shot through with dikes of darker volcanic rock that have more or less welded the pinnacle together. The surrounding rocks are shales and siltstones of the Chinle Formation.

The second photo is of Navajo Mountain, described at the end of yesterday's post. I had intended to include a photo, but to my surprise found very few of them in my archives. It strikes me that as I travel through the plateau country that the mountain looms in the background of nearly every part of our route. It comes into view soon after we leave Grand Canyon National Park out to the east. We drive south of the mountain as we approach Navajo National Monument and Cedar Mesa. It rises to the south when we drive through Escalante-Grand Staircase National Monument. And yet for all that, I hardly ever snapped a shot! But here is one from Cedar Mesa. The mountain is covered by sedimentary rock that has domed up over the volcanic rocks underneath.

Fire Down Below II - a Geological History of the Colorado Plateau

Continuing a long-running adventure, we pick up the tale of the geology of my cherished corner of the United States, the Colorado Plateau....we've come through 2 billion years of the story, and are now only 20 million years from the end!

Volcanic activity had not been a prominent part of the geology of the Colorado Plateau for hundreds of millions of years, throughout Paleozoic and Mesozoic time, except for the volcanic ash that drifted in from elsewhere and ultimately provided much of the color of the Morrison and Chinle formations. That all changed in Cenozoic time. As outlined in yesterday's post, there was a vast outpouring of lava and tephra across much of the western United States around 30 to 20 million years ago, which resulted in the formation of numerous volcanic necks, calderas and the subject of today's post, laccoliths.

There are some real oddities in the landscapes that make up the Colorado Plateau, islands of rock with alpine snowfields and deep green forests that seem more like the Rocky Mountains than part of a desert. These strange out-of-place mountains include the La Sal Mountains (top photo), the Abajo Mountains, Navajo Mountain, the Henry Mountains (center photo), the Sleeping Ute Mountains (bottom photo), and a number of others. The highest, the La Sals, top out at over 12,000 feet. They are a beautiful addition to the landscape; imagine Delicate Arch in Arches National Park (top photo) without the dramatic backdrop of the La Sal Mountains in distance.

How can such mountains develop out of an otherwise flat landscape?

They aren't volcanoes, not in the sense that we normally think of them, but they are volcanic in origin. The mountains are made of a semi-coarse-grained igneous rock that has a number of names, but diorite will do for the moment. 'Coarse-grained' usually means the rock results from slow cooling of the magma miles underground, but the rocks forming the core of these mountains are sort of a hybrid, showing a fine enough texture that they were probably a few thousand feet below the surface, rather than several miles beneath the surface. G.K. Gilbert was studying exposures of these rocks in the Henry Mountains in the 1870's and he realized that although there were complexities, the rocks tended to squeeze between sedimentary layers, and to make room, they pushed the overlying layers upward into a domelike structure, much like a blister pushes the skin upwards. He called these igneous 'blisters' laccolites, and eventually they came to be known as laccoliths.

A simple laccolith, courtesy of http://geology.utah.gov/teacher/tc/tc0108.htm

Navajo Mountain appears to be an ideal example of a simple dome-style laccolith. It certainly looks the part. The more complex mountain ranges include multiple laccoliths at different levels, sometimes branching out in several directions. They often probably constituted the plumbing systems of volcanoes that developed on a surface many thousands of feet above, but have since been eroded away.

Fire Down Below - a Geological History of the Colorado Plateau

I've been seeing a fair number of geobloggers giving apologies for not blogging recently, and then I notice I haven't posted anything in nearly two weeks. It's obviously field season! I've actually been home for a couple of weeks, but other projects took precedence, not the least of which was a lot of weeding. I spent most of the last month discussing the active volcanoes of the Hawaiian Islands, and I will return to that project soon. For the moment I am taking the blogger's perogative to change the subject. I have come close to completing a year-long project on the geology of the Colorado Plateau, and there is a matter of some serious volcanic activity in Cenozoic time across the plateau country.

The Plateau is above all about horizontal rocks. They may form cliffs, but it a simple truth that the landscape is dominated by sedimentary layers of sand, silt, clay and lime. Throughout Paleozoic and Mesozoic time, thousands of feet of rock were laid down, but in Cenozoic time things were changing. The land was rising and buckling so that the last sediments formed on the plateau were in freshwater lake basins of somewhat limited extent that form the colorful rocks of Bryce Canyon and Cedar Breaks today. The Laramide Orogeny, which had lifted the Rocky Mountains, was also responsible for the vast changes in this region as well. The tectonic conditions responsible for the deformation was to have one other effect on the land: volcanism.

Driving across this strange and scenic landscape, one is struck every so often by the sight of rocks just sticking up into the sky. The ancients were pretty sure that these were the remains of monsters who had been frozen in stone. Although geologists have a wonderful time debating the reasons, the fact is that from about 30 million years to around 20 million years ago, volcanism swept across the region, leaving behind all manner of volcanoes, calderas, volcanic necks, and strange laccolithic mountains (see the next post).

Today's pictures include a couple of volcanic necks, including Shiprock in New Mexico. These are the cores of volcanoes that have been exposed by intense erosion.

Post #300! A Pahoehoe Party!

It's my 300th post! In celebration, I am throwing a pahoehoe party with some pictures from our recent Hawaii field studies journey (and one older picture from the pre-blogging days). If you have been following my last few posts, you have learned a bit about the current ongoing eruption of Pu'u O'o on the flank of the Kilauea volcano on the Big Island of Hawai'i. The flows have continued unabated for more than two decades, starting in 1983.

Pahoehoe is a term used by the Hawaiians to describe lavas with a smooth shiny surface, that reminded them of shark skin. We were lucky enough on this trip to witness a number of breakouts from the lava tube system that were forming pahoehoe surfaces. They were far up on the pali, and best viewed in the evening twilight, as can be seen in the top photo. Our view the next day was astounding, as our helicopter flew right over the breakouts just 1,500 feet above.

The center photo shows the active breakouts, with the main lava flow snaking along the right side of the picture, and two breakouts at the top and bottom. Despite the bright sunlight, the orange glow of the lava can be seen in places. The shiny, silvery appearance results from the thin coating of glass on the surface of the lava where it cools first. It is a blasted hellish surface.

We didn't have a chance to stand next to an advancing flow, but in 2002 I was able to get a photo of the front of a lava stream on the coastal plain. The lumpy surfaces expand as they fill with the hot basalt and then the lava squeezes out the edge of the bulbous masses. It is hypnotic to watch, and there is little to fear, since the only way it will get you is if you stand really still and let it surround you (for all I know, this is how some photographers I know will buy the farm!).