Category Archives: Plate Tectonics

Draining the Colorado Plateau

Posted on by Leave a comment

Grand Canyon

About 600 Million years old, the Colorado Plateau has been relatively stable throughout it’s history.  It has uplifts, interesting laccolithic mountains, lava flows, and In the last six million years or so, produced some of the most spectacular scenery on the earth.  Canyons.  Many canyons carved into many layers of rock, much of it red.  The canyons grew upstream from the Grand Canyon, the most spectacular of the canyons.

Some of the Colorado Plateau, Bryce Canyon

The Plateau is big, about 130,000 square miles.  It could swallow some of those little Eastern states with hardly a belch.  Utah east of the Wasatch mountains just east of Salt Lake.  Colorado west of the Rocky Mountains, south of the Uintah mountains and north of that rough country in northern New Mexico and Arizona.

For much of its history the Colorado Plateau was drained interiorly, no outlet to the oceans and surrounded by highlands.  The rivers then flowed north from now gone highlands in Arizona into a succession of mostly fresh water lakes.  The lakes left signatures such as the Green River Formation with its oil shale and landslides.  The Piceance and Uinta basins are examples. I grew up on a margin of the Piceance Basin.

Colorado River

Around six million years ago, the Colorado River flowed south, found its way onto the Basin and Range Province and eventually to the Gulf of California.  It is ironic such a powerful river responsible for carving all those canyons seldom reaches the sea, diverted onto land by recent despoilers, us.  The shift from internal drainage to the Colorado River with all the canyons carved by the river and tributaries is something of a mystery.

The answer is elevation change.  The Colorado Plateau ended up higher than the Basin and Range province to the west.  Was the Plateau uplifted or did the Basin and Range subside?  The change in elevation is relatively recent and gave the Colorado River the opportunity to begin draining the Plateau.

The Basin and Range With Stretch Marks

The Basin and Range is known as an extensional region.  As the Pacific Tectonic Plate slides northward along the North American Plate, it is stretching and pulling the Basin and Range to the west.  As it pulls apart, some big blocks stay at about the same altitude while adjacent blocks drop down to fill the void.  Thus we have basins and ranges.  There are theories that the entire province subsided as well.

This is probably not the case, as the crust under the region is thinner, a function of stretching, not uplift.  With uplift, we would expect the crust to be thicker.  The crust is thicker under the Colorado Plateau, suggesting uplift created by the remnant of the Farallon Plate subsiding and allowing lighter rock to rise and allow uplift.

I won’t go into detail about the various layers in the crust responsible for all this.  It is complex and all the stuff is way down there. Some are thicker, some thinner, as some rise their chemistry and density changes, and my eyes start glazing over.  Take it this way, that big thick and somewhat dense Plateau has stayed in one place and has had several periods of uplift.

The most recent uplift left the Plateau higher than the Basin and Range.  A stream on the western margin cut its way into the highland to the east and the Plateau started draining through the new canyon.  What a canyon it is, 6000 feet deep.  As the river cut its way down, all the tributary streams followed suit by making their own canyons.  The region is arid and has lots of cliff forming rocks, so deep, narrow canyons formed, some so narrow you can stand on the bottom and touch each wall.

I wanted the Basin and Range to sink, but is too thin and lightweight so it stretches.  The Colorado Plateau is also being pulled by that Pacific Plate, but instead of stretching, that big thick slab is rotating clockwise.  I am going to dig around and try to figure out why the Plateau stays in-one piece with all the activities going on all sides, but it is for later.  My working hypothesis is since the Colorado Plateau houses the center of the universe, it has stayed intact out of respect.

Posted in Basin and Range, Colorado Plateau, Colorado River, Grand Canyon, Plate Tectonics, Stories

The Enigma of the Colorado Plateau

Posted on by Leave a comment

 

Although I have lived somewhere along Front Range Colorado most of my life, I still think of the Colorado Plateau as home.  I am dedicated to the region because of its Geology.  All that rock is just sitting there looking at you.  There aren’t too many plants to get in the way.  I grew up looking at the red rock of the Colorado National Monument just across the river from home.

Thing is, the red rock is mostly from the Mesozoic.  There is a lot of boring buff colored rock from the Cenozoic, notably the Green River Formation.  The Green River Isn’t much to look at, but it has a huge present day impact.  Look at my piece on the Formation in Categories.

I am currently interested in the time after the Green River Formation was deposited.  The literature on this time on the Plateau is relatively sparse, mostly because it was more a time of erosion, not deposition.  A large part of the deposition was in fresh water lakes with only remnants of the sediments today.  Those red rocks forming canyons and arches get all the press.

For most of its existence, the Colorado Plateau had no outlet to the sea.  Then, a bit less than six million years ago. The Colorado River decided to head for  Baja California.  There have been lots of explanations for this happening, but they boil down to elevation change.  The Plateau ended up higher than the Basin and Range province to the west.  That area was once a highland, but big tectonic forces changed everything. Then, a stream eroding east carved enough of a canyon to capture the Colorado Plateau drainage which became the Colorado River.

As you are aware, the Pacific Plate is colliding with the North American plate.  The collision is not at right angles, the Pacific Plate is headed northeast.  As it slides along, it pulls on the North American plate, somewhat pulling it apart.  The area east of the Sierras known as the Basin and Range is being stretched.  As it stretches, big blocks called grabens drop down, leaving mountain ranges between them.  The area ended up lower than the Colorado Plateau.

The Basin and Range Stretched

It’s not entirely clear if The Plateau was uplifted during this time by remnants of the same forces responsible for creating the Rocky Mountains.  It seems to me the uplift was earlier, but my reading on the issue is not conclusive. The fact is the Plateau ended up higher than the Basin and Range.  Erosion then began digging away in the area west of Grand Canyon, hooking up with the internal drainage of the Plateau.  Canyon cutting began.

All those spectacular canyons are younger than six million years.  An interesting sidelight is that the stretching of the Basin and Range continues, and the Colorado Plateau is slowly rotating clockwise.  The question for me is why is the Plateau still in one piece?  Compression, stretching, rising magma, new mountains, collapsing Nevada, huge volcanic fields, Arizona mountains wearing away, all this stuff going on all over the neighborhood, but this one slab mostly stays in one piece. Why?

My personal hypothesis is, as the Plateau houses the Center of  the Universe, it necessarily stays in one piece.  I haven’t found anything in the literature to support this, so we will look at some other explanations.  By the way, the Center of the Universe is within a 100 mile radius from the confluence of the Green and the Colorado.  Let me know if you find it.

In the meantime we will see what the geologists have to say.  Usually they reach some consensus, except here.

If you want to explore all this on your own: Carving Grand Canyon, by Wayne Ranney and Geological Evolution of the Colorado Plateau of Western Colorado and Eastern Utah… by Robert Fillmore.

Posted in Basin and Range, Canyonlands, Colorado Plateau, Colorado River, Geology, Grand Canyon, Green River Formation, Plate Tectonics, Stories

Plate Tectonics

Posted on by Leave a comment

Western Colorado’s Grand Valley

As a Western Colorado Native, having lots of geology looking down on me sparked my interest in the field.  I have to know, so knowing how the Bookcliffs, Grand Mesa, and the Colorado National Monument got there stirred my curiosity.  Plate Tectonics is responsible for that stuff poking up all over the place.

Back in the Permian when I took geology at Mesa College, the orthodox explanation for mountain building was something called isostasy.  Push down in one place, and something will pop up nearby.  New Orleans and the Mississippi Delta is sinking under the weight of all that mud coming down the river.  New England is rising a little because that heavy glacial ice sheet melted.  Geologist tried to make isostasy work in places like California with little success.

The early twentieth century saw some new thinking.  Alfred Wegener proposed that continents move around on our sphere.  He was laughed at when he gave papers on the idea.  Yes, Africa and South America look like they once fit together, but how can an entire continent move?  That is a lot of mass to be sliding around.

In the nineteen sixties new thinking started to change attitudes.  Why are there identical fossils on the African and South American coasts?  The real game changer came when oceanic exploration found the mid-oceanic ridges with young basalt near the ridges and steadily getting older farther away.  The only explanation was a spreading seafloor.  Things are on the move.

After college I subscribed to Scientific American magazine.  It seemed like a new article appeared every month explaining how physical features are the result of magma (molten or hot and plastic rock) on the move.  There are seven big (North America, Asia) plates and a number of smaller ones being affected by the rock coming from that spreading seafloor.

Subduction

As the oceanic sea floor impacts the boundary of a continent, something has to give.  The more dense seafloor basalt tends to dive under the continent.  They can form trenches almost seven miles deep next to a subduction zone where the plate dives under the lighter rock of a continent.  The subducting rock is wet, and changes chemically forming lighter rock that often belches up as volcanos.  Earthquakes occur as the plates bump against one another, dip, or slide.

The island arcs off Asia are the current example.  Java, the Philippines, Japan, New Guinea are all volcanic islands getting ready to smash into Asia.  India already has, creating the Himalayas at the suture.  Lots of shaking there, too.

Around 1.75 billion years an island arc docked (yes, geologists use that word) on the Wyoming Craton.  The craton has some rocks as old as six billion (abbreviated as 6 ga) years old.  Many of the rocks are around 3 ga.  The oldest Colorado rocks are around 1.75 ga.  Just outside Morrison, Colorado is the Great Unconformity.  The red rocks are about 60 ma (million years).  The dark gneiss and schist just barely up Bear Creek canyon are those 1.75 ga guys.  Lots went on between those dates, but there it is all eroded away.

Snowy Range, formed by Colorado smashing into Wyoming

The Snowy Range in Wyoming is a result of the join-up.  The coastal ranges in California that like to shake and burn and belch fire and rock formed from the collision of the Pacific and North American plates.  As you are probably aware, L.A. is headed for Anchorage.  Don’t worry it’s going to take a while.

The Rocky Mountains are kind of a strange story.  Usually mountain building occurs at plate boundaries, like the Andes and the Cascades.  What is known as the Laramide Orogeny that created the Rockies happened about 800 miles inland.  The idea is that for some reason about 80 ma the pacific plate scooted under the lighter continental rocks before diving. The Rockies came up a bunch, the Colorado Plateau, my homeland, not so much.

The Rockies are now sitting still but the plateau is moving clockwise, pulled by the pacific plate sliding north against and under the north american plate at the continental boundary, as things should happen.  The Basin and Range province, Nevada mostly, is being pulled apart.  For some reason the Colorado Plateau wants to stay in one piece while western Utah, Arizona, and Nevada are coming apart.

All this motion is happening at about the rate your fingernails are growing.  It doesn’t seem like much, but after a few million years we are talking big moves.  Stay tuned.

My favorite book on these topics is Annals of the Former World, by John McPhee.  It is a big book composed of sections covering the territories along I-80.  Great reading.

Posted in Colorado Plateau, Earthquakes, Geology, Golden Fault, Plate Tectonics, Stories, Utah, volcanoes, Yellowstone Volcano