Category Archives: volcanoes

Plate Tectonics

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.


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.


Roman Empire

Roman Empire

As I moved along in my history studies I heard a lot about developing a historical perspective.  It means acquiring a long view about historical processes.  For example, how the civilizations on the Italian peninsula evolved from Etruscan influences to Greek colonies to the start of the Roman city state with a political system somewhat modeled on Greek city-states to a republic, evolving with the growth of the empire and the need to defend the borders which tended to generate a more autocratic government which became increasingly corrupt and unable to check the invasions of Germanic tribes which led to collapse and the Dark Ages.  How is that for a synopsis?

Just about every statement I made about Rome has been debated, supported, refuted, and revised for centuries.  It is my perspective, however, and provides a framework, however tenuous, for my thinking about the development of European civilization.  Many of the conflicts in what was once the Roman Empire and it’s fringes have roots over two thousand years old.  I draw on my views of ancient empires in thinking about current developments in our world.

Geology also requires developing perspective; probably more than history because of the vast expanse of time.  We think of two thousand years as a long time historically, but it is less than a blink in geologic time.  The earth is over four billion years old.  What we can think of as written geologic history, the  evolution of life forms leaving a fossil record, is 600 million years old.  The

2013 Colorado Flooding

2013 Colorado Flooding

2013 Colorado floods, viewed as a rare catastrophe, is only one of many thousands of similar events that carved canyons, moved rivers, filled basins that continue to subside, and provided habitats for ever-changing life forms.  Bring back the mastodon and the Sabre-Toothed Tiger!

Geologists think that Colorado was formed by island arcs similar to those archipelagos south of Asia colliding with the Eurasian continent.  Here, island arcs from the south docked on the Wyoming Craton, some of the oldest rocks in North America.  This happened a very long time ago, before life emerged.  If you want to see this transition I recommend the

Wyoming Stromatolite

Wyoming Stromatolite

Snowy Range road in Wyoming.  You can find stromatolites, fossilized algae.  These fossils predate the Paleozoic, and are older than anything in Colorado.  Yes, geologists use the word docked, if you can imagine India docking on Eurasia. The transition between the Wyoming Craton and the younger stuff is called the Cheyenne Belt.

Now here is some serious history.  Since Colorado became one land mass, mountain ranges have come and gone, oceans have advanced and retreated, new life has evolved and gone extinct, and the whole deal has skidded around on the earth.  All the skidding has moved at about the rate your toenails grow.  That is not very fast, but after a few million years it begins to add up.

Trail Ruts at Guernsey WY

Trail Ruts at Guernsey WY

One of the problems of moving in next to Wyoming is that Yellowstone is there.  200 miles north of Denver is Guernsey, Wyoming, home of some interesting human history.  Guernsey is on the Platte River and the Oregon-California Trail.  Hundreds of thousands of people traveled the trail seeking new opportunity.  The trail stays close to the river in most places, but at one point the banks narrow and the trail climbs up about 20 feet through what looks like soft sandstone.  The Rock is so soft the wagon wheels formed axle deep ruts.

Yellowstone Volcanic Caldera

Yellowstone Volcanic Caldera

The rock is not sandstone.  It is volcanic tuff, deposited by ash clouds from the last time the Yellowstone Supervolcano blew, about 640,000 years ago.  On the highway it is about 380 miles.  Four feet of hot ash from 380 miles away.  It must have gotten pretty nasty here in Denver.  That volcano is going to erupt again.  We don’t know when, but the magma continues to bulge and recede in the area around Yellowstone Lake, which formed in the caldera.  Now that is historical perspective.  I live in a place that is going to be cooked and buried some day.

For those of you in coastal lowlands, along the West Coast, near St. Louis, Salt Lake, or anywhere between Tibet and Australia, there will probably be cataclysms occurring sooner than Yellowstone erupting, but nobody knows for sure.