The Wind River Mountains

Wind River Mountains from high above

When I was young and saw the San Gabriel Mountains, beautiful and alluring behind our house, I asked my parents how they got there.  God created them some ten thousand years ago, seemed at the time a final explanation.  In school, when I asked the same question, geology and evolution conflicted and confused my young head, and I’ve been searching ever since.  It’s been a lifelong trek of the world’s mountains and deserts looking for satisfaction the way a rabbit looks for food.  

I have been down a rabbit hole these past three months since the outbreak began, deep inside the earth, while the surface reeled with a plague.  Now emerged, I wish to share a tale of wondrous sights and events, discoveries difficult to explain, yet worthy of gathering into a story—a true story as best I know.  I hope you find my absence from concerns on earth’s surface partly explained in reading this underground adventure.  

Beginnings of the North American Crystalline Shield
 might have looked like this, with magma rising
into overlying sedimentary rock.

Geologists told me, with a fair amount of certainty in their voices, that I don't need to crawl in the dirt when stories already abound on earth's formation.  They said that that most of the continental United States is underlain by a thick slab of mostly granite.  Long ago the slab was hot, gooey magma rising from the deep.  It heated the sedimentary rock into which it intruded, changing sandstone to metamorphic rock, crystalline like granite.  They called it the North American Crystalline Shield, and said it is very thick, fifteen to thirty miles thick.

But I crawled down there anyway, and while in the depths of the word, I asked for pictures and details of how this might have happened. But all they said was that shock waves from earthquakes had brought them messages, evidence of rock types carried on the waves.  They gave me this cartoon showing how it might have looked in the early stages. 

Not finding their explanation completely satisfying, I dug in their books for something not just convenient, but verifiable.  Within geologic jargon I began seeing a dim image as through a frosty looking glass.  

To me, it looked more like this.

If I had brought my camera into the rabbit hole, I would show you graphic vistas of melting, bending, distorting rocks as this great Shield of magma disturbed and swallowed existing layers forming the foundation of America that everything else is built on. To me it looked more like this work of art.

Radiometric Dating

While studying one of these books, a man joined me wearing khaki pants and knobby boots.  He said that during World War II, as part of America’s effort to make an atom bomb, a fringe benefit of that dubious goal was the discovery of radioactive decay of certain elements and how that knowledge might be used to learn how old rocks are.  I will not try to explain the particle physics that he so eloquently expounded, except to say that some elements change to other elements and do so very slowly, as summarized in this picture.  The most useful of these for this adventure is uranium 238. 

He said that when the universe was 80% of its present age, the shield of magma beneath America began to cool.  When it cooled to the "closure temperature" where radioactive decay begins, a clock was started that, if we could read it, would tell us when the magma cooled.  Within zircon crystals in the granite, uranium 238 began changing to lead.  Very slowly and predictably, one atom at a time, it changes to lead at a rate such that half of it changes in 4.5 billion years.  He said that he measured the amounts of uranium 238 and of lead in zircon crystals from the Wind River Mountains and deduced the time when the magma cooled as 2.6 billion years ago.  Then with a big grin, he said "Radiometric dating has revolutionized geology."

I apologize for beginning my story at the bottom of everything American and working my way up to the surface.  My rabbit hole adventure revealed the evidence from the top down, but for the sake of story I’ll proceed upward from the North American Crystalline Shield.

Just 80 million years ago, while dinosaurs still walked the land, I saw a great inland sea that flooded what is now Wyoming and other states. I felt rumbles of earthquakes from under the sea and saw bending and breaking layers of sedimentary rock that rested above the great Shield and below the inland sea.  And I saw even the Shield itself bending and cracking. 

Another geologist entered the room and overheard our discussion about radiometric dating.  He said that all of this distortion was caused by movements of great plates that covered the earth.  Tectonic plates, he said, were sliding on the semi-fluid interior of the world.  They slid beside each other causing violent jerks,  and sometimes a plate slid over the top of another plate. 

The Farallon Plate subducting
under the North American Plate

One of these moving pancakes of rock, the North American Plate, is host to most of the United States.  It's like a great ship on which most Americans ride.  But west of it, another plate was moving eastward from under the Pacific Ocean.  It pressed so hard against the North American Plate that with much shaking and ripping, it plowed its way underneath.  We call it the Farallon Plate, he said, and the process by which it slid under the North American Plate, subduction.   

I was so engrossed in the adventure by now that I interject cartoons, realizing that the scene could be improved with pictures, if only I had brought my camera.  

Acretion of Pacific Ocean Islands and Ridges

As it moved eastward, the Farallon Plate carried with it all that the ocean floor supported and carried most of it down into the depths of the world as the plate subducted.  But lumpy places on the ocean bottom—high ocean ridges and islands—lodged on the edge of the North American Plate as the Farallon Plate bent on its journey down under.  With much ripping and cracking, these accretions were scraped off, becoming part of the North American Plate, forming what would become California, Oregon and Washington.  All this tearing disturbed the motion of the Farallon Plate, causing its point of subduction to move westward.  

Farallon Plate underplating the North American Plate 

For a time, the Farallon Plate continued to plunge downward, but because of the accreted land, now transferred to the North American Plate, and following the course of least resistance, its direction changed to horizontal, underplating the North American Plate.  It traveled horizontally for some six hundred miles before plunging into the molten core.  

As the North American Crystalline Shield rose,
the sedimentary rock above it rose also.

As the Farallon Plate slid underneath it, the North American Plate suffered much disruption to its longtime peaceful stability.  Even the thick, hard granite shield, which composed the lowest part of the North American Plate, bent and cracked forming a series of hills and basins.  The sedimentary layers above it were, of course, bent and faulted also.  

I was told that the thick, hard upper plate (fifteen to thirty miles thick) was deformed by contact with the much thinner Farallon Plate (two to five miles thick) moving horizontally beneath it.  I asked how this could happen, because common sense seemed to say it couldn’t.  I was told that many geologists still wonder if a thin subducting plate could cause such disruption to a much thicker plate above.  They wonder if this is the full story.  A new study by Craig Jones, for example, a Caltech geologist, now at the University of Boulder Colorado, has published a new theory.  He suggests that an unusually thick landmass under Wyoming, up to 180 miles thick, created an obstacle for the underplating Farallon Plate, causing it to bend and fold like a rug being pushed under a bed, increasing its thrust against the upper plate. 

Rising Mountains and descending Basins,
all the while erosion

In any case the North American Plate rose in some places and lowered in other places, forming basins .  Eroded material from the high places flowed into the basins, which tended to level the mountains as they rose.  

All this distortion happened from 75 to 55 million years ago.  We learned about it from seismographs receiving earthquake waves that carry information about the materials the waves pass through. Recently, GPS has shown minute changes in positions of thousands of points over time—points once thought to be fixed.

The Farallon Plate, in its long reach under America, finally became unstable, and broke several times during its eastward movement and the westward movement of its point of subduction.  The plate is completely gone now, totally subducted.  

Now, it is the Juan de Fuca Plate and the Pacific Plate that perturb Americans.  I thought of asking these people how they know the Farallon Plate ever existed.  But I reconsidered.

Getting back to the Wind River Mountains, we return to the rising and falling of places in the North American Plate—the building of mountains and the depressing of basins.  With thick sediment above it, the North American Crystalline Shield rose while suffering major breaking and bending.  It rose under what was becoming the Wind River Mountains.  In other places, it descended, forming basins on both sides of the emerging range.

The rise of mountains always stimulates erosion, and the sedimentary layers were slowly worn away as the Wind River Mountains rose, until the crystalline core was finally exposed to the sun.  Peaks rose thousands of feet above the sedimentary plane, from 75 to 55 million years ago.  

Erosion continued stealing material from the mountains, and might have carried debris all the way to an ocean.  But rivers could not flow that far by gravity, because basins were also formed during the mountain building event, and these were filled with sediment as the mountains rose. 

Eventually the process of orogeny (mountain building) ended, and the high granite mountains became gradually buried by erosion in their own debris, except for a few peaks standing above the plain.  

It was not until about 30 million years ago that another orogeny raised the buried granite mountains a second time, along with the sediment surrounding them.  The sediment, being softer than granite, was carried off by erosion, but not to basins this time, because they were full.  It flowed to the Great Plains, covering them with an east-sloping prairie for the bison.  

The Wind River Mountains are still rising at about one foot per hundred years, and they are being worn down by rainfall.  But it was not always rainfall that eroded them.  Ice ages, beginning two million years ago carved the present shapes—U-shaped valleys, high mountain cirques, and polished surfaces. 

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I wish my story were more graphic and exciting.  I believe that in reality it was very exciting and graphic.  But it happened so slowly and was buried so deeply that even from a rabbit hole the vision is blurred.  I can say with some certainty that because uplift happened faster than erosion, a mountain range that was once buried in its own debris, rose again to become a magnificent place.  Tectonic action, glaciers, and rivers joined to form the Wind River Mountains.  I will go there later this month to hike, backpack, and photograph, remembering an adventure down a rabbit hole.  

The story will continue, but will not become classic as Alice’s Wonderland.  It’s too tied up to the ground for that. What kind of world are we born into, and what can we infer from a small sample?  For all their long history of rumbling and bending, I expect to find the Wind River Mountains stronger and more stable than I am—more enduring than I am.  Through the looking glass of these blog posts I hope to see as you see, and to better understand what I see.  Please join me.