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Google Earth view of Wheeler Peak in the Snake Range of
the Basin and Range Physiographic Provence
“Few places in this world are more dangerous than home.
Fear not, therefore, to try the mountain passes.” – John Muir |
From high above the western United States, one of the most striking landforms seen anywhere in the world lies in a broad desert. Composed of alternating mountain ranges and valleys, all running generally north-south, the view from an airplane looks like an army of caterpillars marching toward Mexico. From the eastern escarpment of the Sierra Nevada Mountains on the west to the Rocky Mountains on the east, and from southern Oregon on the north to Mexico on the south, this pattern of basins and ranges dominates the landscape.
In a college geology class, I was told that the land had stretched.
Common wisdom among experts at the time deemed stretching as the reason for raised mountains and sunken valleys.
Here on a field trip, a professor brought a wooden device he had made to show what stretching looks like.
As he pulled and “stretched” the device, blocks of “land” rose while others fell,
“Land area always increases with normal faulting,” he said.
No other place in the world is like it. Basins and ranges, like waves and troughs on a wind blown sea of time, 150 separate mountain ranges with high peaks and low basins, one of them below sea level in Death Valley, span the region. It has been called the Basin and Range Physiologic Provence for a hundred years; and stretching has been held as its cause for nearly as long. But exactly how this strange landform got this way is still in debate.
Since the Basin Range is on my way to the Wind River Mountains, I plan to spend a few days there.
I want to climb one of the mountains, Wheeler Peak at 13,063 feet in the Snake Range of eastern Nevada, near the Utah line.
If you have followed my many treks over the years, I think you get the picture.
If you read my last blog post and followed me down a rabbit hole leading to the source of the Wind River Mountains, you can predict that this venture down another rabbit hole seeks the source of the Basin and Range Physiologic Provence. Please hold on, we’re going down under again.
In his unkempt office, deep in underground thought, a man in knobby boots spoke with academic dignity.
“The geologic history of the Basin Range Province includes an event unique in all the world, one that tore the country apart,” The old geologist spoke as a convert to the “new geology” where everything needs explanation in terms of tectonic plates.
“We know that Nevada stretched in the east-west direction and that 'normal' faults are a result of stretching."
He pointed to a schematic cross-section on the wall showing how stretching causes cracks which release great blocks of earth’s crust allowing them to sink by gravity. “We call the faults that allow this to happen, ‘normal faults’ because they allow blocks to move by gravity, not by tectonic pressure."
I had to ask the obvious question, “What caused the crust to stretch?” I have since come to wish I hadn’t asked, because what followed has come to seem more wondrous than anything Alice saw in her rabbit hole, and just as impossible. But remembering that Alice saw several impossible things before breakfast, I asked the geologist to continue. He certainly had some clout, being consultant for Great Basin National Park.
“Magma, the consistency of tree pitch or resin," he said, "rose from deep within the earth softening the North American Crystalline Shield and pressing upward on it, upwelled like a blister beneath the overlying crust,” He made a gesture of a great blister rising upward.
He showed diagrams of magma coming up through cracks in the North American Crystalline Shield and oozing outward under sedimentary rock. (You remember the great Shield from my first blog post, how it rose a few billion years ago under everything that is now American; and then it hardened into granite.)
But unlike the mountain building event of my last blog post, where 75-55 million years ago the Wind River Mountains were pushed up, the crust in the Basin Range was not eroded very much in the uplift.
Rather it was forced outward, stretched and made thinner, a kind of taffy pulling, he said.
The land stretched to twice its width and became half as thick, widening Nevada by 155 miles, he said.
And I think he understood my bewilderment.
I had to wonder: was the top of the Shield so slippery that material above simply slid down a gentle grade? “Perhaps it was the Shield that stretched,” I ventured, “and it carried the overlying rock with it.” But he quoted a line from the noted DiPetro book that supports his theory.
“Alice thought the whole thing absurd, but they all looked so grave, she did not dare to laugh.” ~Lewis Carroll
I thought the whole thing absurd, but he looked so grave, I did not dare to laugh. Stories told as truth, as his was, and that seem impossible, are surely more intriguing than fiction. It was such an incredible idea to me that I was silent for a minute or two. I fully expected to crawl deeper into the rabbit hole after this encounter.
Recall that at the Laramide Orogeny (the mountain building event that raised the Wind River Mountains along with most of the Rocky Mountains) ended about 55 million years ago.
Stresses relaxed after that, and the mountains eroded to a nearly flat plain.
Then beginning about 30 million years ago another orogeny raised the Wind River Mountains again, exhuming them from their own erosional debris.
This was about the time that radiometric dating shows the beginning of "stretching" of the plain that would become the Basin Range.
I would not be down underground if it were not for an interest in unknowable things. If geology appeared a closed and known field of study, I would not pursue it because I would not feel needed. And to feel needed, if only to pass out napkins at a poetry workshop, is necessary for wellbeing.
“I am degenerating into a machine for making plans. I am learning nothing in this trivial world of men. I must break away and get out into the mountains to learn the news.” – John Muir
I once thought trigonometry a subject I could search for years, even invent some new aspect of it, or at least explain something better than others had.
Then I reached what a teacher told me was the end of trigonometry.
With sadness in having nothing more to learn, I have searched ever since in places where the end of discovery cannot be found, or at least where it seems unattainable.
Geology is such a field, and one that I have subjected you to over the years, if you followed my many trips and blogs.
Now I realize that the angles and distances of trigonometry depend not only on principles I learned, but upon how fast I am traveling and on how far away those angles and distances are.
I could have stuck with it.
Notice how the normal faults curve toward the horizontal as they descend in the diagram he showed me.
Bending allows material to slide on top of the Shield, according to much accepted text, the very thing I couldn’t swallow.
The Shield and the sedimentary material above are both parts of the North American Plate, and plates are supposed to move as units, not slip horizontally within themselves.
So I asked the question. and here’s what I learned:
The old geologist waited a while after my question, then, with staid dignity, looking rather pleased with himself, said “The tectonic mechanisms responsible for lithospheric extension in the Basin Range are controversial, and several competing hypotheses attempt to explain it.” He went on to explain in terms that I perhaps sensationalize in the following summary:
Magma, the consistency of pitch or resin, intrudes between nearly horizontal layers of existing rock. It lubricates like grease. Even so, layers will not slide without force to move them. When the Shield rose like a huge blister, pressurized magma intruded between the shield and the upper crust, and flowed down around the blister, providing gravitational force by which the upper crust slid on the grease. And that’s what caused the upper crust to stretch. It became thin as it stretched.
“Alice had begun to think that very few things are really impossible.”
Perhaps some things she had learned in geology brought her to this. So many unimaginable events have change my mind about logic and its limits that even this fantastical explanation seemed worthy of study.
Recall that a long period of compression of the North American Plate was due to subduction of the Farallon Plate, causing the first rise of the Wind River Range.
Great pressure may have caused shrinking and thickening of the crust.
At about the time this stress ended, stretching of the Basin Range began.
If the plate became twice as thick during compression, then it might have returned to its present thickness by relaxing of pressure.
Under this scenario, it was not stretching, but release of pressure that gave us today’s topography.
I could tell at least three more stories from the rabbit hole, all addressing how the Basin Range stretched. But I have told enough conflicting stories that are unbelievable. All of them are based on evidence and all are plausible. They are not unlike poetry.
Soon, I hope to be on the summit of Wheeler Peak, looking across several of the basins and ranges. Even there, I do not expect to learn how this strange landform came to us, but I will see it in person, and wonder.
Alice thought, “Shall I ever get any older than I am now? That’ll be a comfort, never to be an old woman, always to have lessons to learn.”
Also on this drive,
and of interest geologically, are the hot springs at Thermopolis, a town along Wind
River, north of the Canyon. The Wind
River Basin left by the uplift of about sixty million years ago has filled with
sediments which have hardened into rock, and the rock has bent deep in the crust
and cracked. It is thought, but not proved,
that magma has entered some of the cracks and is heating ground water, causing
it to rise and producing hot springs at the surface. 
