Geology of the Eastern Sierras

or

Exploring the Particular

We used to think we understood mountains; now we know that we don’t know anything at all. The previous discussion of the definition of mountain created a big mental mess. To clarify misconceptions let us explore the geology that has established these ridges and peaks in the Sierras.

Although I was only creating a 3D topography map of the area, the geology was beneath. The geology defined the limits of the topography, just as the bones define the limits of the skin. For this reason my Kearsarge Pass Topo Sculpture is called, ‘The Bones of the Earth: A Tribute to Mother Nature’.

Let us look at the creation of the Eastern Sierras to understand why we have intersecting ridges rather than peaks and valleys like sedimentary mountain ranges. We will view this development through a novice’s eyes, mine. Using McPhee’s book, Reconstructing California, as a source, we will explore where the Sierra Nevadas came from. (A T’ai Chi partner, Bill Powell, a UCSB Religious Studies Professor, specialty Chinese Buddhism, whose real passion it turns out is mountains, turned me on to this book. Another unlikely source has provided me with some more clues.)

Going west to east, most of California is divided into three parts

“Physiographic California, for much of its length, is divided into three parts …: the Sierra Nevada, highest mountain range in the Lower Forty-eight; the Great Central Valley, essentially at sea level and very much flatter than Iowa or Kansas; and the Coast Ranges, a marine medley, still ascending from the adjacent sea. In this cross-section, the Coast Ranges occupy forty miles, the valley fifty miles, the mountains ninety.” (McPhee, p17)

 

This configuration applies to California from Santa Barbara/Bakersfield North. Although the Coastal Ranges continue, more or less, all the way to the tip of South America, the Sierra Nevadas start abruptly just south of Bakersfield and continue 500 miles to the north. The Great Central Valley, containing both the San Joaquin and Sacramento Valleys, begin at the same place in the south near Bakersfield and ends at Shasta Lake 500 miles north about the same latitude as Eureka.

Sierras, a raised trap door

We’ve dealt with the Coastal Mountain Ranges, now we will look at how the Sierras were created.

“Often likened to a raised trapdoor, the Sierra has a long and planar western slope and … a plunging escarpment facing east. … The 19th century geologist Clarence King compared it to “a sea-wave’ – a crested ocean roller about to break upon Nevada. … Hinged somewhere beneath the Great Valley, and sharply faulted on its eastern face, the range began to rise only a very short geologic ago — perhaps three million years, or four million years— and it is still rising, still active, continually at play with the Richter scale and occasionally driven by great earthquakes.” (McPhee p18)

 

Hence our Sierras only began to rise about 3 million years ago. There was nothing west of the Rocky Mountains as recently as three million years ago. What are the Sierras that began to rise?

“This is the Sierra batholith. Geologists reserve that term for the largest bodies of magmatic rock. A batholith, as defined in the science, has a surface of at least forty square miles and no known bottom. For the latter reason, it is also called an abyssolith. The one in California has a surface of about twenty-five thousand square miles. … Geophysicists … say that [the bottom] is six miles down. If so, the batholith weighs a quadrillion tons, and its volume is at least a hundred and fifty thousand cubic miles.” (McPhee, p28)

This is one huge rock - a quadrillion tons. This is a jump in the order of magnitude. We have jumped from the billions in my childhood: 10 billion people on the earth, the earth is 4 billion years old, the universe 20 billion years old, to the trillions in my adulthood: a trillion dollar national budget. Now we have a quadrillion tons. That’s heavy. No wonder there are no individual mountains. It is one enormous magmatic rock that somehow emerged. How was it created?

“Batholiths develop not as single chambers of magma but as contiguous balloons of molten rock called plutons. As red-hot rising fluid, the great Sierra batholith came into the country in successive pulses during a hundred and thirty million years between early Jurassic and late Cretaceous time. There were three peak periods —the first nearly two hundred million years before the present, the second at a hundred and forty million years before the present, the third at eighty. The most extensive is the ’80 pulse.’ All this went on ten to thirty kilometers below the earth’s surface, where continental crust and subducting ocean crust (coming under the continent) were melting. Through the Maastrichtian time and nearly all the Cenozoic epochs, the cooled and cooling magma lay buried. The topography above changed and changed again, like a carousel of slides. And eventually, recently, the batholith came up, to serve as the lithic medium of the erosive sculpting of Olancha Peak, of Wheeler Peak, of Mt. Whitney.” (McPhee, pp. 28-9)

What’s this? The Sierra batholith was not created when it came up? Indeed it had been finished some 80 million years before and had been just sitting there under the surface waiting to rise. Wait a minute? Isn’t a mountain like a volcano? It wells up and overflows, or perhaps it pushes up from underneath. Doesn’t the mountain create itself?

“Remember about mountains: what they are made of is not what made them. With the exception of volcanoes, when mountains rise, as a result of some tectonic force, they consist of what happened to be there. … If a great granite batholith happens to be there, up it goes as part of the mountains. And while everything is going up it is being eroded as well, by water and (sometimes) ice.” (McPhee p19)

 

This huge batholith, this huge rock, was created under the earth before it rose to the surface. Another misconception about mountains. Somehow I thought that the rock of mountains was created when it became a mountain rather than before. And to sit under the surface for some 80 million years. Wow that’s patience.

A Collision of Plates cooks the Earth

How was this batholith created? What are these three pulses that are referred to?

“At the end of Permian time …Sonomia [an island terrane] docked against western North America. The suture is on the longitude where Golconda, Nevada is now. … It was an event that happened about two hundred and fifty million years before the present. Sonomia was an island arc. North to south it might have stretched two thousand miles.” (McPhee pp. 33-4)

 

This island arc, Sonomia, docked onto the North American Plate about 250 million years ago. How does that relate to the Sierra Nevada Mountain Range.

“To create the magma, you must in some way melt the bottom of the crust. Subduction — one plate sliding beneath another —will cause things to melt. And so will a collision that compresses and thickens terrane. After a continent-to-continent collision, the crust might double; a batholith will come up within thirty million years. In deep burial, the heat from such radioactive and universal elements as uranium, potassium, and thorium is trapped. The heat increases until the rocks melt themselves and their surroundings. Granite should be forming under Tibet at present, where India has hit the Eurasian Plate in a collision that is not yet over. Under California, both thickened crust and plate-under-plate subduction contributed to the making of the batholith, at first after Sonomia came in and sutured on and deformed itself and again after Sonomia was hit from the west and further deformed.” (McPhee p35)

 

So first Sonomia docked onto North America in a tectonic collision, generating a tremendous amount of heat. That was about 250 million years ago. What next?

“Toward the end of the middle Jurassic— in the high noon of dinosaurs, about a hundred and sixty-five million years ago—an island arc like the Aleutians or Japan had moved in from the western ocean and docked here. This was the third terrane at this latitude: the one that followed Sonomia and smashed into it with crumpling, mountain building effects that propagated eastward. …In aggregate the three terranes extended the continent by at least four hundred miles. The third on, suturing here had doubled the width of what is now California. North-south, the third terrane probably came near to being a thousand miles long. … Its width, including the part that is under the Great Valley, is about a hundred miles now. This ten thousand square mile piece of ground … is known in geology as the Smartville Block.”(McPhee p81)

 

First an unnamed terrane began the first melting under the surface. Then came the island arc, Sonomia, about 250 million years ago and then finally the Smartville Block about 165 million years ago. Thus this huge magmatic rock was melted and deformed by these incredible collisions. The heat is generated by the friction as one tectonic plate goes under or collides with another tectonic plate. The heat was so intense that it created gold where the Smartville Block collided with the North American Plate.

“If you look at a map of the Mother Lode… you are, for practical purposes, looking at map of the Smartville suture.”

 

In summary, California was created by a series of large island terranes docking onto the side of California.

“California is in large part a collection and compaction of oceanic islands.” (McPhee p108)

 

The last one, the Smartville Block, subducted under the Great Valley creating such heat that it further melted the crust into granite. Why is this melted magmatic rock sprinkled with black specks?

“There were small black shapes within it, like raisins. Thousands of them. Alien pebbles. These were bits of the country rock that the batholith intruded. They had fallen in to the magma while it was still molten or, if cooler than that, sufficiently yielding to be receptive. They had been softened and rounded but not melted and destroyed.” (McPhee. p29)

 

So now we have this huge granite block under the surface of the earth. What happened next? Volcanic flows.

“In geologic ages just before the uplift, volcanic andesite flows spread themselves over the terrain like butterscotch syrup over ice cream. Successive andesite flows filled in local landscapes and hardened flat upon them. As the trapdoor rises — the andesite flows tilt with it.” (McPhee p18)

The rise and sculpting of the Sierras

In our recreation we now have this volcanic crust on top of the granite block batholith. But it still is under the surface of the earth. How did it make it on top?

“A few million years ago, when lands to the east of us began to stretch apart and break into blocks, producing the province of the Basin and Range, the Sierra Nevada was the westernmost block to rise, lifting within itself the folds and faults of the Mesozoic dockings, the roots of mountains that had long since disappeared. The chronology at Emigrant Gap ends with the signatures of glaciation on the new mountains—the bestrewn boulders and dumped tills, the horns, the aretes, the deep wide U of the Bear Valley.” (McPhee pp. 35-6)

I’m a little stupid. Let me try to repeat this in my own words. With repeated subductions and collisions the surface of the earth was melted and shaped. Then came lava flow. Next this huge mass, unwilling to be pulled by the stretching of the east, instead rose with pulling pressure.

But that’s not the end. Now that our behemoth has risen the forces of water come into play. No more compaction, no more melting. Now it is time for erosion to make its mark. Water in its liquid state is truly destructive. Over the millennia it creates huge erosional valleys. However when it changes from liquid to solid and back again in a seasonal fashion, it breaks rock apart and crumbles it into tiny bits. When the solid becomes fluid, as a glacial flow, the rock itself is scoured by the river of ice.

“In recent time, alpine glaciers dug in to the country and dozed away much of what was left of the volcano. …The story had repeated itself through much of the Sierra during the same band of time: other volcanoes extruding andesite and shedding mud, their remains disturbed by ice. It was a surface story, a latter-day account. The brecciated mudflows and andesite lavaflows had come to rest on rock that was older by as much as five hundred million years— rocks with a deep and different story.”

 

In other words, the underlying rock with its own history is now changed by erosion. This is, however, only the surface story. It is like looking at the war in Bosnia without considering any history. It is so easy to get distracted by appearances. To understand the development one must begin to look through appearances to the underlying structure.

“To try to sense a structure …one must develop a talent for ‘seeing through the topography’ and into the rock on which the topography was carved.” (McPhee pp. 20-21)

 

The problem many times is that the eye sees only what it wants to see or only what the mind will let it see.

“The eye sees what it is trained to see. Or … ‘the eye seldom sees what the mind does not anticipate.” (McPhee p94)

In my case, I had a conception of mountain, which intruded upon my perception. My mind led me to see what I wanted to see, that which would confirm my theories. I didn’t see that which contradicted my theories.

Further I had nothing invested in my subconscious theories that I was holding onto. I stood to gain or lose neither money, nor property, nor life over my conception of mountain. I had neither lectured upon, nor written about my conception of mountain, nor even spoken to anyone about what I considered a mountain to be. So I couldn’t even lose face. And yet with nothing invested I still wanted to hold onto my conception of mountain, presumably because it was secure and comfortable. I had grown accustomed to my world. Think then how much harder it would be to let go of your conceptions if there would be anything invested in the concept or idea. The mind leads the eyes. The eyes are biased spectators. In the following sections we will see how those with serious investment in their theories hold on desperately, ignoring all facts to the contrary.

 

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