Sierra Navada Notes & General Information

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Here are notes and other bits of information that I found useful and wanted to record/preserve.

GEOLOGY
The formation of the Sierra Nevada Mountains involved subduction, where the Pacific Plate (more specifically the Farallon Plate) the ran into and was force under the North American Plate. The sinkin oceanic plate then melted, sending magma upward which eventually formed the mountain range. The following is a excerpt from a book that I found very informative about California's geology and in particular the Sierra Nevada.

Roadside Geology of Northern and Central California
by David Alt and Donald W. Hyndman
(find this book on Amazon.com)
pages 17-24

SIERRA NEVADA GRANITE BATHOLITH
The word granite is an old miner’s term that originally referred to almost any massive granular rock composed mostly of feldspar and quartz in crystals large enough to see without a magnifier. A batholith is simply a large expanse of granite, enough to cover more than approximately 40 square miles.

A chain of volcanoes rises parallel to every oceanic trench, for reasons that go back to the oceanic ridge, and hinge mainly on water. The volcanoes depend upon the extremely hot fluid released as serpentinite reverts to peridotite. Add water in any form to any kind of rock, and you lower its melting temperature. In this case, that happens directly above the depth where the sinking slab of lithosphere gets hot enough to break down its serpentinite. The released water rises into the overlying mantle, partially melting it to make basalt magma, which in turn rises into the overlying crustal rocks and partially melts them to make granite magma.

The magmas finally erupt through a chain of volcanoes aligned above the zone where the sinking slab loses its water. The volcanoes may rise anywhere from 50 to 200 miles from the oceanic trench, depending upon how steeply the descending oceanic crust is sinking. Long after the volcanoes die, erosion reveals enormous masses of granite that crystallized
 from unerupted magma in their roots. The basalt magma that helped provide the heat to melt the granite magma leaves dark inclusions in the granite, big freckles of black rock a few inches across.

The Sierra Nevada batholith was one of the first in which geologists recognized numerous huge masses of slightly different kinds of granite packed together like marshmallows in a bag. At first glance, the varieties of granite in those masses, known as plutons, look alike, in the same way that all house cats look alike. In either case, a closer look reveals small variations on the common theme. Granites differ in color, grain size, texture, and the proportions of the different minerals. The various plutons are distinctive enough that you can recognize them in the field and draw their boundaries on a map.

Most of the plutons in the Sierra Nevada batholith range from a few miles to a few tens of miles across. According to some estimates, the Sierra Nevada batholith contains more than one hundred separate granite plutons. Age dates show that they were emplaced between about 225 and 80 million years ago, as big blobs of magma rose from the depths.
The Kiamath block is the original northern end of the Sierra Nevada, now detached and moved about 60 miles west. It contains just a few masses of granite, each one quite separate from the others. Granite becomes progressively more abundant from the northern Sierra Nevada southward, and the individual masses become more closely packed. The southern end of the range consists almost entirely of granite.

Many geologists now contend that the traverse from north to south down the Sierra Nevada is the equivalent of a vertical section through the crust. They believe that the rocks you see at the surface in the southern part of the range are like those that exist at depth in the northern part. Come back in 50 million years to see if deeper erosion of the northern Sierra Nevada has indeed exposed a nearly continuous mass of granite.

Weathering Granite
People who wander the Sierra Nevada see enormous amounts of weathered granite, which normally forms bouldery outcrops and vast amounts of sandy debris. The first step in the weathering process is that water penetrates fractures in the granite, reacts with the rock, and weathers it to soil. Water weathers angular blocks most rapidly at corners, where it attacks from three directions; less rapidly at edges, where it attacks from two directions; and least rapidly on the flat surfaces. So weathering preferentially rounds off the corners and edges, leaving a round mass of unweathered granite within the soil, a core stone. Next, something destroys the plant cover—a fire, or perhaps a period of very dry climate.

Then erosion by rain splash and surface runoff removes the soil, leaving the rounded core stones at the surface. Granite core stones are typically large because the fractures in the rock tend to be widely spaced. Rocks with more closely spaced fractures weather into smaller and much less conspicuous core stones.

The grains of feldspar and mica in granite swell as they react with water and turn into clay. Then the rock falls apart, for the same reason that a building would fall apart if its bricks were to swell, some more than others. Geologists call the piles of loose and partly weathered mineral grains grus, a German word more or less domesticated into the geologic variant of English. Grus is common and conspicuous in the lower elevations of the Sierra Nevada, where no glaciers scraped it off

Roof Pendants
The great expanses of granite in the southern parts of the Sierra Nevada batholith contain occasional masses of older metamorphic rocks. Early geologists interpreted them as pieces of the older rocks that hung down into a sea of molten magma beneath them, so they called them roof pendants. Later fieldwork showed that most are actually remnants of the older rocks caught between the separate granite intrusions that make the batholith, screens of metamorphic rocks that separate the intrusions. But geologists still call them roof pendants, the demands of tradition being what they are.

Most of the roof pendants were strongly heated in their proximity to the masses of granite magma and are now metamorphic rocks recrystallized almost beyond recognition. But it is possible to determine, in at least a general way, what those rocks were before they recrystallized and to reconstruct the bedrock that existed before the granite magmas arrived.

Rise of the Sierra Nevada
The Sierra Nevada was a broadly rolling lowland until a few million years ago, when movements on the faults along its eastern face began to raise it, evidently because the westward development of the Basin and Range finally established the Sierra Nevada fault. Gently tilted remnants of the old rolling lowland conspicuously survive in the broad upland fiats of the western slope.

The steep eastern front of the Sierra Nevada rises immediately west of the Owens Valley, the westernmost basin of the Basin and Range. As the Sierra Nevada moves west on a fault, it emerges from beneath the burden of the slab to the east, which includes the Owens Valley and the White and Inyo Mountains. That unloads the earth’s crust just east of the Sierra Nevada, permitting the range to rise because it floats up. Meanwhile, volcanic rocks erupt along the western edge of the Owens Valley, probably because the drop in pressure on the hot rocks at depth permits them to partly melt.

In time, a new fault will probably develop farther west. The eastern part of the Sierra Nevada will shear off along it and move east to become part of the Basin and Range, and a new version of the Owens Valley will open west of the newly detached slice. As that happens, what is now the gentle western slope of the Sierra Nevada will continue to rise, and a new crest will appear west of the present crest.

GLACIERS
Pleistocene time, the past 2 million or so years, saw an unknown number of ice ages come and go—eight according to some authorities, twelve according to others, perhaps as many as twenty according to a few. It seems clear that ice ages were times of cold weather and heavy rain and snowfall. Although theories abound, no one actually knows what caused them. Nor does anyone know whether the earth will see more ice ages, why the last one suddenly ended, or when the next one may come.

Ice age glaciers grew all along the crest of the high Sierra Nevada and flowed down the valleys, especially to the west, but to a lesser extent to the east. So the upper parts of major Sierran valleys are glacially eroded, and the lowest parts are filled with glacial sediment. The largest glaciers grew where the crest is highest and caught the most moisture, and they eroded the deepest canyons.

Glaciers polished large areas of granite bedrock in the high Sierra Nevada and left them covered with patterns of parallel scratches. Glacial ice is not really the clear blue stuff so often featured in advertisements for gin or vodka. It is actually full of rocks, sand, and mud—so much in places that it is hard to tell where the glacial ice ends and the frozen mud begins.

So the glacial ice that scrapes across bedrock outcrops is full of abrasive particles in widely assorted sizes. The finer particles polish the bedrock, while the larger objects gouge grooves into the polished surfaces. The grooves exactly record the direction of ice movement. If you find sets of grooves that cross, they record a change in the direction of ice movement.

Actually, the grooves record the direction of ice movement with an ambiguity of 180 degrees. Did the ice move from left to right, or from right to left? Geologists resolve that question by gently stroking the surface with their hands. It feels much smoother if you move your hand in the direction the ice was moving. People who have not already done so might try fondling rock outcrops, quite possibly the least famous of all outdoor sports.
Looking at glacially polished and striated surfaces is sobering if you consider that those surfaces have been there since the last ice melted, about 10,000 years ago. The processes of weathering that break rock down into soil operate very slowly indeed. Soil is not a renewable resource within any humanly meaningful span of time.

Glacial ice also freezes tight to the bedrock and plucks out blocks broken along the fractures that all rocks naturally contain. The combination of glacial rasping and plucking straightens the original stream valley and steepens its walls. The ice scours hills into rounded knobs. It shapes rock outcrops into streamlined forms with smoothly rasped surfaces that face into the direction of ice flow and raggedly plucked surfaces that face down the direction of ice flow. Each glaciated valley heads in a broad basin called a cirque, with a gnawed peak rising above it.

Glacial ice dumps its load of debris where it melts. Most of it is in deposits of till, a disorderly mixture of all sizes of material with little internal layering. Till looks like something a bulldozer might have scraped together. Melting ice also drops scattered boulders, which are called erratics because most of them are unlike the bedrock beneath. You see them here and there on glaciated surfaces throughout the high Sierra Nevada.

Anything made of till is a moraine. Many moraines are ridges made of debris that melting ice dropped along the edge of a glacier. Plot them on a map, and you have a precise picture of an ice age glacier. Geologists who do that generally find two sets of glacial moraines, one much farther down the valley than the other. The outer moraines apparently formed during an ice age of approximately 100,000 years ago—the exact date is a matter of much dispute. The inner set of moraines date from the last ice age, a lesser event that reached its maximum about 15,000 years ago and ended about 12,000 years ago. No one has clearly recognized in the landscape the mark of whatever ice ages preceded those two.

Melting ice sent torrents of water heavily laden with outwash sediment down glaciated valleys, many miles beyond the farthest moraine. Most outwash deposits survive as stream terraces within the lower valleys and as smooth alluvial fans and plains beyond the valleys, mainly on the floors of the Owens Valley and the Great Valley.






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