Yosemite and Sierra Nevada Geology
Yosemite Valley is one of the most spectacular examples on earth of the power of geologic activity. It has been referred to as the "global masterpiece" of four awesome sculptors: earthquakes, glaciers, wind, and water. Let's learn more about this awe- inspiring geologic event - one that has taken millions of years to sculpt, and that is still changing today. We will be referring to resources provided by Yosemite National Park geologists - and talking to them online in a live chat.
Formation of the Sierra Nevada Range
200 million years ago there were two ancient tectonic plates at work in California. The oceanic plate was being pulled under the continental plate and this triggered ancient volcanic activity. Back then the Central Valley and Sierra areas were underneath the ocean (which is why your Camp ammonite fossil might have been found in California's foothills). On the floor of this ocean formed sedimentary rock washed into the ocean from the lands to the east, from what is now Nevada. When the tectonic uplift and volcanic activity took place, it started folding the soft sedimentary rock and compressed it, which started changing the sediments until they became a harder metamorphic rock that can still be seen on the surface today. This uplift and folding created a north-south low mountain range, and beneath it welled up more and more magma, working its way towards the surface.
The magma's activity created a large underground granite rock mass called a batholith after 5 million years of ongoing geologic activity. This granite batholith is today the foundation of the Sierra Nevada. After another 65 million years, the batholith began to reach the surface of the earth as the softer sedimentary and metamorphic soils above it were eroded away by winds and rain.
Then, the plate activity changed. Instead of having the oceanic plate pulled under - subducted - the continental plate, they started moving along side one another along what we now call the San Andreas Fault. This movement created the pressure that lead to even more of the uplifting of the batholith to become a major mountain range. As the mountains lifted up, the water run off grew faster on the more vertical inclines, and new types of erosion began to happen, creating V-shaped canyons.
In these V-shaped canyons, snow packed down so deep and solid that it did not melt year round, and grew larger and denser every year. This took place when the entire climate of California changed, and North American was experiencing an ice age with cold temperatures. These masses of hard packed snow formed glaciers. As gravity pulled these glaciers downhill, they carved U-shaped valleys, sheared rock faces, drug along massive boulders that scarred the rock underneath, and the glaciers shaped what we now see in Yosemite Valley.
When a volcano forms, it is bringing hot molten lave up from the core of the earth. If this rock cools below the surface, it is called plutonic. If it cools by reaching the surface it is classed volcanic. The magma that explodes out of a volcano is cooled by contact with the air and is volcanic igneous rock. The magma that cools while still buried under the cooler earth crust is plutonic igneous rock.
From what we have just read above, the Sierra Nevada then is primarily what type of rock? It did not explode in volcanic eruptions... it cooled beneath the surface.
That makes it plutonic igneous rock.
We have read that the granite forming the foundation of the Sierra Nevada is a batholith. Where do you think scientists got this word, batholith?
Batholith is a word that comes from two Greek words: bathos meaning deep, and lithos meaning rock. Deep rock.
The Sierras are not one huge mass of granite, they are made up of many large masses of igneous rock called plutons. Where do you think the word pluton comes from?
Pluton comes from the name Pluto, who was the Greek good of the underworld.
Over 95% of the rock in Yosemite is granitic rock. Granite is composed of crystals you can see with your eye - they are white, gray, black or even pink. Up close it looks like chunks of salt and pepper all compressed together. Granite is often used in outdoor sculptures, garden benches - and even faces huge skyscrapers in big cities. Granite is actually made up of two minerals - feldspar and quartz, with a little bit of biotic mica and hernblende giving it its darker peppery flecks. The size of the crystals vary, as does the combination of colors, and scientists group the types of granite together by measuring the proportion of minerals found in the rock, and the size of its individual crystals.
North America has gone through several ice ages over the millions of years of the earth's development. The most recent ice age that shaped Yosemite is called the Tioga Glaciation and it began 30,000 years ago. When it ended 10,000 years ago, the mammoths were still roaming California and living out on the Channel Islands. And people were first beginning to inhabit California. There are still a few small glaciers in the sierras, and believe it or not, they date back to this same time, 10,000 years ago. This means that snowflakes that are at least 10,000 years old are hard-packed into the small one square mile glaciers people can still visit today. Amazing.
An earlier glaciation, 1.2 million years ago, filled Yosemite Valley to the brim and excavated the valley to the form we know today. This glacier advanced down the deep narrow canyons created by rivers. In places such as Yosemite Valley, glacial ice traveling through was thousands of feet deep. Half dome projected 900 feet above the ice, but many peaks to the north were completely engulfed. The grinding, gouging action of the tremendously heavy weight of the ice eroded the canyons and valleys, widening and deepening them into U-shaped troughs. Later glaciations were not as deep, but also contributed to the polished, gouging, and breaking down of the great granite batholith into the familiar sites we see today in Yosemite Valley.