With stunning vistas of shear rock walls, cascading waterfalls, rounded granite domes and jagged spires, it’s no surprise that Yosemite is one of the most popular national parks in the U.S. While most visitors are drawn to Yosemite Valley, the park has numerous other wonders beyond the valley and even the park boundaries in the Sierra Nevada (=snowy mountain range in Spanish). Extending for about 650 km (400 miles) along the eastern side of California, and with a width around 110 km (70 miles), the Sierra encompasses nearly 25% of California’s land area. It is a recreational playground built by fire and carved by ice.
A classic view of Yosemite Valley from the Valley View overlook at the western end of the valley. On the left is El Capitan, at >900 m (>3000 ft), the highest sheer rock wall in Yosemite and one of the highest in the world. For this reason, and because of the massive nature of the granite, it is a popular destination for rock climbers. Even those without technical skills can summit Half Dome (center of photo) via a cabled route that scales the dome’s back side. NOTE: you can click on any image in the blog to get a larger view of it!
The story of Yosemite starts about 500 Ma (Ma=millions of years ago), when the western coastline of North America was in Nevada, and marine sediments were being deposited in shallow ocean water. We know about these ancient environments because their remnants have been preserved around the edges of the granitic rocks that intruded into them (see map below).
Starting in the early Mesozoic, around 200 Ma, the west coast of North America became an active plate boundary as an oceanic (Farallon) plate began to be subducted beneath the continental (North American) plate. As in the Cascade Range today, subduction caused rock to melt and magma to rise to create vast granitic plutons underground and a chain of volcanoes at Earth’s surface. Granite ages in the Sierra range from about 200–80 Ma, with the peak around 100 Ma, the average age of granitic rocks in Yosemite Valley.
This geologic map of California has colors categorized by age and rock type—in the legend they are listed from oldest at the bottom to youngest at the top. Most of the Sierra Nevada consists of granite, represented by the red color. The blue color represents rocks that are Paleozoic to Mesozoic in age (~500–150 Ma)—they are the pre-existing rocks that the granite intruded into. Notice the blue strip along the northwestern edge of the Sierras—this Western Foothills belt is a heterogeneous mix of volcanic islands, marine sediments, and slices of oceanic crust that were accreted to the continent prior to the granite’s intrusion. These rocks continue to the northwest in the Klamath Mountains and even into the Blue Mountains in eastern Oregon! The connections are obscured by younger activity; for example, the salmon-colored volcanic rocks of the Cascade Range (Lassen and Shasta volcanoes) and the Modoc Plateau cover the area between the northern Sierra and the Klamath Mountains.I created this profile (cross sectional view) to show what it looked like while the Sierra was being built. The Farallon plate was being subducted beneath the North American continent, much like the Juan de Fuca plate is being subducted beneath the Pacific Northwest today (see previous two posts). The green and black colors represent the igneous oceanic crust, whereas the red and brown colors represent marine sediments that were deposited on top of the igneous rock as the Farallon plate moved away from a mid-ocean spreading ridge toward the oceanic trench. Some of these materials were scraped off onto the overlying continent and are the rocks that make up San Francisco today. Chert is a sedimentary rock made of shells of siliceous micro-organisms and red clay.
In the proto-Sierra there was a chain of volcanoes similar to the Cascade Range today. Magma generated by melting deep beneath the surface rose upward (red vertical line). Some of the magma made it all the way to the surface and was erupted in volcanoes; much of the magma remained underground in chambers around 10 km (6 miles) deep where it cooled and solidified into granitic rock. At that time, the California’s Central Valley was a deep oceanic basin where thick sequences of marine sedimentary rocks accumulated. They are now buried beneath younger sediments, as shown by light yellow color (Quaternary = past 2.8 million years) labeled Great Valley in the geologic map above. Tertiary is the geologic period from ~65–3 Ma.
What we have in central California is a fossil subduction zone, where we can see at Earth’s surface the components that are buried beneath water or volcanoes in active subduction zones like Cascadia or the west coast of South America.This reconstruction shows the location of tectonic plates 100 Ma (modified from Engebretson et al., 1985). The profile above (“How it looked in the Mesozoic Era”) is a cross section from the Pacific plate, across the mid-ocean ridge (MOR), to the Farallon plate that was being subducted along the entire western edge of North America. The plate boundary (subduction zone) was located under the ocean where San Francisco is today, and the coastline was located farther east. As the Farallon plate was consumed beneath North America, the Pacific plate moved eastward and eventually impacted the continent and changed the boundary from subduction to transform, an evolution that will be explored in my next blog post.Because the magma was rising through the North American continent, it picked up lots of silica and produced the silica-rich type of rock called granite. This photo shows the minerals that make up granite—the clear minerals are quartz, the white minerals are feldspar, and the black minerals are biotite, a form of mica. Typically, the lighter the color of the rock, the more silica it contains. Because the magma solidified into rock at about 10 km (6 miles) beneath Earth’s surface, it cooled slowly and had time to form large crystals.
In contrast, magma that makes it all of the way to Earth’s surface and is erupted at volcanoes cools quickly so the size of crystals is very small and they need to be viewed under a microscope. The volcanoes that formed at the surface, above the granite, have since been eroded away, as the granite was uplifted to the surface.When magma cools underground, the resulting bodies of crystallized rock are called plutons (after Pluto, god of the underworld). In the Sierra, continued intrusion of magma over a vast distance during at least 100 million years produced hundreds of plutons that coalesced and even intruded into each other. Since the plutons have been uplifted to the surface, and the overlying sedimentary and volcanic rocks have been eroded away, we are left with an impressively continuous display of granite rock, as seen in the photo above, which is a view northeastward from Ten Lakes (central part of Yosemite NP) across the Grand Canyon of the Tuolumne to the high peaks of the eastern Sierra. Such large extents of granite rock are often referred to as batholiths (=deep rock).
Near the end of the Mesozoic Era, around 80 Ma, magmatism ceased due to changes in subduction geometry, and then resumed about 25 Ma, only to cease again as the plate boundary changed from subduction to transform. In the interim, uplift and erosion had removed the former volcanic chain and exposed the underlying granitic plutons at Earth’s surface by ~60 Ma. The lack of a high mountain range by ~30 Ma is demonstrated by lush vegetation and a vast megafauna in Nevada that could not have existed in the rain shadow created by the Sierra Nevada today. In addition, sediments and lava found west of the Sierra were able to flow from Nevada without the impediment of a large mountain range.
The current Sierra Nevada did not begin to form until ~5 Ma, when faults along the eastern edge of the Sierras became active. These faults are the western edge of the Basin and Range Province (see geologic map above) that is extending (pulling apart) the continent between the Sierra Nevada and the Wasatch Range in Utah.
Faults have caused the east side of the Sierra to be uplifted to elevations of ~4300 m (>14,000 ft), creating an asymmetric range with a short, steep eastern escarpment and a long, gentle western slope. This uplift is continuing today. (Diagram from Huber, 1989.)
As the Sierra was uplifted, streams cut valleys into the granitic bedrock. But another erosive agent was poised to make its mark on the mountains. About 2.8 Ma, Earth entered an Ice Age with alternating glacial and interglacial periods. The Sierra’s high elevation led to alpine glaciation, most recently during a peak about 18,000 years ago.
This map shows the extent of glacial cover in Yosemite National Park during the Tioga glaciation that was at its peak 18,000 years ago. Note the Grand Canyon of the Tuolumne (shown in photo above) and Yosemite Valley, which were formed initially by stream erosion, but then modified by glacial erosion. You can click on the map to get a larger image.We can imagine what Yosemite looked like 18,000 years ago by traveling to the Juneau Ice Field in Alaska. Here the Taku glacier is >1500 m (>5000 ft) thick and is eroding an underlying valley that will look much like Yosemite Valley after the glacier melts.
What is the evidence for glaciation in the Sierra? The following photos are examples of how geologists have been able to recreate the glacial history.
As glaciers flow over rocks, they scour the rock surfaces and can even produce a shiny polish. They grind grooves into the rock that indicate the direction the ice was moving. Jay is “surfing” the rock surface in the direction of the ice flow. Photo taken in Ten Lakes region in the central part of Yosemite National Park.The grooves in the granite surface (“surfing” photo) show an azimuth but how did Jay know to surf to the right rather than to the left? That is because of other nearby features called chatter marks. These crescent-shaped features form when glaciers skitter over a rock surface and pluck out pieces of the rock as it moves. The chatter marks are convex in the direction toward which the ice was moving.Hanging valleys are another hallmark of glacial erosion, and are where the magnificent waterfalls occur in Yosemite Valley. This photo is of Bridalveil Falls, located at the western end of the valley. A glacier occupied the main valley, and tributary glaciers created elevated valleys where they flowed into the main valley. Both the main valley (see photo at the top of this blog) and the tributary valleys have the U shape that is characteristic of glacial erosion.Another glacial “calling card” is erratics, rocks that get carried downhill in glacial ice and get dropped when the ice melts, often a long distance from where they originated. This photo at Olmsted Point shows erratics on a glacially-scoured rock surface. Also shown are extensive fractures in the granite bedrock that contribute to weathering and erosion of the granite. If we turn our head to the left, we would be looking westward down Yosemite Valley.This view of the east side of the Sierra shows a moraine that formed during the Tioga glaciation. The yellow arrow points to the terminus of the moraine, a ridge of sediments that was ground up by the glacier, carried down the valley, and left behind after the glacier retreated westward and then melted completely. Less-well-defined ridges away from the mountain indicate that ice was able to travel even farther during older glacial periods.This photo of a rock wall in Yosemite Valley demonstrates processes occurring today. At the bottom of the wall is an edge where a sheet of granite has slid off, a process called exfoliation. Large sheets of rock fracture because of pressure release as erosion removes the overburden from a rock that formed at high pressure deep in the Earth’s crust. This process helps to give domes their rounded appearance.
In the center of the wall are white areas that are the sites of recent rock falls. Weathering turns the granite surface a grey color, so the white color indicates recent rock removal. Rock falls are a continuing hazard in the valley that geologists monitor. For information about this, and other ongoing research in the park, see: https://www.usgs.gov/news/yosemite-science.This map shows rockfalls that have been documented in Yosemite Valley from 1857 to 2011. There are many, so be careful when you are near the rock walls in the valley!
There are many other geologic stories to tell about the Sierra Nevada—for example, gold emplacement and mining, a watershed that encompasses 40% of California’s land area, active volcanism on the east side (Long Valley Caldera and Mono Craters)—that will have to wait for a future post!
Park history. Yosemite is one of the earliest parks. Yosemite Valley was first deeded to California by President Abraham Lincoln in 1864. Largely due to the activism of naturalist John Muir, President Theodore Roosevelt decided to preserve the area around the valley as federal land; it became the national park we know today in 1906, when California ceded the valley to become federal land. Yosemite is named after the Native American word (uzumate) for grizzly bear, a type of wildlife that is no longer found in California.
