Yosemite Geology 101

Learn how volcanoes, glaciers and erosion from millions of years ago shaped today's gorgeous landscape of Yosemite National Park.
By Staff,
Glacier Point

Yosemite National Park sits on the rooftop of the Sierra Nevada Mountains, which stretch for over 400 miles from sparkling blue Lake Tahoe down into the Mojave Desert. The park's epic landscapes, from its gorgeous valleys and dramatic waterfalls to the iconic granite domes and the mountain peaks of the high country, were all shaped by the same powerful geologic forces.

Erupting Volcanoes and an Ancient Sea

Although it might be hard for many park visitors to imagine, scientists believe that the Sierra Nevada Mountains were born 200 million years ago at the bottom of an ancient sea. Their story began off what is now the west coast of California, when an oceanic plate began to slide underneath the land mass of North America in a geologic process called subduction.

This tectonic-plate activity was often violent, causing volcanoes to erupt above the water's surface. At the same time, some of the molten rock cooled into vast deposits of underground granite, called plutons. Today, you can see still evidence of ancient and more recent volcanic activity around Mono Lake, just east of Yosemite's high country via the Tioga Pass.

The Sierra Nevada Mountains Rise

About 65 million years ago, erosion wore down the cooled volcanic landscape into low rolling hills. At the same time, erosion also exposed the underground granite lying beneath the surface.

Geologists now estimate that the Sierra Nevada Mountains began to be formed in earnest about between 10 and 25 million years ago. Strike-slip movement along California's famous shake, rattle and roll fault lines caused a dramatic uplift of the region's underground granite batholiths.

Starting about 5 million years ago, rivers and streams started cutting down through the mountains, creating steep V-shaped valleys and canyons. However, the canyons and valleys that you'll see in Yosemite today look very different from say, the Grand Canyon. And that's all because of the activity of glaciers.

Glaciers During the Last Ice Age

At the end of the planet's last ice age, just over 1 million years ago, massive glaciers moved down from the Sierra Nevada high country, uniquely carving Yosemite's river valleys into wider U-shapes (as first correctly theorized by the naturalist John Muir). Some of these ancient glaciers traveled for over 50 miles along the canyons cut by such rivers as the Tuolumne and the Merced.

When the last glaciers retreated from the Yosemite region about 10,000 to 15,000 years ago, they left behind transitory lakes - for example, Mirror Lake in Yosemite Valley - which will eventually disappear over time, after being completely filled in by sediment and being transformed into meadows. These last glaciers also polished the park's iconic granite domes, such as Half Dome (to learn more about how these domes were formed and are still being shaped, click here).

Yosemite Today: Still Rising and Falling

The Sierra Nevada Mountains are still rising at a rate of a about a few inches per year. However, this persistent uplift is almost negated by the forces of erosion, which wear and tear down the Sierra Nevada Mountains at almost the same rate. Erosion is also one of the factors in the rockfalls that still happen in the Yosemite Valley to this day.