Mother Lode Geology: Mighty Sierra begins to rise

The Dardanelles, the volcanic peaks seen in the distance, are part of the same lava formation that forms Table Mountain. Originating from a volcano somewhere still farther east, the flows extend all the way to Knights Ferry on the Stanislaus River, a distance of at least 100 miles.

Editor’s note: This is part of an ongoing series about the geology of the Mother Lode.

The Miwok and other first Americans believed Coyote created the Earth. He took handfuls of dirt and spread it about, making valleys and mountains. He sang and he danced, and rivers and lakes, pines and cedars and oaks, birds and animal people appeared. Different tribal bands disagree about who helped him; some say Coyote worked with Turtle, others with Silver Fox or with Frog. But in every story, when the job was done they celebrated. They sang and danced.

This is a story about that creation, a journey through time. It is about how the Mother Lode came to be, about the Gold Country and the lands that surround it. It also is the story of the Sierra – Spanish for mountain range – Nevada, Spanish for snowy, and how it came to be, how the land changed over millions of years, how it came to look the way it does today, how those changes shaped its history, and what might become of the region, indeed much of California, millions of years from now.

It’s a complex story. Let’s start.

There was a time more than 200 million years ago when the place where the Mother Lode is now was occupied by an ocean. Most of what was then North America was much farther east. The ancient ocean teemed with many creatures we would recognize today and many others we can only imagine.

Mary Hill, a geologist and the author of “Geology of the Sierra Nevada” (University of California Press, 1975, revised 2006), wrote, “Very little is left of the creatures that lived in those ancient seas. Fossils are scarce in the Sierra, largely because most remnants of life were destroyed when the layers of rock that entombed them were pushed upward, bent, twisted and faulted during the several episodes that marked the creation of the range.”

The ocean where the Sierra Nevada is now was in time replaced by a high mountain range. It’s been called the ancestral Sierra Nevada or the Nevadan orogeny and formed between 180 million and 140 million years ago. It included mountain peaks as high or higher than 20,000 feet, nearly twice as high as today’s Sierra Nevada. Millions of years of erosion ground down those mighty mountains to a hilly plain, averaging about 3,000 feet above sea level. The ocean still lapped the coast west of today’s Highway 49.

After a long time, however, a new epoch of mountain building began.

The several episodes Hill described as creating the Sierra Nevada began with the successive formation of huge underground pools called magma chambers, made up of 2,000-plus degree Fahrenheit molten rock. Volcanoes sprout from magma chambers, rise through cracks called chimneys to break the surface of the land or the seabed, build volcanic cones and, in the case of the Sierra Nevada, spew silicate ash and sometimes lava, and cause mud and rock flows all around. Volcanoes shaped the region over many millions of years. Evidence of those volcanoes and their eruptions exists throughout the Sierra and its foothills.

When the deep magma pools cool and the rock hardens, they are called plutons. Taken together they are called a batholith. The Sierra batholith is a mass of more than 80 plutons.

The magma chambers form at depths miles below the Earth’s surface. In several pulses beginning more than 150 million years ago, such chambers began to form in a north-south line where the Sierra is today. More continued to form at intervals over the next 60 million years.

When these massive pools of molten rock cooled and hardened they became granodiorite and other granitic rocks; they are the plutons that together form the 400-mile-long Sierra batholith – but even when they cooled these great rock formations were still miles underground. Granitic rocks abut or underlie much of the Mother Lode.

Molten rock in the Tuolumne Meadows area of Yosemite National Park hardened 88 million years ago; El Capitan about 102 million years ago; and its neighbor across Yosemite Valley, Half Dome, about 92 million years ago; the magma of Sentinel Dome hardened into rock about 95 million years ago.

Next installment: An ocean trench.


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