Mother Lode Geology: An old world becomes new

The land here came about because tectonic forces drove islands against the continent, because the rock underlying the ocean bed sank beneath the continent and created plutons, and because volcanoes spewed vast amounts of lava and ash across the landscape.

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

Geology forms the land, which shapes the history – and perhaps the future – of a place. It certainly did that in the Mother Lode, the central Sierra Nevada counties where generations have enjoyed the riches that the land provides.

Once, the area was under water in an ancient ocean. Much later, but not that long ago in geologic time, the Mother Lode was deep beneath the Earth’s – not the ocean’s – surface. The rocks and dirt that would have been above us back then are now down in the Central Valley. And most of the people who live in the Mother Lode today live on or very near earthquake fault zones; don’t worry, though, they haven’t shaken in eons.

Geology is all about change. The land here came about because tectonic forces drove islands against the continent, because the rock underlying the ocean bed sank beneath the continent and created plutons, and because volcanoes spewed vast amounts of lava and ash across the landscape.

Those things are important to our story. But there’s much more.

• Once upon a time, there was an ocean here where one heavy part of the Earth’s surface plunged beneath another part of the Earth, a place called a subduction zone. In fact, there were many subduction zones over time.

• The sinking rock, called the lithosphere, sometimes brought islands and other rocky materials along with it, as if on a conveyor belt, and pasted them onto the growing continental shoreline.

• When the shoreline overran a subduction zone, the subduction process stopped. But often a new subduction zone would then form offshore. The former subduction zones sometimes became earthquake fault zones where heated, mineral-rich liquid squeezed up from deep below the surface and hardened in the cracks and fissures, forming veins of quartz and other minerals, such as serpentine, the state rock.

• The subduction process created the Sierra Nevada, but not the mountain range we know by that name today. It was an ancestral Sierra Nevada much different from and much higher than the one we see, something more like today’s Andes in South America.

• The granitic rocks that we see as today’s Sierra Nevada, called plutons and visible in places like Yosemite National Park, were originally huge bubbles of molten rock that formed the deep basement of that older mountain range. They were created in successive pulses over millions of years.

• Over many millions of years, that first mountain range eroded and was replaced by a high plateau, a plain with broad rivers that emptied into a western sea. The rivers spread gold along their channels. The plutons, the rocks of today’s Sierra Nevada, remained embedded deep beneath the plateau. Massive amounts of earth and rock sat atop them.

• About 10 million years ago, the batholith began to rise steeply on its eastern edge, and it continues to rise today. This rise dramatically changed the course of the ancient rivers, leaving most of their old channels dry, creating layers of round river rocks in the earth as well as new streambeds that cut through the new landscape.

Many of the old riverbeds are now visible in road cuts, sometimes on mountain ridges in what geologists call “inverted topography,” where what once was lower is now higher up. Gold seekers mined the new riverbeds and then the old ones, too. Today, well drillers tap into the old channels for another valuable resource: water.

• As the Sierra Nevada rose, erosion followed on an unimaginable scale. Massive rock and mudslides occurred. Volcanoes added even more excitement, thickly layering the landscape with lava and ash. Evidence of that is still visible today in road cuts and elsewhere.

The mud and rocks and volcanic ash also filled the broad, flat Central Valley with rich soil, and the heavy erosion helped to reveal landmark features like Tuolumne County’s towering Table Mountain, once an ancient river channel that filled with successive lava flows.

• Ice, advancing and retreating, reshaped the landscape, too.

• The erosion also exposed the long-dormant earthquake fault zones that run basically north-to-south throughout the Mother Lode and far beyond it.

• The most prominent of the fault zones, the Melones Fault Zone, is the primary source of gold-laden quartz that the earliest miners called “la veta madre,” Spanish for the Mother Lode. Some zones yielded gold, copper, silver and other mineral finds of high value.

• The geologic processes that formed the region continue, and the future of the Mother Lode over many millions of years is a geologic story at least as interesting and dynamic as its past.

Next time, we’ll consider the beginning.

Contact Buzz Eggleston at buzz.calent@gmail.com.

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