Visitors to Yosemite National Park in California’s Sierra Nevada Mountains often ask Park Geologist Greg Stock the same questions: “What’s up with this mountain range? Is it growing or shrinking? How old is it?” But 150 years ago, people considered the same questions, though the reason they were asking them happened to be very different—those visitors were out to strike it rich on California gold.
Mountain ranges, those places where the horizon wins its war with the sky, appear so imposing that they demand explanation even to those unaccustomed to geologic curiosity. Many ranges have origins simple enough that school kids can recite the basic sequence of events. Two tectonic plates smash together like an extremely slow-motion car accident, with a crumple zone in the middle. Or in another flavor of convergence, an oceanic plate is subducted beneath its partner, fueling volcanoes. The first can describe the Himalayas; the second, the Andes.
But the Sierra Nevada Mountains, naturalist John Muir’s brilliant muse, remain a geologic puzzle not so easily solved despite the work of generations of geologists. The North American Plate and the Pacific Plate don’t collide along most of the California coast, they slip past each other horizontally. The Sierra is not an active line of rock-pouring volcanoes like the Cascades to the north. Yet there it is, with all the subtlety of a sledgehammer—Mount Whitney, the highest point in the Lower 48, towering 10,000 feet above neighboring Owens Valley, which is just about 50 miles from Death Valley, the lowest point in North America. The Sierra is a defining feature of the most populous state in the US, but we still don't quite understand it.
Over the years, ideas about the history of these mountains have ranged from no change in height since the dinosaurs roamed (and shrinking today, if anything) to mostly being raised in the last 10 million years and probably still growing. The “middle ground” isn’t necessarily a good bet in science, but in this case it seems there’s a good chance both these views are partly right. The catch is, the story of the Sierra is a play in three acts.
How the West was wondrous
The earliest history of the Sierra isn’t contentious. During the Mesozoic Era, spanning from about 252 to 66 million years ago, the western edge of North America looked very different. The wandering barges of crust that would dock with North America to build what is now the Pacific Northwest hadn’t even arrived yet. And while California slides horizontally along a boundary with the oceanic plate today, it was a bona fide subduction zone back then. The North American continent was converging with oceanic crust there, driving the denser oceanic plate downward into the mantle. That means like much of the rest of the “Pacific Ring of Fire” today, a line of volcanoes stood watch over the coast. The West is often described as having looked like the modern South American Andes.
The beautiful granites that comprise much of the Sierra Nevada—including Yosemite National Park’s iconic Half Dome—were the product of that volcanism. The ash and lava that erupted from those volcanoes has largely eroded away. What's been left behind is an expanse of igneous guts that cooled slowly underground beneath the volcanic peaks, creating hard rocks like granite. Because the Earth’s crust floats, in essence, on top of the denser mantle, the removal of rock from the surface by erosion can result in the crust popping up like a ship off-loading heavy cargo. The crust is normally quite thick at a mountain range, and elevation can be maintained for a long time in the face of erosion this way. This has helped push the granite upward from its former position deeper in the crust.
The volcanoes sputtered and died around 70 million years ago, but not because the tectonic plates stopped converging. Instead, it was the angle of the diving oceanic plate that brought magma production beneath the Sierra to a halt. Rather than diving downward quickly, it flattened out—the result of a couple of massive seafloor plateaus getting sucked down the subduction trench. The production of magma at these plate boundaries occurs because water in the ocean plate heats up and gets released into neighboring mantle rock, lowering its melting point. If the plate doesn’t dive deep enough, it doesn’t get hot enough, and this doesn’t happen—meaning no magma for volcanoes.
This “flat slab” of oceanic crust dragged against the underside of the North American plate far inland, increasing the resistance between the passing plates. The added stress pushed up mountains across the American West, including the Rockies. For a long time, the Sierra Nevada Mountains were just the western edge of a wide mountainous region that continued much farther east. After 20 million years or so, the flat slab started to fall away from the continental plate again, fueling some interesting eruptions far inland from the coast.
And then things really got crazy. North America had been gobbling up the Farallon Plate, and it finished its meal around 30 million years ago. There was a mid-ocean ridge between the Farallon and Pacific plates, where oceanic crust is birthed before moving apart in twin conveyor belts. That ridge nestled up to the West Coast even as the crust on the eastern side of it separated from the ridge as it sank down into the mantle beneath North America.
That had several consequences. First, with one of its two plates gone, the subduction zone along what is now California stopped being a subduction zone. (Subduction continues today along Oregon, Washington, and into British Columbia, feeding the Cascade volcanoes.) Instead, the continental and oceanic plates began sliding past each other laterally, creating the San Andreas Fault. That meant no more compression squeezing up mountains. With the vice loosened around that thickened, compressed continental crust, it began to relax and spread out.
This extension pulled much of the mountainous region apart, cracking and stretching it to form the distinctive pattern of ridges and valleys—like ribs sticking through the skin of the Earth—from Nevada to Mexico. The western boundary of all that turmoil is a fault that sharply defines the edge of the Sierra Nevada Mountains. The mountains that had sprung up behind the Sierra, eclipsing that range in height, dropped away. From then on, the Sierra stood alone.