The Sierra Nevada: Mountains on the Move
July 27, 2004
In all human cultures, mountains are seen as ageless terrains that defy the elements, providing the perfect eternal abode for spirits and gods. We tend to view mountains as pinnacles of stability that have always existed and always will. Beginning in the early 19th century, as the modern methods of scientific inquiry developed, geologists became increasingly skeptical of the notion of ageless mountain systems. Today, we can demonstrate beyond a shadow of a doubt that such presumptions are utter nonsense. The geological evidence clearly demonstrates that mountain systems rise and fall in continuous cycles of uplift and erosion that began shortly after the Earth coalesced from cosmic debris more than 4 billion years ago. In addition to cycles of vertical ascent and decline, mountains are also not as laterally immobile as we might suspect. Because they develop on and within the mobile lithospheric plates comprising the outer Earth, all mountain systems can glide around the planet like a bump on a driftwood log. Evidence of both kinds of movement, vertical and lateral, is abundant in the Sierra Nevada. Some of the evidence is found in the geological record, where the history of the Sierra is recorded in rock millions of years old. Other, even more compelling, evidence of mobility comes from the application of modern high-tech mapping techniques that can actually demonstrate that the Sierra is clearly a mountain system on the move. The ascending SierraThe rise of the modern Sierra is documented in the sediments that were shed from the ascending mountains into the surrounding basins and flanks. By studying the deposits of sand, gravel, and silt eroded from the soaring summits, it is possible to re-construct the history of uplift much like we might be able to understand how a house was built by rummaging through a nearby construction Dumpster. Based on such evidence, the uplift of the modern Sierra Nevada does not appear to have been a simple event. The mountain system was elevated at different rates, in different places, at different times. Nonetheless, it is clear from the geological data that the Sierra, in general, is a young mountain system, having risen in an incremental process that began about 5 to 10 million years ago. In contrast, the uplift of the Rocky Mountains, also a complex geological event, began about 60 million years ago and the Appalachian system in the eastern United States began to rise more than 325 million years ago.
The spectacular ruggedness of the Sierra is a reflection, at least in part, of its relative youth. And it’s far from being “finished.” The recent uplift of the Sierra appears to have begun to the south, in the area between Mt. Whitney and Tehachapi Pass, and migrated north. The ascent of the mountains was related to the stretching forces that broke the crust to east into the familiar pattern of alternating mountains and valleys that geologists call the Basin and Range province. The Sierra Nevada can be considered the western-most mountain system in this rugged terrain of alternating linear mountains extending from eastern California to central Utah. It appears that the uplift began around 10 million years ago in the central and southern Sierra. Prior to that time, ancient river deposits suggest that there was no mountain barrier that prevented direct drainage between western Nevada and the Pacific Ocean. In addition, plant fossils about 8 million years old in western Nevada are of a type that could not have survived in the modern desert environment. This suggests that there was no Sierra standing in the way of moisture-laden Pacific storms during that time. Without the Sierra rain shadow, western Nevada was then cloaked with a dense subtropical forest, a strikingly different flora from the sparse sagebrush desert of today. In the Verdi-Truckee region, the geological evidence has led some geologists to suggest that the most active phase of uplift may be even younger than elsewhere in the Sierra. The ancient drainages on the west slope of the Sierra, reconstructed from the old stream deposits they left behind, appear to have remained intact until about 3 to 4 million years ago, when tilting and faulting along the crest disrupted them. Sediments were shed from the rising mountains into the Verdi area as early as 10 to 12 million years ago, but there appears to have been a surge of uplift in the region around 3 million years ago. In the Tahoe region, this recent uplift was accompanied by the eruption of numerous volcanoes.As the mountains rose, erosion immediately began to attack the rocks of the rising summits, wearing down the Sierra even as it ascended. The slow process of erosion did not match the rate of ascent, so the mountains continued to gain elevation, even though the crests were being simultaneously worn down by the elements. Gravel and sand shed from the mountain peaks was flushed by rivers across the volcanically active region to the east, where it was mixed with ash and rubble. Eventually, the sand, mud, and gravel settled out in layers along the flanks of the rising mountains. Such materials can be seen in the bluffs around Boca Reservoir, in the hills just west of Reno, and in the railroad cuts in Verdi. Nearly all of these layers accumulated between 12 and 3 million years ago, as the local mountains were soaring to the east. The erosional refuse is commonly sandwiched between layers of volcanic ash and hardened lava flows, indicating that volcanic activity continued in the region as the Sierra was being lifted.Eventually, of course, erosion may gain the upper hand, if (or when?) the rate of uplift diminishes. Then, our mountains will gradually be planed down to await the next cycle of uplift. However, even if the Sierra was to stop rising today, it would take millions of years for erosion to significantly alter the modern landscape. But, such things do happen in the long span of geological time. In fact, the uplift of the modern Sierra in the relatively recent geological past is the second time a mountain system has developed here. The Ancestral Sierra Nevada was elevated about 110 to 85 million years ago, before it was eroded to a low, rolling highland some 50 million years later.
Measuring movementDeciphering the history of uplift of a mountain range from the sediments that accumulated along its flanks is a traditional geologic practice that has provided insights on the development of mountain systems for more than a century. Measuring the lateral motion of a mountain with respect to the continent involves a completely different approach. It was impossible to measure such motion accurately until technological advances of the last decade were applied to the undertaking. For the past 12 years, scientists have been developing a network of Global Positioning System (GPS) stations across western North America. These devices, like the GPS units in cars and boats, rely on satellite signals to measure location with incredible accuracy. In the case of the geodetic GPS network in western North America, locations can be measured with a margin of error of a fraction of an inch or less. By measuring the precise positions of GPS stations over a period of several years, even imperceptibly slow movements can be detected.Coordinated by the Space Geodesy Group of the Harvard-Smithsonian Center for Astrophysics, a consortium of scientific organizations have shared data acquired from this geodetic grid during the 1990s to track the actual motion of dozen of stations located in western North America. Combining the GPS data with another satellite-based technique known as Very Long Baseline Interferometry (VLBI), scientists can now accurately measure the surface motion that results from forces generated deep within the earth. For residents of the Sierra Nevada region, the results of these high-tech approaches to observing the slow lateral motion of the crust are very intriguing. Drifting northAlong the Pacific coast of California, it is not surprising to learn that GPS and VLBI data demonstrate a steady, and relatively rapid, drift to the northwest. This is due to the location of the coastal region to the west of the San Andreas Fault system. This sliver of California is sliding toward Alaska at a rate of about 50 millimeters (about 2 inches) per year due to the continuous sliding motion that occurs between blocks of the crust in the fault zone. On the opposite side of the Sierra, in the Basin and Range province, the motion is much slower, about 3 millimeters per year, and directed toward the west with respect to the rest of North America.
Given the abundant geologic evidence that the crust in the Basin and Range is subject to stretching forces, the measured surface motion of locations in Nevada is not surprising: the crust there is slowly being stretched away from the stable part of North America like warm taffy. However, between the coast and the central Basin and Range province is the Sierra Nevada Block, our mountain home. The new data clearly demonstrate that the “Sierra Block,” which includes the Central Valley of California, it is currently following a unique trajectory compared to the surrounding terrain. The Sierra Block is moving northwest, nearly (but not exactly) parallel to coastal California, at a rate of about 12 millimeters (approximately a half-inch) per year. The Sierra Block is also following a slightly more westerly trajectory than is coastal California. We may interpret this motion as the consequence of friction, or “drag,” against the northwest-moving Pacific plate to the west, coupled with a westward nudge from the widening Basin and Range province to the east. Within the Sierra Block, the velocity and direction of motion appears to be remarkably uniform, suggesting that the block will remain intact as a coherent unit while the crust adjacent to it is broken apart into smaller fragments by the unrelenting geological forces currently affecting the region.So, how fast is 12 millimeters per year? Depending on your diet, metabolism, and heredity, your fingernails will normally grow from 20 to 50 millimeters per year. So, we are perched atop a mountain that is moving along at a speed of about half the rate of human fingernail growth.This motion is, of course, exceedingly slow by human standards, but it is measurable with the tools of modern technology, and it may have some interesting longer-term implications for future landscapes.Five million years from now, the Sierra Block will have migrated about 40 miles to the northwest. Truckee will have drifted with the moving mountain block to a location near modern Blairsden. Meanwhile, by that same time, coastal California will have slipped 156 miles to the northwest, and will have started to separate from the mainland. The eventual result of this slow process will be the fragmentation of the western edge of North America into several isolated pieces that will move away from the continent in various directions and at differing rates. Coastal California will eventually collide with the southern edge of Alaska, while the Sierra Block will move outward from the continent as large island reminiscent of Madagascar, while has similarly been severed from eastern Africa.No one alive today, or even their distant descendants, will ever see these dramatic changes in landscape. What sorts of humans, if any, will inhabit the Sierra Block when it has evolved into an island in the northern Pacific is an open question. But, what we will notice from time to time are the periodic trembles produced when rocks snap as geological forces continue to drive the Sierra Block farther and farther to the northwest each year. Thus, the Sierra will never be “finished” because, like all other mountain systems on Earth, it is a perpetual state of change. Over the long run of geologic time, the mountains will be lifted, worn down, and lifted again as they continue sliding around the globe in response to the deeper convulsions of our geologically active planet. The stability and permanence of our mountains are only illusions. The Sierra Nevada is, without question, a mountain on the move. Frank DeCourten is dean of the Sierra College Truckee Center.