TECTONIC HISTORY OF LONG VALLEY CALDERA
The great Scottish naturalist James Hutton (1726-1797), often regarded as the father of modern geology, coined the phrase, “The present is the key to the past.” This catchphrase of modern Earth science reminds us that in order to understand the geologic history of our planet, we need to observe and understand how present-day processes, operating over unimaginably long geologic time spans, can lead to the landscapes and geological structures observed at Earth's surface. So we can use modern geology to determine what happened millions of years ago. Geologists invoke this idea all the time when they look at faults, mountains, rifts, and volcanoes to try to understand the processes that teamed up to create those features.
In the three centuries since Hutton's time, our understanding of Earth's processes has grown through remarkable new observations of Earth's surface and interior. A suite of observations of present-day Earth processes—mostly from the oceans—has led to one of the greatest scientific revolutions in history, the theory of plate tectonics. Just as Hutton predicted, the plate tectonics theory has helped us understand the long-term evolution of Earth's landscape in places like Long Valley.
According to the theory of plate tectonics, Earth's surface is broken up into a mosaic of ‘tectonic plates.' and these plates are in constant motion. There are eight large plates and about a dozen smaller plates. The geologic processes observed at Earth's surface are governed by the relative motion of these plates. In some areas, neighboring plates are moving away from each other creating divergent plate boundaries; in others, plates are coming together to create convergent plate boundaries; at other sites, the plates are simply sliding past one another at transform boundaries. Over the immense lengths of geologic time, these motions, measured in millimeters per year, can lead to the creation or destruction of whole oceans, formation of mountains, and the rifting of whole continents... In short, the process of plate tectonics teaches us that the surface of Earth is utterly dynamic; a time-lapse movie of Earth would show a planet in constant motion.
Of course, without highly specialized instruments, we can't see this movement every day. It takes the accumulation of many years for us to see the results of tectonic plate movement on Earth structures and processes. So geologists have to look at evidence of plate movement such as the types of rocks, the structures such as folds or faults in the rock, and remnants of living things—fossils left behind in the rock strata—to reconstruct what plates looked like 10, 50, even 100 million years ago (or more!).
From such geologic observations, Earth scientists have been able to reconstruct that around 270 million years ago all of the world's continents were joined together in one single ‘supercontinent' called Pangea. About 215 million years ago, Pangea began to break up and the plates began to move into positions much like the globe we know now. What is now Long Valley was located near the western edge of this supercontinent. Much of the subsequent history of the North American continent involves the breakup of Pangea, and resultant changes in the world's ocean. Specifically, the basin of the Atlantic Ocean has been created during this time, while the Pacific basin has been shrinking. That process of oceanic shrinkage has taken place primarily through the process of plate consumption, or subduction, of the oceanic crust west of North America.
From roughly 180 until 20 million years ago, the tectonic motion along the western edge of the Americas was dominated by subduction of a sea-floor plate that no longer exists—the Farallon Plate. That segment of oceanic crust separated the Pacific sea floor plate from the North American Plate. Subduction of the Farallon plate under North America led to the rising of magma bodies and the creation of the Sierra Nevada volcanoes. These volcanic magmas also brought with them the gold, silver, and other precious metals that drew prospectors to California in the 1800's.
The last bit of the Farallon plate subducted under North America and brought the North American and Pacific plates together some 20 million years ago. As illustrated above, this event has created a “slab gap,” a zone in which there is no subducted sea floor slab between the continent and the mantle. So the underside of the western North America continent is now exposed to the very hot mantle! The heat from the mantle has caused the continent to thin and stretch, in some places by as much as 170 miles! This stretching and thinning is what has created the Basin and Range province.
Some scientists believe that the volcanoes in the Long Valley region are related to this crustal extension associated with the western margin of the Basin and Range province. Mono Basin is the westernmost basin in the Basin and Range province, and the Sierra Nevada Mountains the westernmost range.
Perhaps more importantly to Californians, the new change in plate configuration has placed the state in a new state of stress, dominated by the shearing motion between the North American and Pacific plates. It is this lateral motion between the two plates that has created the San Andreas fault. This fault is a big part of the reason there are so many earthquakes in California!
This motion created the Eastern California Shear Zone and Owens Valley Fault Zone. These zones accommodate part of the relative motion between the Pacific Plate to the west and stable North America to the east.