On April 18th, 1906, at 5:12 AM, the city of San Francisco awoke to the terrifying power of the San Andreas Fault. A powerful earthquake tore through the city, causing the destruction of over 80% of San Francisco and the loss of over 3,000 lives. However, this was only one incident in the 28,000,000 year history of one of the most powerful tectonic forces on planet Earth.

The San Andreas Fault is the meeting place between the North American and Pacific tectonic plates. These two masses continuously grind on each other in opposite directions, slipping about an inch every year. Occasionally though, they can suddenly slip feet at a time, resulting in devastating earthquakes across the plate boundary.

This specimen is a pair of geologic samples from Tejon Pass in California, where the boundary is exposed to the surface. In this location, you can actually see the gray metamorphic rock from the ocean plate and the brown sedimentary rock from the land plate colliding with each other.

The material from either side of the boundary is encased in a resin sheet, with both pieces enclosed in a handsome, glass-topped riker box case measuring 4 1/2" x 3 1/2". A small information card is also included.

On the morning of April 18th, 1906, a powerful earthquake woke the city of San Francisco at precisely 5:12AM. Fires resulting from the quake destroyed 80% of the city, killing more than 3,000 people and leaving hundreds of thousands homeless. It was as violent an introduction as one could imagine to the presence of the 28 million year-old San Andreas Fault.

Known as a transform boundary, the edges of the San Andreas Fault slip past each other in a horizontal fashion. In the case of the 1906 earthquake, it took less than five seconds for the Pacific and North American plates to slip 20 feet past each other along a 296 mile rupture. The results were devastating.

Disconcerting as this may be, the 1906 quake is not the largest quake possible along the San Andreas or even the largest that's occurred in the past. All along the fault's 800 mile spine, we can see the history of two continental plates grinding past each other. To understand the destructive potential of this fault, it's helpful to take a look into the deep history of the forces that have helped shape the western quarter of North America.

Nearly all of the land here was accumulated over the course of 200,000,000 million years as one ancient, oceanic plate known as the Farallon Plate slowly sank beneath the North American Continental Plate. Under the intense pressure, these terranes became sutured together and formed new land. When the central portion of the Farallon Plate finally slipped all the way under the North American Plate, the Pacific Plate came into contact with the North American Plate for the first time, forming the San Andreas Fault.

Even more troubling than the plate’s history is its near-future, with scientists long predicting that the San Andreas Fault is overdue for the “Big One”. Along the southern end of the fault, the two plates move northward and southward about an inch and a half each year, steadily building energy. Over the past three centuries, 27 feet of discrepancy have grown between them, ready to slip and release a devastating earthquake on Los Angeles and the surrounding region.

Over the past thirty million years, the fault has carried material hundreds of miles, depositing Mojave Desert granite up into Northern California and severing the Neenach Volcano field.

This specimen consists of pieces from both the Pacific and the North American Plate, recovered from the Tejon Pass in Southern California. Much of the San Andreas fault is covered by sediment and is inaccessible, but here the evidence of the fault is visible to the naked eye. 

At Tejon pass, one can see the fault expressed in the contrasting colors of the rocks visible. The Pacific Plate has deposited gray igneous monzonite, while the North American has brought brown sedimentary sandstone. These two types of rock do not form together naturally, but instead have been brought together by the power of the fault. 

Running between them is the fault gouge, rock that’s been directly exposed to the grinding of the fault, rendering it blackened and pulverized. Here at the roadside, you can literally see the power of our planet at work, scraping two plates together in a slow but never ending tectonic smash.