Life on the Plates' Edge
The Earth’s crust is composed of about 20 large plates. These plates move about atop a slowly roiling mantle which has the consistency of thick molasses. The Presidio area is located on the edge of two of these crustal plates, the North American and Pacific plates. This marginal location is responsible for the rugged terrain, periodic earthquakes, and the unusual types of rock exposed here.
The Presidio lies on the North American Plate, but the boundary with the Pacific Plate, the San Andreas fault, lies only five mile west of the Pacific shoreline at the Golden Gate. The Pacific Plate is moving northward at the same speed that your fingernail grows (1 inch or 2.5 cm a year). The rocks of the Presidio started rising out of the ocean only 4 million years ago, when a small shift in plate motion caused pressure to increase along the plate margin at a left bend in the San Andreas fault. In contrast, the San Francisco Bay lies on a stable or slowly down-dropping area formed between the San Andreas and Hayward faults, both of which are zones of slip between the North American and Pacific plates. Major earthquakes occur several times each century on these or less well-known faults in the San Francisco Bay Area; releasing strain built up between the creeping plates. These earthquakes destroyed the original adobe Presidio on several occasions and led to the near destruction of San Francisco in 1906.
Last updated: February 28, 2015