Geology: GFNMS

The seafloor landscape of the Greater Farallones National Marine Sanctuary is active, varied and beautiful – much like the land above the water. This underwater landscape is the cumulative result of millions of years of the earth’s history, involving many phenomena, including the building and subsequent wearing down of mountains, movement during large and small earthquakes, changes in sea level, and the never-ending processes of wind and water erosion and deposition of bottom sediments.

The sanctuary straddles the boundary zone between two of the earth’s major tectonic plates: the Pacific and North American. The Pacific Plate is slowly moving northward relative to the North American Plate at an average rate of about five centimeters (two inches) per year. Most of this motion occurs in catastrophic bursts of movement – earthquakes – along the San Andreas Fault system.

Near San Francisco, the San Andreas Fault system is a complex zone of faults about 80 kilometers (50 miles) wide. It stretches from the sanctuary as far east as the cities of Vallejo and Livermore. Two of the system’s major faults, the San Andreas and the San Gregorio, merge into one fault zone in the sanctuary just offshore from the Golden Gate Bridge.

The San Andreas Fault zone splits Bolinas Lagoon, Tomales Bay and Bodega Bay. It is responsible for the long, straight, narrow shape of Tomales Bay.

West of the fault, Bodega Head, the Farallon Islands and Point Reyes Peninsula are made of granitic bedrock believed to be originally from the southern Sierra Nevada. These landmasses and the surrounding submerged lands ride on the Pacific Plate northward relative to the adjacent North American Plate. Most of Marin County is east of the Fault, which is on the North American Plate.

The famous 1906 San Francisco earthquake, with a magnitude of 7.8 on the Richter Scale, began on a segment of the San Andreas Fault that lies underwater adjacent to the sanctuary, just a few miles offshore of San Francisco. During this one earthquake, the Pacific Plate underlying the sanctuary lurched 4.5 to 6 meters (15 to 20 feet) north relative to the North American Plate to the east.

Sediment cores from eastern Bolinas Lagoon (North American Plate) indicate that this portion of the lagoon became deeper as a result of this earthquake, due to shaking-induced compaction and basin subsidence. The lagoon has since been filling in with sediment from terrestrial watersheds and the ocean, increasing the amount of salt marsh and frequently-exposed mudflat habitats while decreasing the amount of submerged habitat. It is thought that when another large earthquake occurs in this area, the east side of Bolinas Lagoon may again deepen and experience a natural shift toward submerged habitats.

Interesting geology, knobs formed by the differential erosion of sedimentary rock. This is just at the base of the cliff jutting southwest-most at Pillar Point.

Offshore of Tomales Bay and Bolinas Lagoon, the sanctuary is characterized by a gently seaward-sloping continental shelf area that is approximately 55 kilometers wide. The western edge of the shelf is currently in 90 meters of water depth and coincides with the shoreline of the last ice age.

As recently as about 10,000 years ago, at the end of the last ice age, the great sheets of ice (glaciers) that covered much of the earth’s northern hemisphere began melting – causing worldwide sea level to rise. Sea level rose to its present-day position within 5,000 years. In that time, the continental shelf portion of the sanctuary was flooded and Tomales Bay, Bolinas Lagoon, Estero Americano and Estero de San Antonio became estuaries.

Since much of the sanctuary seafloor is too deep for human divers, scientists have learned much about it by using acoustic instruments (those that use sound sources and listening devices) and underwater cameras and video equipment. Data collected from the sanctuary continental shelf reveal various features on the seabed, including linear trends of biologically diverse rocky outcrops and several types of sand wave features, such as giant undersea sand dunes. West of the continental shelf, the water depth steeply increases over the rugged seafloor, indented by numerous submarine canyons.

Monitoring

The following list includes some of the projects underway in the sanctuary. Please click on the Projects tab at the top of this page for more information.

usSEABED Database
The USGS has spearheaded a major ongoing effort to gather diverse geologic data about the continental shelf and organize it through a GIS database. This project, called usSEABED, is an innovative system developed to use both lab-based analyses and numeric data from descriptive data together in a unified database. The database provides unprecedented coverage of the sediment and rock types sampled from the seafloor as well as a searchable catalog of other observations, such as micro and macro flora and fauna and sedimentary structures.