Oceanography and Geology of the Gulf of the Farallones 



Regional Setting 



John L. Chin and Russell W. Graymer 



The Gulf of the Farallones lies in the off- 

 shore part of the San Francisco Bay region 

 just beyond the Golden Gate. The bay 

 region is home to more than 6 million 

 people and has long been regarded as 

 one of the most beautiful regions in the 

 United States. It annually draws hundreds 

 of thousands of tourists from all over 

 the world to enjoy its hilly, often fog- 

 shrouded terrain and its mild, Mediterra- 

 nean climate, as well as to experience its 

 cultural richness and diversity. The histori- 

 cal settlement of the bay region is inti- 

 mately related to its geography, natural 

 harbors, and mild climate. 



The San Francisco Bay region extends 

 from the Point Reyes Peninsula southward 

 to Monterey Bay and from the coast east- 

 ward to the Sacramento-San Joaquin Delta 

 and also includes the offshore area out to 

 a water depth of about 1 1 ,500 feet, which 

 is the outer edge of the Continental Slope. 

 This offshore area, where many residents 

 of the region and tourists go to watch the 

 annual migration of whales, as well as to 

 boat and fish, includes the Gulf of the Far- 

 allones and the Farallon Islands. Although 

 the sea floor in this area cannot be visually 

 observed, scientists have learned much 

 about it by using acoustic instruments 

 (instruments that utilize sound sources and 



listening devices) and underwater cameras 

 and video equipment. 



The landscape features of the San 

 Francisco Bay region are the cumulative 

 result of millions of years of Earth his- 

 tory, involving the building and subse- 

 quent wearing down of mountains, volca- 

 nic eruptions and resulting outpourings of 

 lava, large and small earthquakes, and the 

 never-ending processes of wind and water 

 erosion and deposition. Many of these 

 processes are occurring today, although 

 some of them occur at rates so slow that 

 little change may be perceived over the 

 course of a human lifetime. The sedi- 

 mentary deposits and rocks of the region 

 range in age from hundreds qf millions 

 of years to those still forming today. 

 The entire spectrum of rocks present in 

 the region records only a fraction of 

 the Earth's history. The rock types range 

 from the salt-and-pepper granitic rocks at 

 Point Reyes and on the Farallon Islands, 

 through various types of volcanic rocks 

 (such as the pillow basalts in the Marin 

 Headlands), to sedimentary rocks, such 

 as the red to brown bricklike chert that 

 forms the cliffs of the Marin Headlands 

 and many of the hills of San Francisco, 

 and the young sand-dune deposits that 

 occur along the Pacific Coast. 



The San Francisco Bay region strad- 

 dles the boundary zone between two of the 

 Earth's major tectonic plates. The Pacific 

 Plate is slowly moving northward relative 

 to the North American Plate, and this 

 motion takes place along the San Andreas 

 Fault and a network of associated sub- 

 parallel faults, collectively called the San 

 Andreas Fault Zone (see chapter on Earth- 

 quakes, Faults, and Tectonics). Rocks east 

 of the San Andreas Fault Zone were 

 mainly formed in approximately their 

 present positions. However, many of the 

 rocks found within the zone, like the Fran- 

 ciscan chert at the Marin Headlands, are 

 ocean-floor rocks that have been scraped 

 off onto the edge of North America over 

 the past 150 million years. 



The oldest of the rocks found west of 

 the San Andreas Fault were formed hun- 

 dreds of miles to the south, some near 

 the present site of Los Angeles, and 

 subsequently transported northward by 

 movement along the San Andreas and its 

 associated faults. Rocks west of the fault 

 are, in fact, still being transported north- 

 ward at about 2 inches per year and after 

 millions of years may reach the area 

 where Alaska is today. 



Mountain ranges in the San Francisco 

 Bay region trend mostly northwest and are 



separated by broad basins and narrow val- 

 leys. Many of the ridgecrests in the 

 Coast Range, the major mountain range in 

 the region, rise above 1 ,000 feet, and a 

 few above 4,000 feet. The highest peaks 

 include Mount Hamilton at 4,373 feet, 

 Mount Diablo at 3,849 feet, and Mount 

 Tamalpais at 2,606 feet. 



As recently as about 10,000 years ago, 

 at the end of the last ice age, great sheets 

 of ice (glaciers) covered much of Earth's 

 northern hemisphere. Because so much 

 water was stored in these glaciers, sea 

 level was about 300 feet lower than at 

 present, and the coastline of the San Fran- 

 cisco Bay region was situated as much as 

 35 miles seaward of its present position, 

 near coastal hills whose tops are now the 

 Farallon Islands. The melting of the ice 

 sheets caused worldwide sea level to rise, 

 forming the Gulf of the Farallones and San 

 Francisco Bay. 



If the vast volume of ocean water that 

 now covers the floor of the gulf could be 

 removed, people would see a varied land- 

 scape. That landscape is as rich in diver- 

 sity and rugged splendor as that on the 

 nearby shore (see chapter on Landscape of 

 the Sea Floor). 



8 Oceanography and Geology of the Gulf of the Farallones 



