Oceanography and Geology of the Gulf of the Farallones 



Landscape of the Sea Floor 



Herman A. Karl and William C. Schwab 



Various forces sculpt the surface of the 

 Earth into shapes large and small that 

 range from mountains tens of thousands 

 of feet high to ripples less than an inch 

 high in the sand. We can easily see these 

 morphologic features on dry land, and 

 most are accessible to be explored and 

 appreciated by us as we walk and drive 

 across or fly over them. In contrast, 

 people can travel at sea without ever 

 being aware of the existence of large 

 mountains and deep valleys hidden by 

 tens of feet to tens of thousands of feet 

 of water. At one time, it was impossible 

 to see the landscape beneath the sea 

 except by direct observation, and so the 

 bottom of the deep oceans remained as 

 mysterious as the other side of the Moon. 

 Methods of using sound to map the fea- 

 tures of the sea floor were developed in 

 the decades after World War I, and we 

 now know that mountains larger than any 

 on land and canyons deeper and wider 

 than the Grand Canyon exist beneath our 

 oceans. 



Images of large areas of the land sur- 

 face are taken by cameras in aircraft and 

 acquired by sensors in satellites. Optical 

 instruments (cameras) record reflected 

 light to produce images (photographs) 

 of mountains, plains, rivers, and valleys. 



However, light does not penetrate far in 

 water, and so photographs cannot be pro- 

 duced that show entire mountains and 

 valleys beneath the sea. Just as under- 

 water cameras can take photographs of 

 only very small areas and objects not 

 far distant, divers and submersibles can 

 observe only very small areas of the 

 ocean bottom. Such methods of observa- 

 tion are analogous to walking around at 

 night with a flashlight and trying to see a 

 mountain or a forest. It is possible in 

 this way to see rocks, pebbles, leaves, 

 and trees along your path, but not the 

 entire mountain and forest. To observe 

 and make images of the mountains and 

 valleys under the sea, scientists have 

 developed instruments that use sound 

 (acoustic energy) as a way to "insonify" 

 (flood with sound waves) rather than 

 illuminate (flood with light) those fea- 

 tures. Computers are used to process 

 the acoustic data so that the resulting 

 sound images (sonographs) resemble 

 aerial photographs (light images). 



Data collected from the Continental 

 Shelf in the Gulf of the Farallones by the 

 U.S. Geological Survey (USGS) reveal 

 various features on the seabed, including 

 outcropping rock and several types of 

 ripples, dunes, lineations, and depres- 



sions (these features are collectively 

 called bedforms). A regional map com- 

 piled from these data established that at 

 least four major discrete fields of bed- 

 forms occur on the Continental Shelf 

 between Point Reyes and Half Moon 

 Bay. These fields are separated by 

 monotonous stretches of flat, featureless 

 sea floor. Of particular interest is a 

 series of depressions floored by ripples 

 with wavelengths of about 3 feet. These 

 depressions, which are common east of 

 the Farallon Islands between Point Reyes 

 and the Golden Gate, form the largest of 

 the four fields of bedforms. The shelf in 

 the study area is morphologically com- 

 plex. This complexity reflects an intri- 

 cate geologic history and a wide variety 

 of geologic and oceanographic processes 

 that operate on the shelf to transport, 

 erode, and deposit sediment. 



Data collected from the Continental 

 Slope by the USGS show that the rugged 

 northern part of the Gulf of the Faral- 

 lones is scarred by numerous small can- 

 yons. Sediment cover is thin or absent. 

 This northern part contrasts markedly 

 with the southern part of the gulf, which 

 is much less rugged and draped by a 

 thin blanket of sediment. Sediment in 

 the southern part appears stable because 



no large underwater landslides were dis- 

 cerned on the sonographs. Conspicuous 

 geomorphic features in the southern part 

 include Pioneer Canyon and Pioneer 

 Seamount. Sediment is accumulating in 

 Pioneer Canyon and on Pioneer Sea- 

 mount, suggesting that this area is depo- 

 sitional, in contrast to the active trans- 

 port environment found on the shelf. 



USGS sonographs made of the Gulf 

 of the Farallones have several practical 

 applications. For example, the evidence 

 of strong currents, as indicated by large 

 ripples in coarse sand, suggests that 

 dredge material and pollutants disposed 

 of at sites on the Continental Shelf could 

 be redistributed over large areas. Also, 

 commercial fisherman can use these 

 images to locate the substrates preferred 

 by bottom fishes and crabs. 



16 Oceanography and Geology of the Gulf of the Farallones 



