National Geographic : 1990 Feb
Interlocking habitats of Monterey Bay ' San Francisco AN Monterey Bay AREAENLARGED BELOW 'V Los Angeles DRAIN THE BAY and a landscape comparable to the Grand Can yon would emerge. Largest submarine chasm along the continental U. S., the Monterey Canyon plunges to 90 meters (300 feet) less than a kilometer off Moss Landing and meanders some 175 kilometers out to sea to depths of more than three kilometers. This map reflects recently declassified data plotted by the National Oceanic and Atmospheric Administration. Carved from granite, shale, and sandstone, Monterey Can yon was born 25 to 30 million years ago nearly 300 kilometers to the south. Pushed along the San Andreas Fault as part of the Pacific plate, it owes much of its magnitude to continual seismic activity-from tremors to major earthquakes-that triggers four main types of erosion (illustra tions below). The 7.1 Loma Prieta earthquake in October 1989 caused seafloor failure in the shallows off Moss Landing. In a process called liquefaction, water permeated the shaking sand and silt; the resulting mud slides flowed into the canyon. Unmanned remotely operated Submarine erosion Turbidity flow/current: Set in motion by earthquakes and storm waves, a cloudlike flow of sediment builds into a scouring current of sandand gravel- the major erosive force in the canyon. SRockfall: A common landslide inthe upper reaches of the canyon, where granite and shale prevail. SSlump: A slow-moving land slide that leaves angular contours without triggering turbidity currents. Freshwater sapping: Water flowing from an aquifer into the canyon loosens rock, causing landslides. Increased water use by the area's growing population has reversed the process; saltwater now invades the aquifer.