In the north, new ocean crust forms along the Juan de Fuca spreading center, as oceanic plates move apart. This spreading center is part of the global ridge system that extends 50,000 km through all the ocean basins. The Blanco Fracture Zone accommodates the offset between the Juan de Fuca Ridge and the Gorda Ridge. Cascadia Channel, a major sediment conduit, can be clearly traced from offshore central Oregon south to where it snakes through the Blanco Fracture Zone, and then heads off the mosaic to the west. The channel transports sediment from the Columbia and Fraser River systems westward more than 500 km across the Juan de Fuca and Pacific Plates. The Cascadia Subduction Zone is active along the Washington, Oregon, and northern California coasts: here the North American continental plate overrides both the Gorda and the Juan de Fuca oceanic plates, causing large earthquakes and creating the string of Cascade volcanoes in Oregon and Washington. The Mendocino Fracture Zone (MFZ) is a right-lateral transform fault that forms the boundary between the young crust of the Gorda Plate to the north and the older, more sediment-covered Pacific Plate to the south and accommodates the relative motion between these two tectonic plates.
South of latitude 40º N, the transform plate boundary is located onshore near the shoreline. The geologic features most visible in the mosaic are deep-sea fans and seamounts. Perhaps the best-studied submarine fan in the area is the Monterey Fan. The continental margin off central California has a narrow shelf and steep slope. The margin extends from the shoreline to depths of 3,000 meters. Monterey Bay is a 40-km (N-S) by 10-km (E-W) reentrant of the coastline that forms a shallow, relatively broad portion of the shelf. The continental slope seaward of Monterey Bay is incised by the large Monterey Canyon system. This canyon system feeds sediment to the Monterey Fan, which covers about 100,000 sq km of sea floor. The Monterey Fan, bounded on the north by the Farallon Fan and on the south by the Murray Fracture Zone, thins to the west, ending more than 400 km from the shoreline.
The large influx of sediment along the central California margin, which created the numerous deep-sea fans, diminishes to the south. A large number of seamounts are visible along the southern California margin draped with only a thin covering of sediment.
The first sidescan-sonar system was developed in 1960 by the Institute of Oceanographic Sciences (IOS) in the United Kingdom for use in shallow water. When the submarine Thresher was lost in 1963, the need for deep-water search capability became clear. In 1969 IOS introduced GLORIA (Geologic LOng Range Inclined ASDIC), the first sidescan-sonar system capable of deep-water geologic surveying. In 1977 they produced the next generation, GLORIA II, which was used to collect the imagery presented on this CD-ROM.
The GLORIA II sidescan-sonar system, used by the U.S. Geological Survey to map the U.S. Exclusive Economic Zone (EEZ), was specifically designed to map the large-scale morphology and texture of sea-floor features in the deep ocean. To map efficiently a large area such as the EEZ, some detail had to be sacrificed. GLORIA cannot resolve human debris such as pieces of shipwrecks, but it can map an area the size of New Jersey in a single day!
Acoustic images of the sea floor are formed by aiming sound waves at the sea floor and recording the returning sound waves. These images are graphic presentations of the interaction of acoustic energy with the sea floor, not optical views of the sea floor. The GLORIA II sonar vehicle is 8 m long, weighs 2.25 tons in air, and is neutrally buoyant in sea water. This torpedo-shaped "fish" is towed at about 50 m depth behind a ship by a heavy cable containing electrical wires that transmit the signals between the ship and the fish. The electronic instruments in the fish send out a pulse of sound that travels through the water and is reflected by the sea-floor sediments. Computers aboard the ship then record the amount of energy (backscatter) that returns from the sea floor and use that data to create the image.
"Sidescan" refers to the shape of the sonar beam, which fans out to the sides and scans a wide "swath" of sea floor. The energy beam is confined to the plane perpendicular to the ship's track, forming a narrow acoustic beam in the horizontal plane and a broad beam in the vertical plane. With every sound pulse the fish emits, it images one strip of sea floor as much as 22 km wide on each side of the fish. As the ship tows the fish along its course, it insonifies successive areas of the sea floor, building a long swath of imagery pulse by pulse. The fish is towed along parallel tracklines back and forth over an area, like mowing a lawn, with a small amount of overlap between adjacent swaths. With proper navigational control, these parallel swaths of sonar imagery can be mosaicked together to produce large areas of continuous sea-floor imagery, similar to the method used in aerial photography.
The mosaic is a gray-scale image of the acoustic backscatter variations of the sea floor. The darkness or lightness of a feature on the mosaic is a function of how much sound is backscattered from the sea floor. White represents the most acoustic backscatter, and black represents the least. The amount of backscattered acoustic energy is controlled by the sea-floor topography, roughness of the sea-floor surface, and the sea-floor composition. The first-time viewer is often distracted by the dark, narrow stripes of the ship's nadir, which are non-data areas. They indicate the path of the sidescan fish, and thus the source of sonar beams; they can be ignored.
Sea-floor topography dramatically affects the amount of backscatter received by the sidescan fish. When viewed from the ship's trackline, a positive-relief feature, such as a volcano, usually appears as a bright zone (the slope facing the sonar beam) with a dark zone (the acoustic shadow) behind it. Conversely, a negative-relief feature, such as a canyon, usually appears as a dark zone (the near wall is shadowed) followed by a bright zone (the far wall is facing the sonar beam).
Hard rock ocean-floor substrates such as seamounts, volcanoes, and oceanic ridges reflect a lot of acoustic energy (high backscatter), and are recorded as bright areas on the sidescan image. Bright patterns also can be caused by certain sedimentation patterns and by steep sediment-covered slopes facing the sonar beam. Sea-floor materials such as soft mud tend to absorb much of the acoustic signal, returning very little energy (low backscatter), and appear on the image as dark areas. Dark patterns also have a variety of causes, but are commonly caused by acoustic shadows and certain variations in sediment type.
The swath files were further processed to apply four radiometric corrections. These corrections included: (1) a shading correction for the attenuation of the sonar energy in water as a function of range; (2) a power correction for very near nadir because of slow buildup; (3) speckle-noise correction; and (4) removal of striping noise.