Continental shelves/West American

Map shows the continental shelf off the California coast. Credit: NOAA {{fairuse}}

Area map shows the three National Marine Sanctuaries of the central California Coast. Credit: NOAA {{free media}}

"A few miles offshore, there are no beaches for the waves to hit, but their bottoms bang into the continental shelf, where the shallow coastal waters drop off sharply into deeper ocean regions. Along the coast of Northern California, the continental shelf is only a few miles wide. In [the image on the right] it is the light greenish-blue area immediately off the coast. The waves of big storm reach down and impact the continental slope west of this shelf."

Pioneer Seamounts

Pioneer Seamount is an undersea mountain, or seamount, in the Pacific Ocean off the coast of central California.

Pioneer Seamount is located at 37° 21.1' North Latitude, 123° 26.1' West Longitude, at the base of the continental slope of North America about 95 kilometers (59 miles) off the coast just southwest of San Francisco, California.

The seamount is a volcano between 10.9 and 11.1 million years old. It is about 12.8 kilometers (8.0 miles) long as well as about 12.8 kilometers (8.0 miles) wide, and has a volume of about 135 cubic kilometres (32 cubic miles). It rises about 1,930 meters (6,330 feet) above the surrounding ocean floor, and its peak is a minimum of 820 meters (2,690 feet) below the ocean's surface. Samples from the seamount consist of highly vesicular alkalic basalt, hawaiite, and mugearite.

The seamount and its volcano once extended above the sea surface, but eroded and sank as the seamount and the seabed at its base were carried further away from the spreading center from which it presumably originated.

Los Angeles submarine canyons

Perspective view looks north over the San Gabriel (A) and Newport (B) submarine canyons. Credit: United States Geological Survey.{{free media}}

"Perspective view looking north over the San Gabriel (A) and Newport (B) submarine canyons. The distance across the bottom of the image is about 17 km with a vertical exaggeration of 6x. Both canyons formed when the San Gabriel River and the Santa Ana River flowed out across the Los Angeles Basin and offshore shelf when it was exposed during lower eustatic sea level. Newport Canyon begins less than 360 m from shore at the north end of Newport Harbor and is composed of individual channels that braid down the slope over a width of about 9 km. San Gabriel Canyon begins as a series of channels that join together midway down the slope and then split into two channels at the base of the slope. The width of San Gabriel Canyon at "C" is 815 m and incises about 25 m into the slope. Lasuen Knoll can be seen in the forground."

Davidson Seamounts

Bathymetric map shows the Davidson Seamount. Credit: NOAA.{{free media}}

1933 map shows the Davidson Seamount. Credit: United States Coast and Geodetic Survey.{{free media}}

The Davidson Seamount is at

Def. a "mountain that rises from the floor of the ocean and does not breach the water's surface" is called a seamount.

Davidson Seamount is an underwater volcano located off the coast of Central California, 80 mi (129 km) southwest of Monterey and 75 mi (121 km) west of San Simeon. At 26 mi (42 km) long and 8 mi (13 km) wide, it is one of the largest known seamounts in the world. From base to crest, the seamount is 7,480 ft (2,280 m) tall, yet its summit is still 4,101 ft (1,250 m) below the sea surface. The seamount is biologically diverse, with 237 species and 27 types of deep-sea coral having been identified.

Discovered during the mapping of California's coast in 1933, Davidson Seamount is named after geographer George Davidson of the U.S. National Geodetic Survey. Studied only sparsely for decades, NOAA expeditions to the seamount in 2002 and 2006 cast light upon its unique deep-sea coral ecosystem. Davidson Seamount is populated by a dense population of large, ancient corals, some of which are over 100 years of age. The data gathered during the studies fueled the making of Davidson Seamount into a part of the Monterey Bay National Marine Sanctuary in 2009.

A seamount such as Davidson is an underwater volcano; this one rises 7,480 ft (2,280 m) above the surrounding ocean floor. Although there are over 30,000 seamounts in the Pacific Ocean alone, only about 0.1% of them have been explored. The aqueous environment of the seamount means that it behaves differently from volcanoes on land. Its surface is composed mostly of blocky lava flows, although some pillow lava, which is the typical lava type of a seamount, prevails at the deeper flank. The summit is composed of layered deposits of volcanic ash and pyroclastic material. These rocks indicate mildly explosive eruptions of gas-rich lava near the summit of the volcano. The base of Davidson is probably buried in a deep layer of muds.

At 26 mi (42 km) long and 8 mi (13 km) wide, Davidson Seamount is impressively large. If it were on land, it would dominate the landscape in a way similar to how Mount Shasta dominates the horizon of northern California. Put in perspective, the size of the seamount is enough to fill Monterey Bay from the Santa Cruz boardwalk to Monterey's Fishermen's Wharf.

Davidson Seamount is part of a group of seamounts off the continental margin, including Guide, Pioneer, Gumdrop, and Rodriguez seamounts, all located roughly between 37.5° and 34.0° degrees of latitude. This group is morphologically unique, and very similar to one another. All the seamounts in the group are complex northeast-southwest trending structures, consisting of parallel ridges separated by sediment-filled troughs. The ridges constructed run parallel to an ancient spreading center which has since been replaced in its role by the San Andreas Fault system. They are unique in this origin, as they are formed from the remnants of an old ocean-ridge spreading center. A series of "knobs" are aligned with the ridges; however the distinctive summit crater, evident in many oceanic volcanoes, is absent. This lack of a collapse crater suggests that magma was never stored in a chamber within the structure, as with most other volcanoes.

Analysis of argon–argon dating studies indicate that Davidson formed between 9 and 15 million years ago, 5 to 12 million years after the formation of the overlaying oceanic crust.

Davidson Seamount was initially discovered and mapped in 1933. Davidson Seamount was the first underwater volcano to be classified as "seamount" by the United States Board of Geographic Names, in 1938, and was named in honor of the United States Coast and Geodetic Survey scientist George Davidson, one of the key figures in the survey of America's west coast.

Because of its great depth, for a long time Davidson Seamount had been preceded only by a sparse few investigations. Davidson is interesting to volcanologists because of its unique geology, and to biologists for its unique ecology. In 2002 the NOAA sponsored the first modern in-depth study of the seamount. The team included scientists, educators, and resource managers, with the goal of documenting species, taking geologic samples, and describing the ocean environment. The expedition documented many rare, previously undiscovered species that exist nowhere else, not even on nearby seamounts, including ancient coral gardens that are vulnerable to human activity.

Recent expeditions to Davidson have focused on its ecology, and specifically on the variety of deep-sea corals, some over 100 years old, that live on its banks. These large colonies are extremely fragile to human interaction. Davidson's proximity to scientific research institutions has helped its exploration, as multiple dives, mappings, and studies have made it one of the better-studied seamounts in the world.

In 2006, another exploration, a collaboration of the Monterey Bay National Marine Sanctuary, the Monterey Bay Aquarium Research Institute (MBARI), and Moss Landing Marine Laboratories was undertaken, mainly to test a model that had been developed to predict the availability of coral and to advance the understanding of the seamount's deep-sea coral. The NOAA outlined a set of 4 goals for the expedition:

• Understand why deep-sea corals live where they do on the seamount
• Determine the age and growth patterns of the bamboo coral
• Improve the species list and taxonomy of corals from the seamount
• Share the exciting experience with the public through television and the Internet

Scientific data on the water currents and food availability of the seamount was collected, as was information on the age and growth patterns of the corals themselves. The British Broadcasting Corporation (BBC) filmed the cruise for their series, Planet Earth. A total of 70 hours of observations and 102 deep-sea animal and rock specimens were collected during the cruise. The expedition, which lasted from January 26 through February 4, made use if the MBARI's research vessel Western Flyer and the Remotely operated underwater vehicle (ROV) Tiburon.

See also

• Geology
• Mediterranean continental shelves
• North east American continental shelves
• North Sea
• North west African continental shelves
• Oceanography
• Paleanthropology
• South east African continental shelves
• South east Asian continental shelves
• West European continental shelves