Atolls

OCEAN BATHYMETRY



CONTINENTAL MARGINS

Atlantic type or passive - Margin where the oceanic crust is the trailing edge of continental crust with both moving together in the same direction.

Pacific type or active - Margin where the oceanic crust is subducted under continental crust. At the margin there is all the activities associated with subduction, hence the term "active" margin.


Continental Shelf - Extension of continental crust into the ocean with a gentle slope, 9 feet per mile (1.7 meters per kilometer).

The continental shelf varies in width, wide at passive margins, narrow at active margins.

The Continental Shelf can reach about 220 miles (350 kilometers) in the Alantic and as much as 800 miles (1,280 kilometers) near Northern Siberia in the Arctic Sea.

The shelf is covered with sediments that can be 9 miles (15 kilometers) thick. The sediments come from erosion of the continental crust on land. The weight of these sediments causes the shelf to be isostatically depressed.

The continental shelf ends at a depth of about 460 feet (140 meters) in most places although in Antarctitca the weight of the glaciers on the continent cause the shelf to be lowered as well. The shelf ends at a depth of 1,000 to 1,300 feet (300 to 400 meters) around Antarctica.

Continental Slope - Sharp change in depth. Greatest change in depth within a narrow range. The shelf break is the point at which the continental shelf ends and the slope begins.

Except around Antarctica as noted above and Greenland, the continental slope begins at a depth of 460 feet or 140 meters. It extends for a distance of about 12 miles or 20 kilometers.

The slope is about 4° or 370 feet per mile (70 meters per kilometer) usually ending at a depth of 12,000 feet or 3,700 meters.

Sub-marine canyons are found in the continental slope as well as the continental shelf. Sub-marine canyons are located on the continental slope: Hudson, Congo, Mississippi Canyons are associated with rivers.

The Hudson Canyon is the largest canyon ont the east coast of the United States. It forms a cut about 500 feet deep for 100 miles in the continental shelf from its origin, the Hudson River. At that point it is 20,000 feet deep. It plunges one mile down the face of the continental slope and meanders for another 200 miles over the continental rise and the abyssal plain of the Atlantic Ocean. Because of its depth there are different and more diverse species of marine organisms found there. For that reason it is a favorite spot for deep-sea fishing boats.

At the base to the continental slope is the continental rise. Materials from landslides and turbiditiy currents tend to accumulate there. Scientists believe that methane gas eruptions may cause these landslides and turbidity currents which can disrupt the transoceanic fiber optic cables.

Tokyo, Monterey, Wilmington, Scripps Canyons are associated with dry river beds when the sea was lower or faults altered the sea floor.

Continental Rise - Gentler slope (about 1/8 that of the continental slope) at the base of the continental slope. The end of continental crust. Usually begins at the end of the continental slope at a depth of 12,000 feet or 3,700 meters and ends at the beginning of the abyssal plain. The continental rise can vary in width 63 to 630 miles or 100 to 1,000 kilometers. Sediments accumulate at its base, especially near rivers with heavy sediment loads like the Ganges-Brahmaputra River which flows into the Bay of Bengal.


Continental Shelf, Slope and Rise.


HOT SPOTS

Hot Spots - Areas of uneven and greater heating of magma in the asthenosphere that penetrate through the crust causing periodic volcanic eruptions on the sea floor. These volcanoes form guyots, seamounts, volcanic islands and atolls.

The USGS web site states the following: "In 1963, J. Tuzo Wilson, the Canadian geophysicist who discovered transform faults, came up with an ingenious idea that became known as the "hotspot" theory. Wilson noted that in certain locations around the world, such as Hawaii, volcanism has been active for very long periods of time. This could only happen, he reasoned, if relatively small, long-lasting, and exceptionally hot regions -- called hotspots -- existed below the plates that would provide localized sources of high heat energy (thermal plumes) to sustain volcanism. Specifically, Wilson hypothesized that the distinctive linear shape of the Hawaiian Island-Emperor Seamounts chain resulted from the Pacific Plate moving over a deep, stationary hotspot in the mantle, located beneath the present-day position of the Island of Hawaii. Heat from this hotspot produced a persistent source of magma by partly melting the overriding Pacific Plate. The magma, which is lighter than the surrounding solid rock, then rises through the mantle and crust to erupt onto the seafloor, forming an active seamount. Over time, countless eruptions cause the seamount to grow until it finally emerges above sea level to form an island volcano. Wilson suggested that continuing plate movement eventually carries the island beyond the hotspot, cutting it off from the magma source, and volcanism ceases. As one island volcano becomes extinct, another develops over the hotspot, and the cycle is repeated. This process of volcano growth and death, over many millions of years, has left a long trail of volcanic islands and seamounts across the Pacific Ocean floor.

According to Wilson's hotspot theory, the volcanoes of the Hawaiian chain should get progressively older and become more eroded the farther they travel beyond the hotspot. The oldest volcanic rocks on Kauai, the northwesternmost inhabited Hawaiian island, are about 5.5 million years old and are deeply eroded. By comparison, on the "Big Island" of Hawaii -- southeasternmost in the chain and presumably still positioned over the hotspot -- the oldest exposed rocks are less than 0.7 million years old and new volcanic rock is continually being formed."

Some scientists do not believe that hot spots at non-plate boundaries are the cause of volcanic islands. These scientists cite the lack of discernible seismic activity. Other scientists believe that there is seismic acitivity but current instrumentation does not exist to measure it.


Click here to read about the hot spot that has formed the Hawaiian Islands.

Click here to read about the geologic history and formation of the Hawaiian Islands.

Guyots and Seamounts.






ATOLLS



ATOLL FORMATION.


Belize Atoll


Corals usually grow in warm waters but ahermatypic corals grow in cold water at a slower rate and are flatter than warm water hermatypic corals. Recently a study of an ancient seamount, the Davidson Seamount, that rises 8,000 feet from the seafloor off the coast of central California, showed a coral garden 4,000 feet beneath the surface at the summit of this seamount. This coral garend fringes the seamount but does not extend to the surface as happens with an atoll.
Click here to read about and see the corals on the Davidson Seamount.

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