|There were attempts to determine the nature of
the ocean floor.
Magellan lowered a
sounding line 750 m in
1521 without reaching bottom. In 1840, the Erebus, under the command of James Clark Ross, attempted to
determine directly the
depth of the South Atlantic. After 4 hours of
hard work, the crew
established the depth
of the ocean at 4375 m
(a figure off by 600 m). During its famous oceanographic cruise, the Challenger
successfully completed a
sounding of 10,000 m.
Other surveys added to the growing knowledge of the ocean floor and suggested that it was not the flat plain originally believed.
However, the 312
million km2 of ocean meant that its floor could not be mapped in detail by the process of sounding.|
Technological advances during and after World War II significantly increased our ability to image and map the ocean floor. One of the earliest instruments, and one still useful today, was the seismic profiler. This instrument is used to determine sea floor topography as well as the shallow structure of the ocean floor. The profiler, which is towed behind a ship, emits low-frequency sound waves that are reflected at the interface between two materials of different composition and density, e.g., seawater and the ocean floor. Thus a profiler can map the contact between water and the sea floor. Because oceanic sediment and the underlying igneous rocks have very different physical characteristics, seismic profiling also can detect the upper surface of the oceanic crust. As a ship moves, it can acquire a continuous topographic profile of the ocean floor as well as a profile of the top of the oceanic crust. Using thousands of such profiles acquired during many ship cruises, topographic maps of the ocean floor were made soon after World War II.
Detailed oceanographic surveys of the ocean floor completed in the mid-1960s revealed a bathymetry of great complexity. The ocean floors were far from the simple monotonous plains geologists had predicted. They consist of six major topographic features:
occur in the deep parts
of the ocean basins and
consist of two components.Abyssal hills are regions of low topographic relief,
whereas abyssal plains are almost flat. In the Atlantic, the depth of the ocean floor varies only a few meters for every
100 km. Subsequent work
has shown that abyssal
plains are actually regions
where oceanic sediment covers
abyssal hills, producing flat, nearly featureless plains. Abyssal plains are much more common and well developed in the Atlantic than in the Pacific Ocean.
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