Review the major physiographic features of the ocean basins.
Investigate the creation of oceanic crust at midocean ridges.
Illustrate the origin of sea floor isochrons and magnetic lineations.
Demonstrate the relationship between sea floor spreading and ocean floor topography.
Show how plate velocities are determined.
Prior to World War II, little was known about the ocean basins. In the mid-eighteenth century, some scientists argued that the oceans were between 25 and 30 km deep. Without modern technology, scientists had few means of sampling or mapping the ocean floor. Lacking data, geologists assumed that the ocean floors were simple, flat plains that had collected sediment over the vastness of geologic time.

Fig. 1: midocean ridges of the world.
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:
  • Midocean ridges
  • Fracture zones and transform faults
  • Seamounts
  • Deep-sea trenches
  • Abyssal floors
  • Continental margins

Abyssal floors 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.
Submarine mountain ranges were shown to dwarf anything seen on land. By the mid-1960s, extensive surveys had shown that these were contiguous and the single largest topographic feature on the planet. The midocean ridge (MOR) system extends for 65,000 km, cuts across all major ocean basins, and covers nearly 25 percent of the Earth’s total surface area. At its widest, the MOR is nearly 1500 km across and reaches heights of 3 km above the ocean floor. New knowledge about the ocean floor coupled with advances in earthquake detection and sampling of the ocean floor required geologists to completely rethink their models of the ocean basins and how they form.



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