(20-50 km) than under oceans (5-10 km). Earth's crust consists of 

 several large rigid plates or blocks that float on the more plastic 

 mantle. These plates move relative to one another and most, if not 

 all, earthquake and volcanic activity occurs along plate boundaries. 

 Mid-ocean ridges represent boundaries of plates that are spreading 

 apart. Peripheral mountain ranges, volcanic arcs, and deep-sea 

 trenches represent the boundaries of adjacent plates that are con- 

 verging (fig. 2-2). When the strength of the Earth's crust is 

 occasionally exceeded locally, sudden ruptures occur that displace 

 rock masses along fractures called faults. Most faulting occurs at 

 the borders of crustal plates. 



Many Earth scientists believe the lateral spreading apart of 

 crustal plates is caused by motion of convective cells in the mantle 

 (fig. 2-3). Molten material in these cells rotates about cell axes, 

 forcing hot basaltic mantle material up through and along the underside 

 of the crust, slowly dragging the crustal plate until a zone of down- 

 ward cell motion is reached. The Mid-Atlantic Ridge is estimated to 

 be spreading on the average of 1.25 cm/yr, whereas the East Pacific 

 Rise is spreading at a rate four times as great. Since new crust is 

 forming, old crust must be destroyed somewhere because the earth is 

 not growing larger at the required rate. A process of subduction occurs 

 at trenches, which are depressions where crustal plates are forced, 

 or sink, into the underlying mantle. Slippage of material into trenches 

 gives rise to earthquakes and volcanic activity, which leads to forma- 

 tion of seamounts and island arcs. 



b. Bathymetry 



Most detailed measurements of ocean depths and submarine 

 topography have been made in the last 50 years, since the development 

 of echo-sounding equipment. This acoustic equipment measures travel 

 time of a sound pulse to the sea bottom and return, producing a 

 continuous trace of the profile of the sea floor along a ship's track. 

 Figure 2-4 shows a bottom profile of a vertical section of the North 

 Atlantic Ocean along a track between Cape Henry, Virginia, and Rio de 

 Oro, Africa. The continental margin of each continent consists of a 

 shelf, a slope, and a rise. Ocean basins are separated by the Mid- 

 Atlantic Ridge. The old volcanic island of Bermuda is also seen. 



After collecting and assembling appropriate bathymetric data 

 from many such tracks, detailed bottom contour charts are prepared 

 which may be used for navigation as well as for other scientific pur- 

 poses. Figure 2-5 is a bottom topography chart of the North Atlantic 

 Ocean. This chart shows the lateral extent of the major physiographic 

 provinces of the ocean floor. 



The hypsographic curve (fig. 2-6) is a plot of the cumulative 

 areal distribution of heights above and depths below mean sea level. 

 This curve is used to show the area of different sea floor zones of 

 the Earth's oceans. A shallow zone at depths from to 200 m occupies 



