understand how the rock floor of the sea was formed when our 

 planet was young, we must penetrate the full thickness of sediments, 

 some two thousand feet, and find out about the soUd rocks beneath. 

 Buried rock masses often have physical properties different, if only 

 in a small way, from surrounding masses of rock. If the density of 

 the buried rock is greater than the density of surrounding rock, 

 then this difference results in minute but detectable variations in 

 gravity around the rock — the greater the density, the stronger the 

 pull of gravity. Magnetic properties of one rock mass also may 

 differ from those of a nearby mass, and this leads to local distortions 

 in the Earth's magnetic field. And so it is possible to locate and 

 identify the character of these buried rocks by making gravity and 

 magnetic field surveys. 



A more powerful tool for identifying the deep-lying rocks is that 

 of seismic surveying. The submarine geologist can detonate an 

 underwater explosive and so generate a series of sound waves that 

 fan out to the ocean bottom and to the rocks underlying the sedi- 

 ments. The echoes that come back to the ship, or move horizontally 

 through a layer of rock to a distant listening point, vary as the 

 rocks refracting them vary. Quite simply, we know that sound 

 waves travel faster through some rocks than through others. So by 

 comparing the acoustic properties of known rocks with those of the 

 sea bottom, geologists can try to identify the rocks making up the 

 sea floor. 



The ocean floor itself presents a landscape of canyons, valleys, 

 trenches, abyssal plains, and mountains. Most of the bottom lies 

 either at an average depth of about a hundred fathoms or else at 

 an average depth of between twenty-five hundred and three thou- 

 sand fathoms. In other words, there are two important levels — one 

 associated with the edge of the continents, the other with the ocean 

 basins. This is not to say that greater depths do not exist; they do, 

 but they are comparatively rare. 



If we could walk out from the coast of any typical continent, 

 say the west coast of Portugal, and stroll toward the center of the 

 ocean, we would find that as soon as we were clear of the immediate 

 shore line and its eroded cliffs we would be standing on a plateau 



Many continental slopes are incised with 

 steep-wailed canyons that are scoured 

 out by turbidity currents which carry 

 sediments out into the ocean basins. Shown 

 here is a detailed map of a typical canyon, 

 the Black Mud Canyon on the continental slope 

 southwest of Brittany, France. The map is 

 based on soundings made by Britain's 

 National Institute of Oceanography. 



194 



