Sea-Floor SpreadingDuring World War II, geologists employed by the military carried out studies of the sea floor, a part of the Earth that had received little scientific study. The purpose of these studies was to understand the topography of the sea floor to find hiding places for both Allied and enemy submarines. The topographic studies involved measuring the depth to the sea floor. These studies revealed the presence of two important topographic features of the ocean floor:\tOceanic Ridges – long sinuous ridges that occupy the middle of the Atlantic Ocean and the eastern part of the Pacific Ocean.\tOceanic Trenches – deep trenches along the margins of continents, particularly surrounding the Pacific Ocean.Plate Tectonics By combining the sea floor spreading theory with continental drift and information on global seismicity, the new theory of Plate Tectonics became a coherent theory to explain crustal movements.Plates are composed of lithosphere, about 100 km thick, that “float” on the ductile asthenosphere.While the continents do indeed appear to drift, they do so only because they are part of larger plates that float and move horizontally on the upper mantle asthenosphere. The plates behave as rigid bodies with some ability to flex, but deformation occurs mainly along the boundaries