COASTLINES AND OCEAN BASINS

• How Ocean Basins Differ from Continents 552
• Coastal Processes 552
• The Shaping of Shorelines 564
• Continental Margins 570
• Topography of the Deep Seafloor 572
• Ocean Sedimentation 578

FOR MOST OF HUMAN HISTORY, the 71 percent of Earth’s surface covered by oceans was a mystery. The large populations who lived at the edge of the sea knew well the impact of the waves, the rise and fall of the tides, and the devastating effects of powerful storms. But they could only guess at the forces that caused these processes. We now know that they result from interactions within the climate system and the solar system. Tides are caused by gravitational interactions between Earth and the Sun and Moon, and coastal surf and storms result from interactions between the atmosphere and the hydrosphere.

And what about the deep sea, which is invisible to humans without the aid of remote observation tools? The nature of the seafloor beyond the shallowest coastal waters remained a mystery until the middle of the nineteenth century. In 1872, the Challenger, a small wooden British warship converted and fitted out for scientific study, became the first research vessel to explore the oceans scientifically. The Challenger expedition discovered great areas of submerged hills and flat plains, extraordinarily deep trenches, and submarine volcanoes.

Today, Earth scientists still search for answers to the questions first raised by these early discoveries. What forces raised the submarine mountain ranges and depressed the trenches? Why are some areas of the seafloor flat and others hilly? Although oceanographers made many important discoveries in the first half of the twentieth century, the answers to most of these questions had to await the plate tectonic revolution of the late 1960s. As we saw in Chapter 2, it was geological observations of the seafloor, not of the continents, that led to the theory of plate tectonics.

In this chapter, we examine the processes that affect shorelines and coastal areas and consider the effects of waves, tides, and damaging storms. Then we move farther offshore to examine the submerged margins of the continents that bound ocean basins, and finish with a discussion of the deep seafloor.