17.2: Topography of the Sea Floor

Topography of the Sea Floor

We examined the topography of the sea floor from the perspective of plate tectonics and in sedimentary rocks but here we are going to take another look at bathymetry Links to an external site. from an oceanographic perspective. The topography of the oceans is shown in Figure 17.2. Notice the variety of blue colors: light blue indicates shallow ocean water whereas darker blues indicate deeper ocean waters.

A topographic profile of the Pacific Ocean floor between Japan and the Pacific Northwest is shown in Figure 17.3. This diagram has a significant amount of vertical exaggeration (the vertical scale overemphasizes the height of the features relative to their width), but it captures the varied topography of the seafloor well. From the deepest trenches (up to 7 miles deep at the Mariana Trench Links to an external site.) to the highest mountains (Mauna Kea Links to an external site. on the Big Island of Hawaii is taller than Mount Everest, when measured from its base to its summit), the floor of the Pacific is not the flat basin we once envisioned. The vast sediment-covered abyssal plains Links to an external site. of the oceans, however, are much flatter than any similar-sized areas on the continents and cover more than 50% of the Earth’s surface (Figure 17.4). These extremely deep (over 10,000 feet) and flat areas of the sea-floor are interrupted by massive underwater mountain chains, seamounts Links to an external site., or MORs Links to an external site. (Figure 17.3). Other major topographic features in the oceans include the continental shelves, slopes, and rises that surround the continental crust.

Starting from land, a trip across an ocean basin along the seafloor would begin with crossing the continental shelf Links to an external site. (Figure 17.4). The continental shelf is an area of relatively shallow water, usually less than a few hundred feet deep, that surrounds a continent. It is narrow or nearly nonexistent in some places; in others, it can extend for hundreds of miles. On the East and Gulf Coasts, where there are passive margins Links to an external site. (no plate boundaries), the continental shelf averages around 85 miles wide. On the West Coast, where there is an active margin Links to an external site., the shelf is less than half as wide. Due to the abundance of light and nutrients from upwelling and runoff, the waters along the continental shelf are usually biologically productive.

At the edge of the shelf is the boundary known as the continental slope, which separates the shelf from the continental rise Links to an external site. (Figure 17.4). The continental rise is very slightly angled, between 0.5 degrees and 1.0 degrees. Deposition of sediments at the mouth of submarine canyons carved into the shelf and slope may form enormous fan-shaped accumulations called submarine fans Links to an external site. on both the continental slope and continental rise.