The topography of the ocean floor is as varied as topography on land. There are mountains and canyons, gently sloping hills and sharp cliffs. But where? Satellite maps can’t show variations in the ocean floor that lie under hundreds of meters of water, and only five to 15 percent of the ocean floor has been mapped by sonar. Gravity, however, can.
Over the past 25 years, David Sandwell and Walter Smith, with the Scripps Institution of Oceanography and NOAA respectively, have been gathering data from military agencies and satellite services on Earth’s gravity field.
Because mountains have a lot of mass, they exert more of a gravitational pull than low-lying valleys. Underwater mountains’ gravity attracts water, creating differences in the height of the surface of the ocean. A sailor wouldn’t be able to tell that the ship was sailing uphill, but the difference is measurable. Using data on the shape and variations in Earth’s gravitational pull, scientists can approximate the topography of the sea floor.
Smith and Sandwell’s gravitational map shows where gravity on the seafloor is stronger or weaker than the global average, giving us an idea of where the underwater trenches, ridges, seamounts, and tectonic plates are located across the world.
Each pixel represents about three miles (five kilometers). The bluer sections are low-lying regions, and the red sections are seamounts. The red slash running between the Americas, Europe, and Africa is the Mid-Atlantic Ridge—the mountain range created where two tectonic plates are moving apart, causing magma from the Earth’s mantle to bubble up and form new additions to the planet’s crust. Meanwhile, some of the deepest underwater trenches in the world are located in the Pacific, like the 6.8-mile-deep Mariana Trench. While the Mid-Atlantic Ridge is formed by tectonic plates moving apart, trenches occur where two plates are moving together, and one is being pushed beneath the other.