Mission Planning Briefing: A Map of the Seafloor From Orbit
1A space agency is getting a new satellite ready that will measure the height of the ocean surface to within a few centimeters. At a mission planning briefing, oceanographer Dr. Marguerite Okonjo explained one of its jobs to the team and to two seventh graders in the agency's summer intern program, Ines Valderrama and Kwabena Thorsen. The ocean surface is not perfectly flat. Above an undersea mountain, the extra mass of rock pulls water toward it, so the sea surface stands up to a few meters higher. Above a deep trench, it sags. Computers turn these bumps and dips into a map of the seafloor.
2To test the map, the team will compare it with depths measured by ships using sonar along a line across the South Atlantic Ocean, from Brazil toward southern Africa (Table 1). Halfway across, the line crosses the Mid-Atlantic Ridge, an underwater mountain range. Along its crest, lava erupts from long cracks and hardens into new seafloor rock. Rock that has just formed is hot, and hot rock takes up a little more space than the same rock does after it cools. Drill cores collected along the line give the age of the seafloor rock.
3The map will also show fracture zones, long, straight valleys in the seafloor. Each one cuts across the ridge and breaks it into pieces that are offset from one another, like steps. Between two offset ridge pieces, the seafloor on one side slides past the seafloor on the other side, and small earthquakes are common there. Beyond the ridge pieces, each fracture zone continues as a quiet scar for more than 1,000 km toward each continent. In the Atlantic near the equator, the fracture zones run nearly east to west. Their western ends point at bends in the coast of Brazil, and their eastern ends point at bends of a matching shape in the coast of West Africa.
4Later, the satellite will map the Pacific Ocean (Table 2). Along much of the Pacific's edge are trenches, narrow valleys in the seafloor more than 8 km deep, with chains of volcanoes and many deep earthquakes on the land beside them. Drilling has found no ocean floor anywhere that is older than about 200 million years, but some rocks on the continents are about 4,000 million years old.