Using Core Samples and Sonar Maps
Another frequently used exploration method is coring, which enables researchers to obtain cross-sections of sea-floor sediment. Coring is done by rapidly lowering a narrow, heavy, open-ended tube to the bottom. The tube plunges into the sediment and encloses a section of it. When the surface crew hauls the tube back up, a flap at the bottom end closes and prevents the sediment from falling out. Variations in the different layers of sediment reveal information about the history of the sea and of Earth's climate.
Core samples from bedrock are obtained by drilling into the rock, using equipment adapted from oil-well derricks. Drilling is done from special ships able to hold a constant position and equipped with sophisticated computers to guide the drill. The rock cores yield valuable insights into the geological history of the deep-sea floor and of the Earth itself. For example, drill cores from the Mid-Atlantic Ridge contained proof that the Earth's magnetic field periodically reverses. The evidence was in magnetic patterns frozen in bands of lava that had flowed from each side of the Atlantic rift and solidified.
Like sonar maps, most maps of the ocean floor based on coring and drilling are compiled from data gathered from widely spaced swaths of the sea floor. Thus, the information they provide is patchy. The most comprehensive pictures of the ocean bottom are made by satellite gravimetry. Satellites in orbit around the Earth send out radar signals to measure the varying height of the ocean surface. These small but detectable variations are caused by differences in the gravity of underwater features. Massive objects beneath the sea exert more gravitational force on the surrounding water than small features do. Therefore, an underwater mountain pulls more water toward itself, forming a hump on the surface that may be up to 60 centimeters (2 feet) higher than the surrounding sea. Over a deep trench, on the other hand, the water forms a shallow trough. The shape of the underlying sea floor can be calculated from these slight variations.