Mid-Ocean Ridges and Rifts (World of Earth Science)
The ocean floor is mountainous and uneven, much like Earth's surface. As oceanographers began mapping the ocean bottom, they discovered that the sea floor is full of vast rising slopes, or ridges, and dramatic open valleys, or rifts. During World War II, oceanographer William Maurice Ewing began mapping the complex ocean bottom with sophisticated instruments such as sonar depth finders and underwater cameras that helped trace the contours of the ocean bottom. Ewing set out to measure and record a massive chain of undersea mountains called the Midatlantic Ridge. When Ewing and his crew began mapping the massive ridge, they encountered a problem: the sonar beams were bouncing back. This problem led to another great discovery. They realized that there were frequent oceanic earthquakes occurring along the ridge. This was an exciting discovery because it opened up the possibility that oceanic earthquakes might be connected to ridges and rifts. Using data from other expeditions, Bruce Charles Heezen (b. 1924) more accurately measured the Midatlantic Ridge as he began mapping the ocean floor. The Ridge measured up to 1.9 miles (3 km) high and 45,954 miles (73,940 km) long. Interestingly, however, he detected a gully in the ridge that led to the Heezen-Ewing theory in 1958, which formally recognized the Midatlantic Ridge as containing a rift. Their discovery sparked interest in other scientists and explorers who questioned the existence of other rifts in ocean ridges.
In the late 1950s, American and Soviet oceanographic vessels began mapping the ocean floor so that their nuclear submarines could navigate deep underwater. The ensuing data provided maps that revealed extraordinary natural phenomena. Submerged peaks and undersea ridges form a continuous mountain chain that reaches up to 10,000 feet (3,048 m) and measures 40,000 miles (64,360 km). This mid-ocean ridge system circles the earth several times and is now known as one of Earth's dominant features, extending over an area greater than all the major land mountain ranges combined. Along a great deal of its length, the ridge system is sliced down its middle by a sharp gully, a rift that is the outlet of powerful heat flows. Temperature surveys demonstrate that heat seeps out of the earth in these mountainous regions of the middle Atlantic, adding to the complexity of the ocean floor. This evidence of heat emitting from Earth's giant cracks and faults helped reveal the existence of earthquakes and volcanic eruptions beneath the ocean. Most of this heat and movement take place in the Atlantic Ocean where the ridge is steeper and more jagged than in the Pacific or Indian Oceans.
In some of the most active volcanic areas another unusual natural phenomenon takes place, discovered by Harry Hammond Hess. Hess studied the isolated mountains rising from the ocean floor and discovered "sea-mounts," which he named guyots in honor of the Swiss-American geographer Arnold Henry Guyot (1807884). Hundreds of these strange undersea protrusions lie under the Pacific Ocean, all of which were probably sunken islands created from volcanic lava. Some of these guyots broke away and gradually wandered further away from the volcanoes. Before oceanographers studied the floor of the great oceans, there was little evidence to support the continental drift theory, which assumed that all the great landmasses were once joined in one supercontinent. Hess's discovery of guyots and other studies of seafloor movement helped reveal the spreading movement of the ocean floor. Hess proposed that hot rock swelled from deep within the earth, constantly forcing the ridges and rifts to part and spread. Later these discoveries of seabed movement helped build on the findings of Alfred L. Wegener's thoery of continental drift.
See also Sea-floor spreading