Much of the seafloor is mountainous and uneven, even more so than the earth's surface. As oceanographers first began mapping the ocean bottom, they discovered that often near the center of ocean basins the seafloor is dominated by a linear mountain chain, or mid-oceanic ridge with vast rising slopes. 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 Mid-Atlantic 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 an interesting discovery--they realized that there were frequent oceanic earthquakes occurring along the ridge.
This was an exciting discovery because it revealed the possibility that oceanic earthquakes might be connected to ridges. Using data from other expeditions, Bruce Charles Heezen (1924-1977) more accurately measured the Mid-Atlantic Ridge in 1952 as he began mapping the ocean floor. This ridge measured up to 1.9 mi (3 km) high and 45,954 mi (73,940 km) long. More interestingly, however, he detected a valley in the ridge that led to the Heezen-Ewing theory in 1958, which formalized the notion that ridges contain a central rift.rift. Their discovery sparked the interest of other scientists and explorers and was one piece in the puzzle that lead to the development of the theory of plate tectonics.
In the late 1950s, American and Soviet oceanographic vessels began mapping the ocean floor so that their nuclear submarines could navigate safely in deep water. The resulting data revealed an extraordinary global feature--submerged peaks and undersea ridges form a continuous mountain chain that rises up to 10,000 ft (3,048 m) high and measures over 40,000 mi (64,500 km) long. This mountain chain, or mid-ocean ridge system, encircles the Earth like the seams on a baseball and is one of Earth's dominant features, extending over an area greater than all the major land mountain ranges combined and covering a third of the ocean basins. Along a great deal of its length, the ridge system is sliced down its middle by a steep valley--a rift that temperature surveys demonstrated were an outlet for strong heat flows--as well as occasional volcanic eruptions. This evidence of heat emitting from the giant fractures in the ocean floor along with the existence of earthquakes and volcanic eruptions beneath the ocean drew the interest of researchers the world over. Much of the earthquake activity takes place in the Atlantic Ocean where the ridge is steeper and more jagged, whereas heat flow is generally greater in the Pacific and Indian Oceans, usually along ridges with very shallow slopes.
As scientists learned in the 1980s, oceanic ridge systems differ in character because of the associated rate of seafloor spreading. Ridge systems with fast spreading rates exhibit low relief, no central valley, high heat flow, earthquakes of magnitude 4.5 or less and are often associated with active hydrothermal vents. Slow spreading ridge systems have a deep central rift valley, low heat flow, and earthquakes of up to magnitude 6.5. Perhaps the best example of a fast spreading ridge is the East Pacific Rise, situated between the Pacific plate and the Nazca plate, with a spreading rate as high as 6.5 in (16.5 cm) per year. The Mid-Atlantic Ridge is a classic slow spreading system, with a rate of about one inch (2.5 cm) per year.
What ever the rate of spreading, much of the volcanism associated with oceanic ridges goes unnoticed. In the early 1990s, researchers discovered a huge lava field on the East Pacific Rise believed to have formed within the previous two decades. It is the largest lava flow known to have occurred during human history. Iceland, on the other hand, sits atop the Mid-Atlantic Ridge allowing easy access and detailed study.
In some of the oceans, particularly the Pacific, there is another unusual seafloor feature related to oceanic ridges that was discovered by Harry Hammond Hess. Based on sonar surveys, Hess recognized isolated mountains rising from the ocean floor, so-called seamounts. Some, which he named guyots in honor of the Swiss-American geographer Arnold Henry Guyot (1807-1884), have very flat tops. Hundreds of these strange undersea protrusions populate the sea bottom in the Pacific Ocean. As it turns out, the origin of these features is related to seafloor spreading.
Generally, at the central rift of a fast spreading ridge the lithosphere is thinner than anywhere else on the planet. The asthenosphere lies as little as 1.2 mi (2 km) below the surface as compared to 3-6 mi (5-10 km) for most of the ocean floor. You might wonder how this is possible. Initially, oceanic ridge mountains often stand 0.6-1.8 mi (1-3 km) above the adjacent seafloor. When new seafloor forms, the rock is very hot and therefore relatively low density, explaining its height. Then as it makes way for new seafloor, the central ridge rock moves away from the magma source, so it slowly cools, contracts and becomes denser. As a result, it subsides. Meanwhile, the plate also thickens from below as cooling of the asthenosphere welds rock on to the base, causing the seafloor to sink even more. Hess reasoned that if a ridge volcano was near or above sea level before subsiding, the top could be eroded flat by wave activity, forming a guyot. After passively traveling thousands of miles away from the oceanic ridge where it formed and subsiding hundreds or thousands of feet, only the tallest of these submarine mountains is high enough to rise above the thick sediment that covers seafloor 10s of millions of years old.
Before oceanographers studied the floor of the great oceans, there was little evidence of a mechanism to explain continental drift, Alfred Lothar Wegener's theory, which speculated that all the continents were once joined in a single landmass, or supercontinent. But 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 constantly convected upwards from deep within the earth, forcing the existing seafloor to part and spread at the oceanic ridge systems. Later these discoveries of sea bed movement became an integral part of the theory of plate tectonics.
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