Mid-Ocean Ridge

Mid-Ocean Ridge, system of mostly underwater mountains that makes up the most extensive chain of mountains on Earth. More than 95 percent of this mountain range lies in the deep ocean. Like the seam on a baseball, the mid-ocean ridge wraps around the globe, extending over 65,000 km (40,000 mi). On average, the crest of the ridge lies 2,500 m (8,200 ft) below the ocean surface. The crest rises above sea level in Iceland and a number of smaller islands and lies more than 4,000 m (13,000 ft) below sea level in the Cayman Trough, a huge depression in the floor of the Caribbean Sea. The individual mountains of the mid-ocean ridge rise some 2,000 to 3,500 m (7,000 to 11,500 ft) above the flat abyssal plains of the deep-ocean floor. See Ocean and Oceanography.

The mid-ocean ridge is sometimes referred to in the plural, as the mid-ocean ridges, because individual ridges in the separate oceans were discovered separately. Scientists soon realized that these individual ridges were part of a single system circling the globe. The mid-ocean ridge provides direct scientific evidence for the process of seafloor spreading, part of the theory of plate tectonics. According to this theory, the outer shell of Earth is made up of thin, rigid plates that move relative to one another.

Mid-ocean ridges are geologically important because they occur along the kind of boundaries between plates where new seafloor is created as the plates spread apart, or diverge. Thus, mid-ocean ridges are also known as spreading centers or divergent plate boundaries. The plates spread apart at rates of 1 to 20 cm (0.4 to 8 in) per year. As plates move apart under the ocean, molten rock, or lava, wells up from tens of kilometers beneath the surface of the seafloor. Some of the molten rock that ascends to the seafloor produces enormous volcanic eruptions of a dark rock called basalt. These eruptions have built the longest chain of volcanoes in the world. Molten rock that rises to the surface and flows out without erupting solidifies on the edges of the plates as they spread apart, creating new oceanic crust, or rocky seafloor material.

In 1783 the volcano Laki, a peak of the ridge that emerges above sea level in Iceland, erupted more than 12 cu km (3 cu mi) of lava, enough to pave the entire United States interstate highway system to a depth of 10 m (30 ft). The scorching lava, with a temperature of some 1200º C (2200º F), as well as the release of 50 million tons of poisonous sulfur dioxide gas into the atmosphere destroyed crops and livestock and caused the deaths of more than 9,000 Icelanders. That figure represented a quarter of Iceland's population at the time.

Most of the mid-ocean ridge lies more than 2,000 m (6,600 ft) beneath the sea, so the majority of its eruptions go unnoticed. In fact, accurate maps of the mid-ocean ridge did not exist until the late 1980s, and much of the ridge is still unmapped. The maps show that the zone of recent volcanic activity is narrow, less than 10 km (6 mi) wide and often less than 1 km (0.6 mi) wide. This narrow region is where new ocean floor is generated, and it is the site of many small to moderate earthquakes. Some of these earthquakes are caused by volcanic eruptions. Others are due to the breaking and ripping of the thin, newly created crust as it is spread to either side of the ridge.

Two of the most carefully studied sections of the mid-ocean ridge are the Mid-Atlantic Ridge and the East Pacific Rise. The latter is called a rise because it has gentler slopes than most of the rest of the ridge. The Mid-Atlantic Ridge runs down the center of the Atlantic Ocean. It spreads apart at rates of 2 to 5 cm (0.8 to 2 in) per year—relatively slow rates of spreading. Along the crest of the Mid-Atlantic Ridge is a deep rift valley. The rift valley is 1 to 3 km (0.6 to 2 mi) deep, about the same depth as the Grand Canyon and just as wide. The East Pacific Rise roughly parallels the western coast of South America. It spreads relatively fast, 6 to 16 cm (2 to 6 in) per year, and in the past, the rate exceeded 20 cm (8 in) per year. At these fast rates, no rift valley forms, just a smooth volcanic summit with a thin crack along the crest.

The mid-ocean ridge is offset (displaced to the left or right) every 50 to 500 km (30 to 300 mi) by transform faults, fractures in the crust that form at a right angle to the ridge. These faults are 50 to 1,000 km (30 to 600 mi) long. The ridge also has many more smaller offsets, which divide the ridge system into a series of segments.

Along the crests of the volcanoes of the mid-ocean ridge are cracks that allow the near-freezing seawater to seep deep into the hot new crust. This water becomes superheated (heated above its boiling point but, because of high pressure, remaining liquid) to temperatures greater than 400º C (750º F). This extremely hot water is so buoyant, relative to the ice-cold water all around it, that it shoots out of the seafloor at high speeds, much like water gushes from a broken fire hydrant. However, rather than being clear, this water looks like thick black smoke. The superheated water looks this way because it contains minerals that have dissolved out of the basalt crust. It also reacts violently with the ice-cold seawater when it comes back up to the seafloor. The black, smoky appearance, which gives rise to the term “black smokers,” is caused by precipitation of the minerals in the plume of hot water as it gushes out of vents in the seafloor. These vents are known as hydrothermal vents. Mineral deposits resembling chimneys and mounds form rapidly around hydrothermal vents. The minerals found in these deposits are mainly zinc, copper, and iron sulfides (pyrite, or 'fool's gold'). Sometimes the deposits include traces of platinum, gold, and silver, but not enough to make the deposits economical to mine, because they are so deep.

The heat from the volcanoes and the hydrothermal vent waters provides energy that makes possible the existence of a most unusual community of deep-sea animals. These animals live completely independently of sunlight. On land and in less deep waters, plants use the energy of sunlight to make organic material (the basic building material and energy source of plants and animals) through a process known as photosynthesis. Animals, in turn, eat the plants, and animals higher up on the food chain eat both plants and animals lower on the food chain. In the vicinity of the mid-ocean ridge, the food chain relies on bacteria that have the ability to oxidize the hydrogen sulfide in hydrothermal vent waters and to convert the carbon dioxide in seawater into organic material (a process known as chemosynthesis). The deep-sea animal community consists of clams, mussels, crabs, vent fish, octopus, and the unusual giant tube worms, which grow as tall as 4 m (13 ft). Conditions in the deep sea are hostile: The pressures are 200 to 300 times greater than the atmospheric pressure at sea level, it is pitch dark, and volcanic eruptions and earthquakes occur frequently.

People have explored less of the mountains of the mid-ocean ridge system than they have viewed of the surface of Venus, Mars, or the dark side of the Moon. Less than 0.1 percent of the mid-ocean ridge has been explored with submersibles or remotely operated vehicles (see Submersible Craft). A vast frontier right here on Earth awaits exploration in the 21st century.