User:Oceanmountains

Mid-Ocean Ridge: the longest mountain range in the world and yet it is under the ocean...

The mid-ocean ridge system is the most extensive chain of mountains on earth, but more than 90% of this mountain range lies in the deep ocean. The mid-ocean ridge wraps around the globe like the seam of a baseball for over 65,000 km. The average depth to the crest of the ridge is 2500m, but it rises above sea-level in Iceland and is more than 4000 m deep in the Cayman Trough. The mid-ocean ridge mountains rise above the abyssal plains approximately 2000 to 3500 m. Mid-ocean ridges are geologically important because they occur along the kind of plate boundary where new ocean floor is created as the plates spread apart. Thus the mid-ocean ridge is also known as a spreading center or a divergent plate boundary. The plates spread apart at rates of 1 cm to 20 cm per year. As oceanic plates move apart, rock melts and wells up from tens of km deep. Some of the molten rock ascends all the way up to the seafloor producing enormous volcanic eruptions of basalt and building the longest chain of volcanoes in the world. The molten rock which does not erupt freezes onto the edges of the plates as they spread apart. In 1783, a segment of the ridge which emerges above sea-level in Iceland erupted more than 12 km3 of lava, enough the pave the entire U.S. interstate freeway system to a depth of 10 m. The scorching lava [~1200 ºC] as well as 50 million tons of sulfur dioxide released into the atmosphere ruined crops and caused the death of more than 10,000 Icelanders, or a quarter of that nation's population at the time. Since most of the mid-ocean ridge is >2000 m deep, most of it's eruptions go unnoticed. In fact, accurate maps of the mid-ocean ridge did not exist until the last ten years and even now, most of it is unmapped. These maps show that the zone of recent volcanic activity is narrow, less than 10 km and often less than 1 km wide. This region of generation of new ocean floor is also characterized by many small to moderate earthquakes. Some of these earthquakes are caused by volcanic eruptions, and others are caused by breaking and ripping the thin, newly created plate as it is spread to either side of the ridge. Two of the most carefully studied mid-ocean ridges are the Mid-Atlantic Ridge and the East Pacific Rise [called a rise because it has more gentle slopes]. The Mid-Atlantic Ridge runs down the center of the Atlantic Ocean. It spread apart at rates of 2 to 5 cm per year, and at these relatively slow rates of spreading, the ridge has a deep rift valley along its crest. The rift valley is 1 to 3 km deep, about the depth and width of the Grand Canyon. In contrast, the East Pacific Rise spreads fast at rates of 6 to 16 cm per year [in excess of 20 cm per year in the past]. At these fast rates, there is no rift valley, just a smooth volcanic summit with a crack along the crest which is much smaller that the Atlantic rift valley. Mid-Ocean Ridges are offset every 50-500 km by transform faults which are 50-1000 km long. The ridge also has many more smaller offsets which define a fundamental segmentation of the ridge system. Along the crests of the volcanoes of the mid-ocean ridge, cracks allow the near-freezing sea water to seep deep into the hot new crust. This water becomes superheated to temperatures of greater than 400ºC. This causes the water to become so buoyant that it shoots out of the seafloor at very high speeds, much like the water gushing from a broken fire hydrant. However, this water is not clear, but looks like thick black smoke. This is because the hot "hydrothermal" water has dissolved minerals out of the basalt crust and also reacts with cold sea water when it comes back up to the seafloor. The black appearance is caused by precipitation of minerals in the plume of hot water as it gushes out of the seafloor. Chimneys and mounds of minerals deposit very rapidly around these "black smoker" hydrothermal vents. The main minerals are zinc, copper and iron sulfides [pyrite or "fool's gold"]. Sometimes there are traces of platinum, gold and silver, but not enough to make these deposits economic since they are so deep. [The author of this article was Co-Chief Scientist of the expedition that first discovered black smoker vents and their deposits in 1979 using the deep-diving submersible, ALVIN, on the East Pacific Rise]. The heat from the volcanoes and the hydrothermal vent waters provide the energy base for a most unusual community of deep sea animals. These animals live completely independently of sunlight, mining the energy of deep sea volcanoes. Instead of relying on plants' use of the sun's energy to make organic material through photosynthesis, the food chain relies on bacteria which have the ability to oxidize the hydrogen sulfide in hydrothermal vent waters and convert large amounts of carbon dioxide in sea water into organic material [this process is called chemosynthesis]. The community consists of clams, mussels, crabs, vent fish, octopus, and the very unusual giant tube worms which grow as tall as 4 m. Conditions are hostile here; the pressures are 200-300 atmospheres [3000 to 4500 lb./sq. in.], it is pitch dark, and volcanic eruptions and earthquakes occur frequently. Like the rest of the deep ocean floor, we have explored less of the mountains of the mid-ocean ridge system than the surface of Venus or Mars or the dark side of the moon. Less than 0.1% of the mid-ocean ridge has been explored with submersibles or remotely operated vehicles.