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Mid-Atlantic Ridge: Definition & Explanation

Instructor: Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

Oceanographers knew the Mid-Atlantic Ridge existed for decades before they found an answer to one burning question: Why was there a 7700 foot high mountain range at the bottom of the Atlantic? Now you can discover the answer in this lesson.

Mountains Out of Place

Rising from the world's seafloors to heights of many thousands of feet is a continuous, sinuous chain of mountains that encircle the planet like the seams of a baseball. For decades, geologists struggled to explain these things they called mid-ocean ridges. Until, that is, they realized that these submarine mountains were another amazing result of plate tectonics.

No, Not Atlantis

The Mid-Atlantic Ridge runs the length of the Atlantic Ocean. Source: U.S. Geological Survey.
Atlantic Ocean basin

The first mid-ocean ridge to be discovered is the one running through the middle of the Atlantic Ocean basin, called the Mid-Atlantic Ridge. It extends from just north of Greenland nearly to Antarctica (and connects with other ridges at each end). In the Northern Hemisphere, it separates the North American tectonic plate from the Eurasian plate. In the Southern Hemisphere, it is the boundary between the South American and African plates.

Krafla volcano in Iceland marks the location of the Mid-Atlantic Ridge. Source: U.S. Geological Survey.
Iceland

The Mid-Atlantic Ridge is submerged for its entire length, except in Iceland, where the ridge rises above sea level. The same volcanic activity seen there also occurs below the ocean along the rest of the ridge.

While it may be tempting to think that the ridge was somehow the source of the legend of Atlantis, I'm afraid it is not. It wasn't discovered until long after the Greek stories were written.

How We Found It

In 1855, a U.S. Navy officer, Lt. Matthew Maury, published a book with a bathymetric (depth) map of the North Atlantic Ocean basin he had drawn based on ships' records and logs that he discovered in the Navy's archives. It depicted a rise in the seafloor near the center of the ocean where the water depths decreased. He didn't know it (neither did anyone else at the time), but his was the first documented evidence for the existence of the Mid-Atlantic Ridge.

Bathymetric map by Lt. Matthew Maury (1855). The ridge is the darker shaded area near the middle of the ocean. Source: NOAA.
1855 Seafloor

In 1872, a research ship, called the HMS Challenger, was performing depth soundings along the proposed path of a new transatlantic telegraph cable. They discovered not just a high spot on the seafloor but a mountain range.

During the late 1920s, a German research vessel, called the Meteor, produced some sonar maps of the ridge and also traced the ridge around Africa and into the Indian Ocean (although those southern parts were actually part of another ridge).

Interesting Topography

The Mid-Atlantic Ridge rises up to 7700 feet above the flatter abyssal plains on either side (yet still remains about 6000 feet below sea level, except near Iceland). Its sinuous path generally mimics the shapes of the coastlines of continents that border the Atlantic Ocean.

Research in the 1950s revealed that the mountain range is rugged and composed of many ridges and valleys, the highest and deepest of which lie parallel and to either side of the central axis, where a deep rift valley is present. The epicenters of many earthquakes are recorded along that valley.

That central rift valley varies in width from around 12-25 miles across at the top, between the crests of the mountains on each side. It averages about a mile deep and at the bottom it is about 2.5-9 miles wide.

Other smaller rifts cross the main valley at nearly right angles, and the central rift is offset along those fractures, which are called transform faults.

The Result of Plate Tectonics

By the early 1960s, scientists had finally put together enough evidence to realize that the Mid-Atlantic Ridge is the topographic expression of a divergent plate boundary, where two of the earth's lithospheric plates are moving away from each other. As they do, new seafloor crust of the Atlantic Ocean is being formed. This process is known as sea-floor spreading.

The ridge is high because hot mantle rocks are rising below it, pushing the earth's crust up. As those rocks get closer to the surface, they melt, and the resulting magma continues on an upward journey, some of it eventually erupting onto the floor of the central rift valley. The remainder of the magma cools and solidifies into new seafloor crust, a rock called basalt. As the plates continue to pull away from each other, more new basalt fills the gap.

Transform faults offset the central rift valley, which lies along the ridge axis. Source: Pim van Tend. Creative Commons, public domain.
transform fault

Remember those transform faults? As the plates move, it is physically impossible for them to do so uniformly along their entire length. Whether because of variations in the rate of movement or the forces that cause the plates to move or just because the earth is spherical, the plates fracture and smaller pieces shift away between the transform faults.

Chimneys on the Seafloor

Sea water that seeps down into the hot basalt rock circulates back and erupts as superheated (several hundred degrees) water geysers that carry dissolved metals and other elements and form dark-colored mineral chimneys, called black smokers. They are found at all mid-ocean ridges.

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