Mid-Atlantic Ridge (black "stitching"), a 10,000-kilometer-long string of volcanoes at the bottom of the Atlantic Ocean, is the longest mountain range in the world. The colors indicate the ages of the rocky crust under the ocean, which is youngest (red) near the ridge and gets progressively older as it nears the continents.
Elliot Lim and Jesse Varner CIRES, University of Colorado at Boulder, and NOAA/National Geophysical Data Center (www.ngdc.noaa.gov/mgg)

Key Concepts

• Eighty-five percent of the earth’s volcanic eruptions occur deep underwater along mid-ocean ridges.
• Lava ejected from those narrow chains of seafloor volcanoes produce the rocky underpinnings of all oceans.
• Until recently, no one understood much about how the molten lava rises up into the ridges.
• Scientists now think they have deciphered the process, beginning with the formation of microscopic droplets of liquid rock in regions up to 150 kilometers deep.

At the dark bottom of our cool oceans, 85 percent of the earth's volcanic eruptions proceed virtually unnoticed. Though unseen, they are hardly insignificant. Submarine volcanoes generate the solid underpinnings of all the world's oceans massive slabs of rock seven kilometers thick.

Geophysicists first began to appreciate the smoldering origins of the land under the sea, known formally as ocean crust, in the early 1960s. Sonar surveys revealed that volcanoes form nearly continuous ridges that wind around the globe like seams on a baseball. Later, the same scientists strove to explain what fuels these erupting mountain ranges, called mid-ocean ridges. Basic theories suggest that because ocean crust pulls apart along the ridges, hot material deep within the earth's rocky interior must rise to fill the gap. But details of exactly where the lava originates and how it travels to the surface long remained a mystery.

   
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