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A rock sample, recovered by drilling 116 meters below the active seafloor hydrothermal vent site at 26°N on the Mid-Atlantic Ridge, shows how the rock has been altered by reactions with seawater at temperature of about 300°C. Pieces of highly altered rock (gray) are cemented together with minerals such as iron sulfides (gold colored) and quartz (white).

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A rock sample dredged from the Mid-Atlantic Ridge shows how seawater flowing between subsurface rocks alters them and cements them together. The rocks' outer rims (gray) have been chemically changed by interaction with hot seawater and can be easily disctinguished from the relatively unaltered interior (brown).

Early Clues: Seafloor Rocks

Rocks dredged up from the bottom of the sea also contained telltale clues. Most rocks from the mid-ocean ridges are black in color, with some white and pale green crystals. But, in the late 1960s and early 1970s, scientists found Mid-Ocean Ridge seafloor rocks with unusual shades of green, orange, and brown.

Scientists figured out why these rocks were different by analyzing the minerals in the rocks and their chemical compositions. They concluded that the original black rocks had been transformed by chemical reactions that could only have taken place in the presence of hot water.

Much the way detectives might use clues to reconstruct how a crime occurred, the scientists figured out how the rocks probably formed. Cold seawater seeped into cracks in the seafloor and was warmed by heat from below. The hot water reacted with the rock, changing the minerals and exchanging elements. Some elements were released from the rock into the seawater; others were taken up from the seawater into the rock. Because hot fluids—like hot air—is lighter and rises, the scientists predicted that the fluids would rise back to the surface and discharge at the seafloor. They predicted that hydrothermal vents might exist, even before any had been seen.

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