8. Seafloor Rocks - Early Clues
Modified with Permission from Dive and Discover
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The plate
tectonics revolution that culminated in the late 1960's fundamentally
rearranged our understanding about how our planet works. It launched
new ways of thinking about Earth’s history and opened new lines
of inquiry—including the idea of seafloor hot springs.
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Here’s the key: Newly made seafloor would likely have cracks in it, because hot rocks contract when they cool. Cold seawater could seep into those cracks, where it would come in contact with hot rocks deeper down. The water would heat up, rise back to the surface, and discharge into the ocean. Geysers on land, such as those in Yellowstone National Park, work in a similar way. |
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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). | |
Scientists figured out why these rocks were different by analyzing the
minerals in the rocks, their crystalline structure, and their chemical
composition. 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 and exchanged chemicals with seafloor
rocks. The
chemical reactions changed the minerals in the rocks and added
minerals to the seawater. Because hot fluid—like hot air—is
lighter and rises, the scientists predicted that mineral-rich
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!
As recently as 1977, no one had seen a seafloor hot spring. They remained solely within the realm of possibility. But the idea that seafloor hot springs might exist grew more intriguing, especially as circumstantial evidence accumulated.
