Exploring Vents: Vent Chemistry & Geology

Between 1980 and 1990, scientists collected hydrothermal fluids from vents in the eastern Pacific and along the Mid-Atlantic Ridge and discovered they were very different from the seawater that percolates into the ocean crust. The reason is that as the seawater percolates through the crust, it is heated by underlying magma or hot rock. Chemical reactions take place between hot seawater and the rocks—just as rainwater reacts with rocks on land and “chemically weathers” them. The seawater gives up certain elements and takes in other elements from the rocks. After these reactions, seawater is no longer seawater—it has chemically changed to hydrothermal fluid.

By looking at ocean crustal rocks that have reacted with seawater at different temperatures, scientists have been able to determine the chemical reactions that are responsible for changing the seawater into hydrothermal fluid. Along much of the mid-ocean ridge, the crust is made up of a volcanic rock called basalt.



Hydrothermal Fluids of a Different Flavor

Volcanic rocks, or basalt, occur along much of the global mid-ocean ridge. But on more slowly spreading ridges, like the Mid-Atlantic Ridge, volcanic activity is much less frequent and so the ocean crust does not have a layer of volcanic rock. Instead, rocks from the next layer down in the Earth—the mantle—form the ocean crust. These rocks contain different minerals and have a different chemical composition than volcanic rocks. That means the chemical reactions that occur between the rocks and the seawater percolating through the crust are different, and so the seawater is changed to a hydrothermal fluid with a different composition. The graph (top left) shows some of the important differences.