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Images: The Cauldron Beneath the Seafloor

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 temperatures of about 300°C. Pieces of highly altered rock (gray) are cemented together with minerals such as iron sulfides (gold-colored) and quartz (white).

A comparison of characteristics and chemical composition shows the distinct differences between seawater and hydrothermal vent fluid, in this case fluid from the TAG hydrothermal site on the Mid-Atlantic Ridge at 26°N.

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 distinguished from the relatively unaltered interior (brown). By comparing the geochemistry of the rim and the interior, researchers can determine the ways in which elements are exchanged between seawater and rock

WHOI Senior Scientist Susan Humphris (foreground) prepares to deploy the near-bottom Argo II optical and imaging system operated by WHOI?s Deep Submergence Operations Group. Towed behind ships at 5 to 15 meters off the seafloor, Argo II collected video and still images of seafloor hydrothermal vent systems.

In a hydrothermal circulation system, cold seawater seeps through the permeable seafloor and deeper subsurface dikes. It undergoes a series of chemical reactions with subsurface rocks at various temperatures to create hot hydrothermal fluid that eventually vents at the seafloor.

Chemical reactions in hydrothermal vent systems are a source of elements (positive values) leaching from the ocean crust to the ocean, and a sink (negative values) for elements removed from seawater and incorporated into the crust. In the chart above, fluxes of elements into and out of seawater caused by hydrothermal activity are compared to element fluxes caused by rivers. Green bars indicate minimum estimates of element fluxes; purple bars represent maximum estimated fluxes. (Adapted from Global Impact of Submarine Hydrothermal Processes, The Final Report, RIDGE/VENTS Workshop, 1994.)