II.

Types of Hydrothermal Vents

Scientists have found hydrothermal vents in many geologically active areas of the ocean floor around the globe. At places where the seafloor crust is spreading apart from plate tectonics, seawater sinks deep into cracks in the ocean floor. The seawater is superheated in volcanic rock in the crust to temperatures up to 400°C (760°F). The water becomes chemically acidic, leaching minerals from surrounding rocks. Carrying mainly iron and sulfur minerals, the dark, mineral-rich solution can emerge in chimneylike structures called “black smokers.” Thick deposits of minerals rich in copper, iron, manganese, and zinc can form. Hydrogen sulfide is the main chemical used by microorganisms around the vents to produce food from carbon compounds.

Another type of hydrothermal vent occurs when seawater reacts with the mantle under the ocean crust. This process is not volcanic and heat comes instead from chemical reactions when rocks rich in magnesium and iron oxidize to produce serpentinite and other minerals. The water that emerges from the seafloor at such sites is relatively cool compared to water at magma-heated spots, and is chemically alkaline. Calcium carbonate and magnesium-containing minerals in the water precipitate out to form large, white, towerlike structures. Hydrogen and methane in the water provide food and energy for microorganisms at such sites.