Mineral sediment precipitated out of the vent fluid

Hydrothermal vents begin to form beneath the sea floor, where super-heated water flows through cracks in the Earth's crust, becoming enriched with dissolved elements from the crust that the water can contain only at extreme temperatures. When it rushes out and mixes with cold sea water, the dissolved elements from the hydrothermal fluid react with elements in the cold sea water, forming new minerals. These minerals precipitate out of the vent fluid and form piles of sediment on the surrounding sea floor, like the mineral sediment in the image above. Image courtesy of the University of California – Santa Barbara, University of South Carolina, NOAA, and Woods Hole Oceanographic Institution.