AMNH - Black Smokers: "

Black Smoker

You've probably seen or heard of natural hot springs on land, like Old Faithful at Yellowstone National Park. Similar phenomena occur under the oceans within midocean ridge volcanoes and are called deep-sea hydrothermal (hot water) vents. They are known as black smokers, like the ones seen above. These black smokers are chimneylike structures made up of sulfur-bearing minerals or sulfides that come from beneath Earth's crust. They form when hot (roughly 350�C), mineral-rich water flows out onto the ocean floor through the volcanic lava on a mid-ocean ridge volcano.

See link above for more info - LRK -

Sulfide minerals grow or crystallize from the hot water directly onto the volcanic rocks at the place where the hot, mineral-rich water flows from the ground. This crystallization forms a hollow, chimneylike sulfide structure through which the hot water continues to flow. As the hot, mineral-rich water rushes out of this chimney and mixes with the cold ocean bottom water, it precipitates a variety of minerals as tiny particles that make the the vent water appear black in color. This is why these sulfide chimney structures are called black smokers. "