Dive and Discover : Hydrothermal Vents : Vent Basics

Hydrothermal vents are one of the most spectacular features on the seafloor. They form in places where there is volcanic activity, such as along the Mid-Ocean Ridge. Water seeps through cracks in the seafloor and is heated by molten rock deep below the ocean crust to as high as 400°C. The hot fluid rises to the surface and gushes out of the vent openings. This hydrothermal fluid carries with it dissolved metals and other chemicals from deep beneath the ocean floor.
Click on the words and numbers to find out more about these remarkable underwater geysers.

1. Cold seawater (2°C) seeps down through cracks into the ocean floor.

2. The seawater continues to seep far below this point in the ocean crust. Energy radiating up from molten rock deep beneath the ocean floor raises the water's temperature to around 350-400°C. As the water heats up, it reacts with the rocks in the ocean crust. These chemical reactions change the water in the following way:
All oxygen is removed.
It becomes acidic.
It picks up dissolved metals, including iron, copper and zinc.
It picks up hydrogen sulfide.

3. Hot liquids are less dense and therefore more buoyant than cold liquids. So the hot hydrothermal fluids rise up through the ocean crust just as a hot-air balloon rises into the air. The fluids carry the dissolved metals and hydrogen sulfide with them.

4. The hydrothermal fluids exit the chimney and mix with the cold seawater. The metals carried up in the fluids combine with sulfur to form black minerals called metal sulfides, and give the hydrothermal fluid the appearance of smoke. Many factors trigger this reaction. One factor is the cold temperature, and another is the presence of oxygen in the seawater. Without oxygen, the minerals would never form.
In white smokers, the hydrothermal fluids mix with seawater under the seafloor. Therefore, the black minerals form beneath the seafloor before the fluid exits the chimney.

Other types of compounds, including silica, remain in the fluid. When the fluid exits the chimney, the silica precipitates out. Another chemical reaction creates a white mineral called anhydrite. Both of these minerals turn the fluids that exit the chimney white.

Diffuse Flow
Hydrothermal fluids do not always flow out of chimneys. In some places, they seep out of the ocean floor vents. The fluids from these diffuse flows are usually much cooler than the vent fluids. They also flow far slower. The diffuse flow fluids mix with seawater below the sea floor, so all of the minerals have been left behind. Some diffuse flows, however, do contain sulfides. Microorganisms feast on these sulfides. These microorganisms become food for the many exotic creatures that live around the vents.

White smoker
White smoker fluid is usually cooler (250-300°C) and flows more slowly than the black smoker fluid. The chimneys generally are smaller as well.

The white color comes from minerals that form when the fluid exits the chimney and mixes with seawater. Unlike the black minerals in black smokers, these minerals don't contain metals. (Click on the numbers to learn how black smokers and white smokers form.)

Chimney
Chimneys, which can be tens of meters tall, are made from minerals rich in metals and sulfur. The hydrothermal fluids carries the metals, which include copper, zinc, and iron, up from the ocean crust. When the fluids mix with the seawater, the metals combine with sulfides and form these black minerals.

The chimneys grow bigger and bigger as long as fluid continues to flow out of them and the minerals continue to form. Scientists have observed some chimneys growing as fast as 30 centimeters per day. Chimneys, however, are fragile, and often collapse if they grow too big. (Click on the numbers to learn how black smokers and white smokers form.)

Black smoker
This is not smoke pouring out of the chimney. Rather it is hydrothermal fluid that is so hot (350 to 400°C), it can melt metal. The fluid carries dissolved metals from deep beneath the ocean floor. When the fluid mixes with sea water, these metals combine with sulfur to form tiny black particles. These particles make the fluid look like smoke.