Introduction to deep-sea hydrothermal vents
Hydrothermal vents are found along the mid-ocean ridge system, a 60,000 km-long, underwater mountain range stretching around the planet like the stitching around a baseball. This underwater chain of mountains is formed by geologic processes associated with seafloor spreading and plate tectonics.
This page gives some background information on hydrothermal vents and the mid-ocean ridges where they are found.
Contents of this page
- Mid-ocean ridges: where tectonic plates move apart
- Extreme conditions
- Hydrothermal fluid and vents
- Chimneys pump hot, toxic fluids
- Warm fluids diffuse from seafloor cracks
- Fluid chemistry supports life
Active volcanoes form along mid-ocean ridges where tectonic plates spread apart from one another (moving at about the rate that your fingernails grow).
Not your typical cone-shaped volcanos, these long seafloor cracks erupt hot lava, adding to the Earth's crust as the plates spread apart. The new crust is warmer, less dense and rises higher than crust on either side of the plate boundary. The overall morphology (shape), looks like a low mountain ridge, with a small valley down its middle along the plate boundary. The shape of this valley varies depending on the rate of plate spreading. In some places, this valley (or axial trough) is shallow and narrow. In other areas, the valley is deep and wide.
Mid-ocean ridges are deep in the ocean, in some places 2,500 meters (a mile and a half) or more beneath the surface. Pressure exerted by this much ocean water is immense, approximately 250 atmospheres (pressure increases by 1 atmosphere for every 10 meters of seawater). Because it is so deep, sunlight cannot penetrate to the bottom: so the ridge is in complete darkness. The temperature is cold (~ 2°C), but can be much greater in areas where lava erupts or where hot fluids are emitted by hydrothermal vents (see below).
Cold seawater seeps down into the crust through cracks in the seafloor. As it gets closer to molten rock, it heats up. In the process of heating, it undergoes several chemical reactions. At first, oxygen is removed from the seawater. Then, as the water seeps further down (and increases in temperature) sulfur and metals such as copper, zinc, and iron dissolve from the surrounding rock into the hot fluid. Eventually, this hot, mineral-rich fluid rises again and gushes out of openings in the seafloor: hydrothermal vents, at temperatures up to about 400°C.
Along the ridge at various locations, super-hot vent fluids jet upwards through oddly shaped chimneys (see picture, left).
How do these chimneys form? When the hot vent fluid mixes with cold seawater, chemical reactions occur. For example, the sulfur in some vent fluid combines with the metals, forming sulfide minerals. When the mixing occurs as the fluid exits the seafloor, the minerals precipitate to form chimney-like structures that project (sometimes for several meters) into the surrounding ocean.
At certain areas along the ridge, often near chimneys, lower-temperature fluids (~8-30°C) seep out of cracks in the ocean floor. Here, the vent fluid has mixed with cold seawater beneath the ocean floor, so chimneys don't form. These areas of seafloor are called "diffuse flow" areas because the warm fluid typically drifts rather than gushes out of the seafloor. The cooler temperature is more suitable for many animals than the super-hot chimneys.
Many organisms are found living near hydrothermal vents, including tubeworms (see picture, left), mussels, crabs, squat lobsters, limpets, alvinella worms, scaleworms, zoarcid fish, and octopus. Given the absence of sunlight, no plants are found here. Instead, microbes (bacteria and Archaea) form the base of the food chain. These microscopic organisms are chemosynthetic, using energy from hydrogen sulfide in the vent fluid, and oxygen and carbon dioxide from the seawater, to create simple sugars. Some microbes live symbiotically with organisms such as tubeworms and mussels. Others are free-living and may be found growing on many different surfaces, including rocks, tubeworm tubes and other animals, and even inside chimneys.
It's a strange and wonderful world down along the mid-ocean ridge!