A hydrothermal vent is a hot spring on the seafloor. Cold seawater sinks through cracks in the crust, is heated by magma and rises back out, carrying dissolved minerals. Vents mostly occur along spreading ridges and at convergent plate boundaries.
Key takeaways
- Vents form where seawater percolates down through cracks near spreading centres or subduction zones, is heated by magma and emerges again [1].
- Hot seawater from vents can top 700 degrees Fahrenheit (about 370 degrees Celsius), but does not boil because of the enormous pressure [1].
- Scientists first discovered vents in 1977 near the Galapagos Islands, surrounded by animals never seen before [1].
- Our map shows the ridges where vents are found, such as the East Pacific Rise, but it has no layer for vents themselves.
How a hydrothermal vent works
Hydrothermal means "hot water". The process starts with ordinary cold seawater. At spreading ridges and subduction zones, water seeps down through cracks and fissures in the ocean crust. Deep down it comes close to hot magma, heats up and rises again, leaving the seafloor as a jet of hot water [1].
On the way, the water picks up dissolved minerals. When it meets near-freezing seawater, tiny particles, mostly fine sulfide minerals, solidify. They build chimney-like structures around the vent [1]. NOAA names two types:
- Black smokers are chimneys of iron sulfide, which is black.
- White smokers are chimneys of barium, calcium and silicon, which are white [1].
The fluid can reach over 700 degrees Fahrenheit and still not boil, because the pressure at those depths is so great [1].
Vents matter for more than geology. When scientists found the first ones in 1977 near the Galapagos Islands, they were surprised to see large numbers of living things nobody had seen before. Those communities depend on chemical processes that come from seawater meeting hot magma, rather than on sunlight [1].
The link to the map is the ridge. Most of the mid-ocean ridge system is a continuous range of underwater volcanoes [2], so the heat that powers vents is there along its length. See seafloor spreading for how the crust is made. The homepage explains how deep the ocean is and where the ridges sit.
An example from our data
Our map has no vent layer. The GEBCO grid is too coarse to show a single chimney, and vent fields are not part of the dataset. What it does show are the ridges where vents are known to form.
On the East Pacific Rise, a fast-spreading ridge, the listed point reads 3,140 m. On the Mid-Atlantic Ridge, which is slow spreading, it reads 1,477 m [2]. The Juan de Fuca Ridge, nearest to Canada, reads 1,512 m. Each figure is the depth of one grid cell near a listed point, not the depth of a vent.
The thing people get wrong
It is easy to imagine a vent as an underwater volcano that erupts lava. A vent is a hot spring, not a lava flow. It gives out hot water loaded with minerals, which is a different thing from molten rock. Vents are also not limited to the deepest parts of the sea. They are tied to spreading ridges and subduction zones [1], wherever the crust is cracked and hot enough.
Hydrothermal vent vs seamount
| Hydrothermal vent | Seamount | |
|---|---|---|
| What it is | A hot spring on the seafloor | A large underwater mountain |
| Size | A chimney or small field | Rises at least 1,000 metres above the seafloor [3] |
| On our map? | No | Yes, 420 named seamounts |
Frequently asked questions
What is a black smoker?
A vent chimney that gives out dark, mineral-rich water. Its colour comes from fine particles of iron sulfide, which formed when hot fluid met near-freezing seawater.
Can you see hydrothermal vents on this map?
No. Vents are too small for the grid. The map shows the shape of the ridges where they are found.
Sources
- What is a hydrothermal vent?, NOAA Ocean Service.
- What is a mid-ocean ridge?, NOAA Ocean Exploration.
- What is a seamount?, NOAA Ocean Service.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).