Although volcanoes seem to be more prevalent on the surface of the earth, approximately 75 to 80% of all volcanic activity occurs under the surface of the ocean. The eruptions of these volcanic areas are almost always low level lava flows but recent research has shown that explosive eruptions do occur in the ocean. There are three different types of volcanic activity in the ocean and these are the mid-ocean ridges, hydrothermal vents, and seamounts.
The ocean floor is in constant movement and at the boundaries of the different plates are areas of volcanic activities. These areas are known as mid-ocean ridges and encompass the largest amount of volcanic activity in the ocean. Parts of the ocean floor are continually being consumed in the magma and also deposited new cooling material on the surface of the ocean floor somewhere else. Miles of volcanic ridge segments and thousands of lone volcanoes exist on the mid-ocean ridge. These areas are prone to periodic eruptions.
Lava will pour from the fissures and may take the form of several different types of lava flows. Also found along these areas are hydrothermal vents which are caused by the interaction of the ridges and ocean water.
Although hydrothermal vents are found in many areas of the ocean floor, they occur in much larger numbers along the mid-ocean ridges. As seawater enters the ridge or through the porous rock of the ocean floor, it comes into contact with the magma and is heated to varying degrees. The heated water will begin to rise and exit the ridge through the fissure or through different vents and openings in the rock.
There are many different kinds of hydrothermal vents and it depends on the temperature that the seawater reaches as well as the mixing that occurs. Well mixed seawater and magma can form a fluid that includes numerous metallic and sulfide containing compounds that will precipitate out of the vent fluid as it exits the vent and back into the cold ocean water. Sometimes chimney like structures are produced at the hydrothermal vents that builds up over time from the precipitating material. Some vents are even good habitats for many types of underwater organisms that like the nutrients that come out of the vents.
Individual volcanoes are present along the ocean floor. These are referred to as seamounts and are circular or conical in shape. The larger seamounts are likely fed by hot spots which are large amounts of magma that are rising from the mantle. They melt through the surface of the ocean floor and can supply magma to seamounts.
Sometimes chains of seamounts are created when tectonic plates pass over a hot spot. This occurrence can also produce volcanoes that come to the surface and produce landmasses such as the Galapagos Islands and the Hawaiian Islands. The reason for this is that the hot spot plumes last for a very long time and continually supply magma which can lead to the creation of a chain of landmasses.