What Is a Submarine Eruption?

Scope of Submarine Eruptions

• While most human volcanic experience relates to volcanoes that erupt on land, most volcanic eruptions occur in the oceans. In fact, scientists estimate that submarine volcanoes number about 1 million, with several thousand considered active. Approximately 75 percent of the magma that reaches the earth's surface each year is produced along mid-ocean ridges. Prior to the 1990s, very few of these submarine eruptions were documented. The exceptions occurred primarily in shallow waters. Japan is home to the largest number of submarine volcanoes, produced by a subduction zone.

Deep Water Eruptions

• In deep waters, the primary characteristic of submarine eruptions is a basaltic flow of lava. This lava is extruded along ridges in incremental amounts. These flows cool very quickly when they come into contact with cold ocean water, forming pillow lavas. These eruptions are rarely explosive because the weight of the overlaying water prevents the explosive release of gas.

Shallow Water Eruptions

• In shallow waters, the submarine eruptions can become much more explosive, forming thick layers of fragmented lava rocks, known as hyaloclastites. This explosivity is due to trapped gases rapidly decompressing, unlike deep water eruptions. Shallow water eruptions can also produce eruptive columns of steam and ash, called Surtseyan eruptions. These eruptions are named for the eruptions that formed the island of Surtsey off the coast of Iceland in 1967. Finally, these eruptions can create large concentrations of pumice that floats on the surface and drifts with the currents.

Characteristics of Mid-Ocean Ridge Eruptions

• The majority of submarine eruptions occur in the deep ocean, along mid-ocean ridges. These eruptions can be identified by a number of characteristics. First, fresh lava flows will be devoid of a sedimentary covering. Second, new lava flows will be accompanied by the presence of organisms, attracted to the warm temperatures of the surrounding water. Third, you will be able to observe warm to hot water shimmering off of the fresh lava flows. Finally, seismic activity, such as earthquake swarms, frequently accompanies these eruptions.

Examples of Deep Water Eruptions

• The first documented submarine eruption occurred around 1986 on the Juan de Fuca Ridge. This deep water eruption was later documented by bathymetric surveys and seafloor mapping. The first submarine eruption discovered in real time also occurred along the same ridge in 1993. It was associated with an earthquake swarm detected by the U.S. Navy's SOSUS (Sound Surveillance System) hydrophone network.

Example of Shallow Water Eruption

• The submarine eruption of Kavachi, in the Solomon Islands, is an example of a shallow eruption. As one of the most active submarine volcanoes in the southwest Pacific, the volcano had a Surtseyan eruption in 1977. The vent reached the surface in 1978, producing a 100- to 164-meter island within two days.