Submarine volcano

Submarine volcanoes are vents or fissures in the Earth's surface located underwater. These geological features allow magma, which is molten rock, to erupt from deep within the planet. They play a massive role in shaping our world. Many of these volcanoes sit near mid-ocean ridges, which are areas where tectonic plates form. These ridges are incredibly productive. It is estimated that mid-ocean ridges alone account for 75% of the total magma output on Earth.

When a volcano erupts underwater, the presence of water changes everything. On land, lava behaves differently because it is not surrounded by liquid. In the ocean, water causes magma to cool and solidify much more rapidly. This fast cooling often turns the molten rock into volcanic glass. As lava flows, a solid crust forms around it almost immediately. New lava then pushes into this outer skin. This process creates unique shapes and textures known as pillow lava.

At extreme depths, the physics of the eruption change due to intense pressure. Once you go below depths of about 2,000 meters, the pressure becomes very high. Specifically, when pressure exceeds the critical pressure of 22.06 MPa, water can no longer boil. Instead, the water becomes a supercritical fluid. This state occurs because the pressure is so great that it prevents the usual phase changes we see at the surface. In these environments, the salt in the seawater can also change the critical temperature and pressure. For a solution with about 3.2% NaCl, the critical point might be 400°C and 29.9 MPa.

Scientists use sound to study these hidden eruptions. They use devices called hydrophones, which are underwater microphones, to listen to the activity. There are two distinct types of sounds generated during an eruption. One sound is created by the slow release and bursting of large lava bubbles. The other sound comes from the quick explosions of many gas bubbles. By distinguishing these sounds, researchers can estimate the volume and composition of the lava flow. In 2009, scientists near Samoa used both video and hydrophones to watch the West Mata Volcano. This allowed them to connect specific sounds to the visual sights of the eruption.

There are many different types of underwater volcanic features. Many submarine volcanoes are seamounts, which are essentially underwater mountains. These are often extinct volcanoes that rise abruptly from the seafloor. Oceanographers define them as independent features that rise at least 1,000 meters above the seabed. There are an estimated 30,000 seamounts across the globe. Most peaks stay hundreds or thousands of meters below the surface. However, some are unusual. The Bowie Seamount in Canada's Pacific waters rises from a depth of about 4,000 meters to within only 15 meters of the sea surface.

Our understanding of these volcanoes is growing through modern technology. The total number of submarine volcanoes is estimated to be over one million, though most are now extinct. About 75,000 of them rise above the seabed. Only 119 known submarine volcanoes have erupted in the last 11,700 years. Scientists use Remote Operated Vehicles, or ROVs, to explore these environments. These robots help us see black smoker chimneys and unique marine life. In 2019, a large pumice raft was found floating between Fiji and Tonga. Satellite images showed this raft came from a volcanic plume during an eruption.

Research also shows that magma can cause trouble without even erupting. In Santorini, Greece, the volcanic system is part of the South Aegean Volcanic Arc. This arc formed because the African Plate is subducting, or sliding, beneath the Aegean microplate. In late January, this area experienced over 28,000 earthquakes in a single month. Scientists discovered that 300 million cubic meters of magma had moved 4km below the seabed. Even though no volcano erupted, the pressure caused many earthquakes. This shows that underwater magmatic systems can create significant unrest even when they stay below the seafloor.