Submarine volcano

Submarine volcanoes are underwater vents or fissures in the Earth's surface from which magma can erupt. Many of these volcanoes are found near areas where tectonic plate formation occurs, especially along mid-ocean ridges, which alone are responsible for about 75% of all the magma that comes out of the Earth. While most submarine volcanoes lie deep in the seas and oceans, some are in shallower water and can sometimes send material into the air during an eruption.

Scientists believe there are over one million submarine volcanoes, though most of them are no longer active. About 75,000 of these rise more than a kilometer above the seabed. Only 119 submarine volcanoes are known to have erupted in the last 11,700 years. These volcanoes play an important role in shaping the ocean floor and the chemistry of seawater.

Near submarine volcanoes, special places called hydrothermal vents can be found. These vents support many kinds of marine life, making these areas important for scientists studying both Earth processes and ocean ecosystems.

Seamounts

Many submarine volcanoes are called seamounts. These are typically extinct volcanoes that rise sharply from the deep ocean floor, which is usually between 1,000 and 4,000 metres deep. Oceanographers define seamounts as features that stand at least 1,000 metres tall above the seafloor. Although their peaks often sit far below the ocean surface, they are still part of the deep sea. Scientists estimate that there are around 30,000 seamounts worldwide, but only a few have been explored.

Some seamounts are special. For example, the Bowie Seamount in Canada’s Pacific waters rises from a depth of about 3,000 metres to just 24 metres below the sea surface.

Effect of water on volcanoes

The presence of water changes how underwater volcanoes erupt compared to volcanoes on land. When lava from a submarine volcano touches water, it cools very quickly and forms a solid crust. This leads to the creation of special lava shapes called pillow lava, which look different from lava on land.

Deep under the ocean, where the pressure is very high, water behaves in unusual ways. At depths below about 2,200 metres, water becomes a supercritical fluid, which makes it hard to detect underwater volcanoes from far away using listening devices called hydrophones. The salts in seawater also change how water reacts to heat and pressure near these volcanoes.

Identifying types of eruptions by sounds

Submarine volcanoes can make two kinds of sounds during eruptions. One sound comes from large lava bubbles slowly releasing and bursting. The other sound is from quick explosions of tiny gas bubbles. By listening to these sounds, scientists can learn about how the lava flows and how it might affect sea life.

In 2009, scientists used a camera and a special microphone, called a hydrophone, to watch and listen to the West Mata Volcano erupt deep in the Pacific Ocean. This helped them understand the different noises made by lava and gas bubbles during eruptions.

Research

Scientists are still discovering new things about underwater volcanoes and where they are located. In the first twenty years of this century, NOAA's Office of Ocean Exploration helped fund studies of these volcanoes. One important project was the Ring of Fire missions near the Mariana Arc in the Pacific Ocean. Using special underwater robots called Remote Operated Vehicles (ROV), scientists looked at underwater eruptions, hot pools of molten sulfur, tall structures called black smoker chimneys, and even special sea creatures that live in these hot, deep places.

Research using the ROV KAIKO near Hawaii showed that a type of smooth lava flow, called pahoehoe, can also happen underwater. The shape of these lava flows depends on how steep the ocean floor is and how fast the lava flows. In August 2019, scientists found a big floating mass of pumice in the South Pacific between Fiji and Tonga. They learned this pumice came from an eruption of a nearby underwater volcano, which they could see from space pictures. This helps scientists learn how to predict when an underwater volcano might erupt in the future.

Santorini: magma pressure

Santorini, Greece is found in the southern Aegean Sea, about 128 nautical miles from the Greek mainland and 63 nautical miles from the island of Crete, the largest Greek island. Santorini lies along the active South Aegean Volcanic Arc, created by the movement of the African Plate beneath another plate. In late January, the area experienced many earthquakes, some very strong. Scientists discovered that a large amount of magma pushed up beneath the seabed, but no eruption happened. This pressure buildup caused many earthquakes and made people leave the area. These events remind us that underwater volcanoes can create problems even without erupting.