Submarine volcano

Submarine volcanoes are underwater places where hot, molten rock from inside the Earth can burst out. Many of these volcanoes are near where tectonic plate formation happens, especially along mid-ocean ridges. These ridges are where most of the hot, molten rock comes out of the Earth. Most submarine volcanoes are deep in the seas and oceans, but some are in shallower water and can sometimes throw material into the air during an eruption.

Scientists think there are more than one million submarine volcanoes, but most of them are not active anymore. 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 help shape the ocean floor and affect the chemistry of seawater.

Near submarine volcanoes, there are special places called hydrothermal vents. These vents support many kinds of marine life, which makes these areas important for scientists studying Earth processes and ocean ecosystems.

Seamounts

Many submarine volcanoes are called seamounts. These are usually old volcanoes that rise sharply from the deep ocean floor. The ocean floor is often between 1,000 and 4,000 metres deep. Oceanographers say seamounts must stand at least 1,000 metres tall above the seafloor. Even though their tops are far below the ocean surface, they are still part of the deep sea. Scientists think there are about 30,000 seamounts in the world, 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

Water changes how underwater volcanoes erupt compared to volcanoes on land. When lava from a submarine volcano touches water, it cools quickly and forms a solid crust. This creates special lava shapes called pillow lava.

Deep under the ocean, where the pressure is very high, water acts in unusual ways. Below about 2,200 metres, water becomes a supercritical fluid. This 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 is from large lava bubbles slowly rising and bursting. The other sound is from quick bursts 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 learning about underwater volcanoes and where they are. In the first twenty years of this century, NOAA's Office of Ocean Exploration helped pay for studies of these volcanoes. One big 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 studied underwater eruptions, hot pools of molten sulfur, tall structures called black smoker chimneys, and special sea creatures that live in these deep, hot 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 moves. 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 in the southern Aegean Sea, about 128 nautical miles from the Greek mainland. It is 63 nautical miles from Crete, Greece's largest island. Santorini is along the active South Aegean Volcanic Arc. This arc was formed when the African Plate moved beneath another plate.

In late January, the area had many earthquakes, some very strong. Scientists found that a lot of magma pushed up under the seabed. But there was no eruption. The pressure from the magma caused the earthquakes and made some people leave the area. This shows that underwater volcanoes can cause problems even without erupting.