Tonga Tsunami: What Are Submarine Or Undersea Volcanoes & How They Erupt | EXPLAINED

Submarine volcanoes are located beneath the ocean’s surface and occur in three ways. The explosion throws rock and ash into the water, and molten lava glows below the water.

A grab taken from footage by Japan's Himawari-8 satellite and released by the National Institute of Information and Communications (Japan) on January 15, 2022 shows the volcanic eruption that provoked a tsunami in Tonga. | AFP

New Delhi: On Saturday, January 15, an explosive eruption of the Hunga Tonga-Hunga Ha’apai volcano occurred, triggering a tsunami in the Pacific Ocean. The underwater volcano also called a submarine volcano, is located in the South Pacific Kingdom of Tonga, an archipelago of more than 170 islands in the South Pacific Ocean 2,000 km northeast of Auckland, New Zealand.

The Geostationary Operational Environmental Satellite (GOES), a joint operation between NASA and the National Oceanic and Atmospheric Administration (NOAA) in the United States, captured the explosive eruption of the submarine volcano located in Tonga, the NOAA said on its website.

Following the eruption of the underwater volcano, a tsunami occurred in Tonga and several South Pacific islands, according to media reports. Waves started crashing into coastal homes and telephone and internet links were severed.

The eruption had a radius of 260 km or 161.5 miles, and sent ash, steam, and gas 20 kilometers into the air, according to the NOAA. This was about seven times more powerful than the previous eruption that occurred on December 20, 2021.

Also, a tidal gauge in Nuku'alofa, the capital of Tonga, measured a 30 cm or one-foot tsunami wave that resulted from the blast.

Types Of Submarine Volcanoes And What Causes Them To Erupt

Submarine volcanoes are volcanoes located beneath the ocean’s surface. The explosion throws rock and ash into the water, and molten lava glows below the water. There are three ways in which submarine volcanoes can take place.

Underwater volcanoes can be formed due to rift zones which are found in all of Earth’s major ocean basins. The second type of submarine eruption occurs due to the collision of crustal plates. There is a third type too when a submarine eruption occurs as a result of a magma plume rising through the Earth’s crust overlying an area of melting in the Earth’s mantle.

Earth’s major ocean basins have rift zones, where crustal plates are being formed. Submarine volcanic eruptions are characteristic of these rift zones, according to the NOAA. The rift zones, also known as seafloor spreading centres, because tectonic plates move away from each other at these regions, mostly lie at depths more than 2,000 meters or 1.2 miles.

These are also known as divergent plate boundaries. 

Therefore, around three-quarters of all volcanic activity on Earth occurs as deep, underwater eruptions. This amounts to more than 70 percent of all volcanic eruptions on Earth. One cannot see the effects of these deep eruptions from the surface of the ocean as they are cloaked from view by thousands of feet of water.

Submarine volcanoes play a significant role in maintaining the ocean’s ecosystem. When spreading centre or submarine volcano eruptions occur, they produce a rock called basalt, which is the principal rock making up the oceanic trust.

The seafloor spreading eruptions are mostly local. However, the Earth’s crust can be deformed as a result of these eruptions. These deformations may closely resemble the eruptions of Hawaiian volcanoes, according to the NOAA.

The submarine basalt flows have a distinctive “pillow shape”, and can also be smooth sheet flows, similar to the basalt eruptions on land.
Though submarine eruptions occur along all seafloor spreading centers, they are most common along spreading centers where the plates are moving apart at relatively rapid rates. For instance, at seafloor spreading centres such as the East Pacific Rise, the spreading rates are 10 to 15 centimetres per year. The minimum spreading rates can be around one to two centimetres per year, observed in areas such as the Mid-Atlantic Ridge.

There are certain regions in the ocean known as subduction zones, where crustal plates collide and one plate progressively dives beneath the other and is ultimately re-melted. Submarine eruptions may take place in these subduction zones, or convergent plate boundaries. These underwater eruptions are different from those occurring along spreading centres.

What Happens After An Undersea Volcano Erupts

While basalt rock is formed as a result of submarine volcanic activity taking place in rift zones, andesite rock is formed due to subduction zone volcanism. Andesite is a product of the melting of the plate being subducted. Andesitic lavas have high viscosity and high gas content, and hence, produce violent eruptions.

Scientists have only recently discovered and observed active deep andesitic eruptions. Since these occur at significant depths, their explosiveness is dampened.

When a magma plume rises through the Earth’s crust overlying an area of melting in the Earth’s mantle, a different type of submarine eruption occurs. Such eruptions are known as hotspot volcanoes and often form chains of volcanic islands and seamounts. These islands and seamounts are older with increased distance from the surface location over the rising magma plume. Basalt rock is produced as a result of hotspot eruptions.

Submarine volcanoes are hidden under an average of 8,500 feet or 2,600 metres below the surface of the water. The global system of mid-ocean ridges is estimated to produce 75 percent of the annual output of magma.

Submarine volcanoes produce around 0.7 cubic miles or three cubic kilometers of lava, according to an article on the Oregon State University website. The magma and lava result in the creation of the edges of new oceanic plates and supply heat and chemicals to some of the Earth's most unusual and rare ecosystems.

It is estimated that there are more than a million submarine volcanoes, and as many as 75,000 of these volcanoes occur over half a mile above the ocean floor.

The rate of plate movement plays an important role in determining the type of volcano that forms and the rate of eruptive activity.
Submarine volcanoes occurring at subduction zones are like their subaerial counterparts, the only difference being that the weight of the overlying water modifies the eruption style.

Some of the Earth's largest volcanoes are a result of hotspots, which leave linear tracks of seamounts across the ocean basins.  

Ecosystems Supported By Submarine Volcanoes

Submarine volcanoes create unique habitats, which makes them even more interesting. Seamounts, which are underwater mountains formed by volcanic activity, are regions of high biological diversity. Their shape helps to deflect food-carrying currents upward and attracts a variety of sessile fauna, and the crustaceans and fish that feed upon them.

Scientists had discovered in the late 1970s that some animals can even metabolise inorganic compounds emitted during volcanic activity.

This forms unique communities around hotspots of hydrothermal venting, similar to geyser activity on land.

In 1977, hydrothermal vents and new forms of life were first discovered at mid-ocean ridges, according to an article on the Oregon State University website. Hydrothermal vents, also called black smokers, are characterised by the presence of water, hydrogen sulfide, and other minerals. The springs have temperatures of about 660 degrees Fahrenheit.

The hot waters host an ecosystem complete with giant clams, mussels, tube worms, and other critters which use sulfur, and not sunlight, to live.

The vents for the hydrothermal fluids produce black "chimney stacks", and hence, are called black smokers. The chimney stacks, made of iron and zinc sulfide minerals and calcium sulfate, can grow to heights of about 40 feet but most are less than 30 feet tall.

Following the submarine eruption at the East Pacific Rise, located in the Pacific Ocean, in 1991, hydrothermal vents were recently formed. These regions host tubeworm colonies.