This image taken by Himawari-8, a Japanese weather satellite, shows an undersea volcano eruption at the Pacific nation of Tonga.

Everything you need to know about Underwater Volcanoes

People have been forced to flee their homes, and streets and buildings have flooded, as tsunami waves crashed into Tonga’s main island of Tongatapu, following a huge underwater volcano explosion.

A tsunami advisory was in effect for Hawaii, Alaska and the US Pacific coast, with reports of waves pushing boats up in the docks in Hawaii.

What are Underwater Volcanoes?

Volcanoes are common occurrences along the boundaries of Earth’s tectonic plates. These boundaries allow super-heated molten rock called magma, along with ash and gases, to rise through Earth’s crust and emerge on the surface, often dramatically. Since many plate boundaries are submerged, around three-quarters of all volcanic activity on Earth actually occurs underwater.

Even though most submarine volcanoes do not produce the spectacular eruption events of some of their land counterparts, undersea volcanic activity is a constant process that shapes the features of the ocean. When magma reaches the level of the seafloor, it meets cold ocean water and quickly cools to form basaltic rock, often termed “pillow lava” due to its rounded shape. This pillow lava, along with slower-cooling magma beneath it, forms the vast majority of oceanic crust.

Tamu Massif is known to be the largest volcano on the planet. Located in the northwestern Pacific Ocean, it stands at a towering 14,620 feet! (Photo Credit:

Frequent eruptions along divergent plate boundaries such as the Mid-Atlantic Ridge form new ocean bottom in a process known as seafloor spreading. This process occurs slowly (1-15 centimeters per year) yet relentlessly, moving the plates—and the ocean floor and continents above them—further apart. Halfway around the world, along the Pacific Ring of Fire, volcanic activity occurs as plates converge on each other, creating subduction zones.

Continued volcanism in one area can build up to form underwater mountains called seamounts or even islands that breach the ocean surface. Many examples of island chains throughout the Pacific, such as the Hawaiian Islands, are formed as one volcanic hotspot erupts over millions of years while the Pacific plate drifts above it.

How Are Underwater Volcanoes Formed?

Submarine volcanoes are formed the same way that terrestrial volcanoes are: either when tectonic plates collide or when they separate.

Convergent Volcano

The convergence of tectonic plates is governed by various forces, such as Earth’s rotational force and tidal forces asserted by the gravity of Earth, the moon and the Sun. When two plates converge and collide, the heavier plate subducts or slides below the lighter plate, thereby forming a trench. The rocks in the subduction zone melt, allowing the sweltering magma beneath to rise from the outer core’s higher pressure to the lower pressure towards the mantle.

Over time, the accumulating magma rises to the edge until eventually, it erupts into the water. However, due to the almost inexhaustible water and crushing pressure around the vent, the magma immediately solidifies upon reaching the surface. As lava is progressively solidified and accumulated, a mountain is sculpted around the vent.

Volcano convergent plate boundary

Such a volcano can be formed when two oceanic plates converge or when an oceanic and a continental plate converge. Aleutian, Kuril, Japanese and Mariana are examples of submarine volcanoes formed when two oceanic plates collided, more precisely, plates below the Pacific Ocean. Whereas, the western coast of South America is a settlement that developed when the oceanic crust’s Nazca plate slid under the South American plate.

This volcano, West Mata, erupted in 2009 about 1,219 meters (4,000 feet) beneath the surface of the Pacific Ocean, between Fiji, Tonga, and Samoa. The orange glow of magma is visible beneath the plume of volcanic gases (mostly yellowish sulfur) emitted by West Mata.

Divergent Volcanoes

A submarine volcano can also be created when two plates separate or diverge. The lava underneath the plates then squeezes and rises through the void created by the rift. Diverging plates are primarily pushed apart by the water’s pressure and convection currents drawn between the mantle and Earth’s outer core.

Typical of submarine volcanoes, the rising magma eventually emanates from the vent and solidifies instantly as it contacts the water above. Gradually, the solidified layers pile up and form a mountain. Over a period of millions of years, the ragged structures can build upon one another and ascend so high that they protrude from the water’s surface and form volcanic “islands”.

Volcano oceanic spreading ridge

The rate of such an evolution is compounded when the rate at which the magma effuses is increased. Larger islands like the islands of Hawaii floating on the Pacific Ocean are formed on what are called “hot spots”. These are spots on plates that experience tremendous volcanic activity because they are essentially gaps, large holes in the plate through which the lava spurts like water spurts from a hole in a filled bottle. The lava streams upwards and solidifies immediately, forming a thick layer that settles on the ocean floor.

Subsequently, as the plate moves forward, so does the plume of magma, thereby causing a trail of thick, stacked layers, of islands, to form, such as Hawaii’s neighboring islands. This chain of islands is known as an island arc. In fact, it is predicted that a new island called Lo’ihi is developing under the ocean 48 kilometers away from the southeast coast of Hawaii. It is predicted to rise to the surface in about 1000 years.

he submarine volcano of West Mata, in the South Pacific, extrudes lava in a passive lava flow about 1,204 degrees Celsius (2,200 degrees Fahrenheit). The lava’s surface cools into a hard shell immediately upon contact with ocean water.

How Do Underwater Volcanoes Erupt?

For a volcano to erupt there must be a catalyst to instigate the occurrence. Without said catalyst the molten rock will continuously form up until such time as the tectonic plate shifts against, cutting off the flow of magma from the earth’s mantle. This is most likely to occur in climes of the world where sudden ocean temperature changes can occur, such as near the equator. What can happen is that a sudden decrease in temperature will speed the cooling of fresh magma before it can clear the vent at the top of the volcano, plugging it.

Facts about Underwater Volcanoes

  • Volcanoes located under water are also known as submarine volcanoes. There are at least 1,500 active volcanoes on the surface of the earth, but it is estimated that there may be more than 10,000 volcanoes in the Pacific Ocean alone.
  • Volcanic eruptions in shallow water can throw underwater material into the air. The Hawaiian islands were formed due to such volcanic eruptions.
  • One of the latest examples of underwater volcanic eruptions is Surtsey island, south Iceland. The earth surface under the sea was literally lifted up during the eruption, and this resulted in the formation of the Surtsey Island!
  • Tremendous heat of magma, the liquid or molten rock, often creates a crack in the surface of the earth, and leads to volcanic eruption.
  • Water exerts higher (about 250 times more pressure) pressure on the surface of the earth than air. This higher pressure can lead to an underwater explosion.
  • Magma is present in the mantle, the layer below the earth’s crust. When a crack relieves the pressure, the magma rises upward along with gases. This is known as the eruption of an underwater volcano.
  • Underwater magma cools and solidifies much more quickly due to presence of water. It often gets converted into volcanic glass.
  • When the magma reaches the surface of the earth, it is known as lava. It cools and settles down in a pile-like structure or tall structures of various shapes. This is how underwater ridges are formed.
  • Submarine volcanoes which are very close to one another, and which are found in the form of a chain or circle, are referred to as the ‘ring of fire’.
  • More research is required to understand the role of these volcanic eruptions in global warming. It has been noticed that these eruptions lead to warmer water and higher CO2 levels.

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