From Hawaiʻi to Indonesia to Iceland, hundreds of islands have been formed by submarine volcanoes. These volcanoes are exactly what they sound like. They are volcanoes located beneath the surface of the ocean.

Submarine volcanoes erupt into water instead of air. For this reason, they behave quite differently than volcanoes on land. For example, it is uncommon for submarine volcanoes to have explosive eruptions. The weight of the water above them creates very high pressure. Instead of explosive eruptions, the volcanoes usually produce passive lava flows. The lava leaks out along the seafloor. Most submarine eruptions do not disturb the ocean surface.

Studying Submarine Volcanoes

Charles Mandeville is a scientist. He works for the Volcano Hazards Program of the United States Geological Survey (USGS). He and his fellow scientists monitor all 169 active volcanoes in the United States. Before he joined USGS, Mandeville studied submarine volcanoes. He became an expert on the famous 1883 eruption of the island of Krakatoa in Indonesia.

Mandeville says there are two main factors that contribute to submarine volcanoes forming islands. One is the supply of magma, or melted rock beneath Earth's crust. The other is tectonic activity. Earth's top layers are the crust and the mantle. They are divided into 15 major tectonic "plates" that cover the planet's surface. These plates are always moving very slowly. Magma sometimes rises up through the gaps between them.

Most volcanic islands are created by passive lava flows on the seafloor. These flows cool and harden into rock. Over millions of years, they build up the height of underwater mountains. Some of these underwater mountains eventually form islands.

Volcanic Island Ecosystems

Formed from nothing but rock, volcanic islands have surprisingly lively ecosystems.

These ecosystems evolve over millions of years, along with the island itself. Life on volcanic islands starts with tiny organisms called bacteria. They are the most basic forms of life.

Species from nearby landforms also contribute to the developing ecosystem. Passing birds might stop to nest on the new island. They might bring seeds from the mainland or other islands. Plant life can float through the ocean to end up on the island's shores.

Since they evolve in such an isolated environment, many organisms are considered to be endemic species. That means they are native to a particular area. The finches endemic to the Galapagos Islands are one famous example of this. These birds are found only in the isolated Galapagos. The Hawaiian Islands are even more isolated. They have more than 1,000 endemic plant species.

World's Youngest Island

One of the world's newest volcanic islands is part of the island nation of Tonga. Tonga is a collection of 170 volcanic islands. They are located in the South Pacific. After an explosive eruption in 2009, a new landmass formed. The eruption covered the nearby island of Hunga Ha'apai in black, volcanic ash.

Days later, there was a second, smaller eruption between Hunga Ha'apai and the new landmass. It combined with rock from the first eruption to fill the space between the two. The result was a single landmass. It was nearly double the original size of Hunga Ha'apai.

Before the eruption, Hunga Ha'apai had rich plant and animal life. The ash devastated its ecosystem. It is unclear whether larger life forms will return to the newly expanded island.

It is also unclear if the island itself will remain. "The wind and the waves are constantly trying to erode that island back below sea level," Mandeville says. New lava flows will be needed to restore the land.

Increasing the height of the island above sea level is critical. It will allow birds from nearby islands to "seed the new island with life," Mandeville says.

In the years since the 2009 eruption, the young island has maintained itself above sea level and experienced significant growth after a series of eruptions in late 2014 and early 2015 added to its landmass. It is still attached to Hunga Ha’apai and is in the very early stages of developing an ecosystem. Other submarine volcanoes near Tonga remain active.