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

Submarine volcanoes erupt into the water instead of air. They behave differently than volcanoes on land. For example, submarine volcanoes do not usually have explosive eruptions. The water above them creates very high pressure. As a result, the volcanoes usually produce passive lava flows. The lava leaks out along the seafloor. Most submarine eruptions do not disturb the ocean surface.

How These Islands Are Formed

Charles Mandeville is a scientist. He works for the Volcano Hazards Program of the United States Geological Survey (USGS). He and his fellow scientists study volcanoes in the United States. Before joining the USGS, Mandeville studied submarine volcanoes.

Two main factors cause submarine volcanoes to form islands, he says. One factor is the supply of magma. This is melted rock found beneath Earth's crust. The other factor is the tectonic activity. Earth's top layers are the crust and the mantle. They are divided into 15 major tectonic "plates." These plates are always moving very slowly. Magma sometimes rises up through the gaps between them.

Most volcanic islands are created by lava flows on the seafloor. These flows cool and harden into rock. Over millions of years, the rock grows higher and higher. Eventually, some of these underwater mountains reach the ocean's surface. Then they become islands.

Volcanic Island Ecosystems

An ecosystem is like a community. It is made up of all the living and nonliving things in an area, including plants and animals. Volcanic islands have surprisingly lively ecosystems. These ecosystems evolve over millions of years. Life on volcanic islands starts with tiny organisms called bacteria. They are the most basic forms of life.

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

Creatures on volcanic islands evolve in isolated environments. As a result, many organisms are considered to be endemic species. That means they are native to a particular area. They are not found anywhere else in the world. The finches endemic to the Galapagos Islands are one famous example of this. These birds are found only in the isolated Galapagos Islands.

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. In 2009, there was an explosive eruption. It caused a new landmass to form. The eruption covered the nearby island of Hunga Ha'apai in volcanic ash.

Days later, there was a second 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 island.

It is also unclear if the island itself will remain. The wind and the waves are eroding the land, Mandeville says. They are wearing away the island back below sea level. New lava flows will be needed to add more land.

In the years since the 2009 eruption, the young island has stayed above sea level and has gotten bigger after several eruptions in late 2014 and early 2015. 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.