From Hawaiʻi to Indonesia to Iceland, hundreds of islands across the globe have been formed by submarine volcanoes. Submarine volcanoes are exactly what they sound like—volcanoes located beneath the ocean’s surface.

Because they erupt into water instead of air, submarine volcanoes behave quite differently than terrestrial volcanoes. For instance, it’s uncommon for submarine volcanoes to have explosive eruptions. The sheer weight of the water above them creates very high pressure, usually resulting in what are known as passive lava flows along the seafloor. Most submarine eruptions do not disturb the ocean surface.

Charles Mandeville is the program coordinator for the Volcano Hazards Program of the United States Geological Survey (USGS). He and his colleagues are responsible for monitoring all 169 active volcanoes—all terrestrial—in the United States. Prior to joining the USGS, Mandeville focused his research on submarine volcanology, becoming an expert on the famous 1883 eruption of the island of Krakatoa in Indonesia.

According to Mandeville, there are two main factors that contribute to submarine volcanoes eventually forming islands: the supply of magma and tectonic activity.

“The first thing you need is a supply of magma,” he says. “Typically, in the origin of most oceanic island volcanoes or submarine volcanoes, you need to melt the Earth’s mantle.”

Most volcanic islands originate from passive lava flows on the seafloor. These passive flows harden into rock and build up the height of the underwater mountain over millions of years. Eventually, some volcanoes reach heights above the seafloor where lower pressure allows for explosive eruptions. Submarine volcanoes that do not reach sea level are called seamounts.

In addition to magma supply, plate tectonics play a large part in determining which submarine volcanoes will eventually form islands. Tectonic activity can sometimes “take the island volcano away from the source of magma that originates in the mantle, because the tectonic plate that the volcano is growing upon is moving,” Mandeville says.

Volcanic Island Ecosystems

Formed from nothing but rock, volcanic islands have surprisingly vibrant ecosystems. These ecosystems evolve over millions of years, along with the island itself. Life on volcanic islands starts with the most basic building blocks—autotrophic bacteria.

“There are bacteria that can make their food from just the chemical elements and parts of the elements that are being emitted from volcanoes,” Mandeville says. “ ... Once you have a microscopic ecosystem established, you have enough of a food supply to support larger life.”

Species from nearby landforms also contribute to the developing ecosystem. Passing birds may stop to nest on the new island, bringing seeds and spores from the mainland or other islands. Plant life can float through the ocean to end up on the island’s shores.

Because they evolve in such an isolated environment, many organisms are considered endemic species—unlike any others in the world. The finches endemic to the Galapagos Islands, described by naturalist Charles Darwin in the 19th century, are one famous example of this. These birds are found only in the isolated Galapagos Islands. The Hawaiian Islands, even more isolated, have over 1,000 endemic plant species alone.

World’s Youngest Island

One of the world’s most recently formed volcanic islands is part of the island nation of Tonga, in the South Pacific. Tonga is an archipelago of 170 volcanic islands. The new landmass formed in March 2009 as an explosive eruption sent steam, volcanic gases, and volcanic ash roughly 800 meters (2,625 feet) into the sky, covering the uninhabited island of Hunga Ha’apai—63 kilometers (39 miles) away—in black, volcanic ash.

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

Though Hunga Ha’apai previously had rich plant and animal life, the ash from the eruption devastated its ecosystem. It’s unclear whether the new island will evolve to be able support larger life forms.

“The wind and the waves are constantly trying to erode that island back below sea level. The only thing that’s going to outpace the effects of the wave and storm erosion is if the magma supply produces enough lava flows and explosive deposits to keep pace with that erosion,” Mandeville says.

Increasing the height of the island above sea level—through lava flows and eruptions—is critical to allowing birds from nearby islands to “seed the new island with life,” he 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.