From Hawaiʻi to Indonesia to Iceland, hundreds of islands across the globe have been formed by submarine volcanoes. These 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 volcanoes on land. For instance, 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 what is 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 fellow scientists are responsible for monitoring all 169 active volcanoes in the United States. Prior to joining USGS, Mandeville focused his research on submarine volcanology. He became an expert on the famous 1883 eruption of the island of Krakatoa in Indonesia.

First, the Magma

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." The mantle is the thick layer of rock beneath Earth's crust; when this rock melts, it becomes hot molten magma.

Most volcanic islands are created by passive lava flows on the seafloor. These 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," Mandeville says. That happens when "the tectonic plate that the volcano is growing upon is moving."

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 ... 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 might 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.

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 and unlike any other organisms in the world. The finches endemic to the Galapagos Islands, described by scientist 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 more than 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. A new landmass formed in 2009 as an explosive eruption sent steam, gases and ash roughly 800 meters (2,625 feet) into the sky. The eruption covered the nearby island of Hunga Ha'apai in black, volcanic ash.

Days later, there was a second, smaller eruption from a vent between Hunga Ha'apai and the new landmass. It 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 is unclear whether larger life forms will return to the newly expanded island.

"The wind and the waves are constantly trying to erode that island back below sea level," Mandeville says. The only way the island will keep from disappearing is if "the magma supply produces enough lava flows" to restore the land.

Increasing the height of the island above sea level—through lava flows and eruptions—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.