A volcano is a special kind of opening in the ground. It lets burning hot material underground escape to the surface. When this happens, it causes an eruption.

The hot material is melted rock called lava. Sometimes it explodes into the sky. Other times it flows out calmly.

There are thousands of islands around the world. Many of them were formed by submarine volcanoes. These are volcanoes below the surface of the ocean.

Submarine volcanoes are not like volcanoes on land. They erupt into the water instead of air. For this reason, they do not usually have explosive eruptions. The water above them creates high pressure. This pushes down on the volcanoes. When they erupt, the lava flows out gently. It leaks out along the seafloor.

Underwater Mountains

Charles Mandeville is a scientist. He works for the United States Geological Survey (USGS). He used to study submarine volcanoes.

Two things cause submarine volcanoes to form islands, Mandeville says. One is the supply of lava. The other is tectonic plates. Earth's top layers are the crust and the mantle. They are divided into 15 major plates. These plates are always moving very slowly. Lava 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 time, the rock grows higher and higher. It becomes an underwater mountain. Some of the mountains 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. Volcanic islands have very complicated ecosystems. They develop over millions of years. Life on volcanic islands starts with very tiny organisms. They are called bacteria.

Sometimes creatures from landforms nearby help an island develop. Birds might stop to rest on the new island. Sometimes they carry seeds with them. These seeds allow plants to grow.

World's Youngest Island

New islands are forming all the time. One of the newest is part of the country of Tonga. It is made up of 170 islands. Tonga is located in the South Pacific. In 2009, there was a large volcanic eruption. It caused a new island to form. Another eruption connected the island to the nearby island of Hunga Ha'apai. More eruptions in 2014 and 2015 made the island grow even bigger.

Before the eruption, many plants and animals lived on Hunga Ha'apai. Then the eruption covered the island in ash. That destroyed its ecosystem. It is not clear if large animals will return to the island.