Volcanoes don’t always erupt above ground for the whole world to see. More than 70 percent of all volcanic eruptions occur underwater. While some knowledge regarding volcanic explosions under the sea has been unearthed, scientists are still groping in the dark when it comes to truly understanding underwater volcanoes. After all, the eruptions are hidden away from view and obscured from sight.
Most submarine volcanoes are located in the depths of seas and oceans, while some are located in shallow waters. Submarine volcanoes are underwater vents or fissures in the Earth’s surface from which magma can erupt. After their eruption underwater, the magma has to contend with the crushing pressure of gallons upon gallons of sea water.
The presence of water considerably alters the characteristics of a volcanic eruption underwater in comparison to those that occur on land. Many submarine volcanoes are seamounts: volcanoes that rise abruptly from a seafloor of 1,000 – 4,000 meters depth.
It is, thus, a creation of fire and water.
The word atoll has its origins in the Maldivian word atolu. The most extensive groups of atolls have taken up residency in the Pacific Ocean, but numerous atolls exist in the central Indian Ocean.
The development of an atoll is fuelled by the eruption of underwater volcanoes. As lava from the eruption piles up on the seafloor, the seamount’s elevation increases; eventually breaking through the surface of the water. The peak of the volcano gives birth to an isolated oceanic island.
Tiny sea animals called corals begin to build a reef around the island. The coral is an animal and not a plant. It is born, it feeds and it dies. Once the coral has created its reef, a variety of underwater lifeforms take up residency in the reef that they have created. They are the architects of the ocean.
The presence of the atoll signifies the final stage in the creation of a volcanic island. As the volcano begins to sink, the coral reefs continue to grow upward. The volcanic peaks continue to sink and erode, until they are below sea level, leaving islets that have built up on the reef.
The type of corals that build reefs are called hermatypic corals. Hermatypic corals create a hard exoskeleton of limestone which consists of calcium carbonate. Billions of these limestone exoskeletons create the reef. If the right elements and conditions are not present, they can neither grow nor develop.
Atolls are often, but not always, uninhabited ‘desert’ islands. Many are remote and difficult to reach. In the 20th century, this isolation resulted in them becoming testing sites for nuclear weapons from the United States, Britain, and France.