Voyager: Are there big volcanoes in the deep sea?

Answer: The answer is yes! Volcanoes erupt on land and under the oceans. Both come in a variety of shapes and sizes. Things that determine the size of a volcano are the amount of magma that erupts, time between eruptions, what the magma is made of, and the environment where eruptions take place. Magma is hot, melted rock that gets pushed up from beneath the earth’s surface.

There are many volcanoes in the Pacific Ocean, with the vast majority lying completely under water. There are about 1 million of these so-called “submarine volcanoes,” or seamounts, and they come in different sizes. The smallest may reach only 100 feet or so above the seafloor.

The biggest volcano on Earth is technically a deep-sea volcano. Mauna Loa, a volcano that occupies about half the area of the island of Hawaii, reaches a height of more than 33,000 feet, making it taller than Mount Everest! However, only the top 13,000 feet are above sea level. Mauna Loa is so tall because it is located over a hotspot – a plume of magma rising from deep within the earth – that has been feeding the volcano for close to 1 million years. In addition, the lava – the name for magma when it reaches the surface – flows smoothly over lava from previous eruptions and slowly builds the volcano over time.

--David Hilton, geochemist, Geosciences Research Division