Islands can be formed by volcanic activity under the sea. Over millions of years, lava erupting from the sea floor builds up.
The lava cools rapidly as it comes in contact with the water, and forms solid rock. Each eruption from the sea floor builds up the solid rock higher and higher to form an undersea volcanic mountain. Finally, the mountain becomes so tall that it rises above the ocean’s surface to create a volcanic island.
Mauna Loa is one of five volcanoes that form the Island of Hawaii. It is the most massive single mountain in the world, because of its sheer bulk.
Mauna Loa has been erupting for at least 700,000 years, and may have emerged above sea level about 400,000 years ago. The volcano’s magma comes from the Hawaii hot-spot, which has been responsible for the creation of the Hawaiian island chain over tens of millions of years.