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Home > Ocean > Oceanography > Volcanic Islands

Geography of the Sea: Volcanic Islands

Another type of mountain in the sea is an island. It rises from the sea floor, but reaches above the ocean’s surface, sometimes just barely. An island is a solitary mountain formed by volcanic activity. Lava erupting from the sea floor builds up on the sea bed over thousands, or even millions of years. The lava cools rapidly as it reaches the ocean water and forms solid rock. Each eruption of the sea floor volcano builds up the volcano a little bit higher. Until one day, the eruptions have built the top of the undersea mountain so high, that it rises above the ocean’s surface.

volcanic islandSometimes, volcanoes occur in close proximity to each other on the sea floor, creating a very large island. For example, the big island of Hawaii is actually five, side-by-side volcanoes that have grown together. The island chain of countries that make up Southeast Asia; Indonesia and Papua New Guinea, the Philippine Islands were all created by volcanic activity on the sea floor. New Zealand, the Island country off the Southeast coast of Australia, was also formed by ancient volcanoes.

A volcanic island in the South Pacific Ocean is also the biggest mountain in the world. Mauna Loa, one of the volcanic islands that make up the big island of Hawaii, is the most massive single mountain in the world because of its sheer bulk. Only about 13,448 feet of Mauna Loa are above sea level, so it may not seem like a very tall mountain. But, when you start measuring Mauna Loa from its true base on the bottom of the ocean, in the Hawaiian Trough, the total height exceeds that of Everest by over 3/4 of a mile. Mauna Loa is Hawaiian for "Long Mountain", probably because of its long, gently sloping shape. Mauna Kea, a neighbor of Mauna Loa on the same island of Hawaii, is actually the tallest mountain in the world. Mauna Kea is about 350 feet taller than Mauna Loa, but its mass doesn't compare to that of Mauna Loa. Mauna Loa takes up a lot of space because it's mass is 9,700 cubic miles of mountain!

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