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			Hawaii - Violent Beginnings Naval Meterological and Oceanography Command 03.23.01 Large and small, awake or sleeping, volcanoes have governed Hawaii's history. Behind these beautiful, exotic islands is the story of molten rock and fire that created this archipelago, or island chain. The story began some 25 million years ago, when the first eruptions began and still today has not ended. The Hawaiian Islands are really tops of very large volcanoes rising from the bottom of the ocean which is over 15,000 feet deep here. A direct connection to the Earth's core. These volcanoes formed over a hot, localized area of magma from the Earth's mantle. This spot is now believed to be located directly under or just south of the island of Hawaii. Each of the volcanoes went through several stages of development. In the first stage the volcano starts to grow up from the ocean floor and is entirely underwater. An underwater volcano is called a seamount. Erupting from the vent, the opening in the Earth's surface, pools of lava begin to form and cool. Then as more and more magma oozed from the vents over millions of years, the pools became ridges, and grew into crests. The seamount continues to grow until finally, miles high, it break's the ocean's surface and forms an island. This type of volcano, with its slowly formed, gently sloping sides, is known as a shield volcano. As long as lava erupts from the vents, a shield volcano will continue to grow. The Pacific Plate moves very slowly to the northwest. The result is that contact between the submarine vents and the volcano conduits, or channels, for magma is gradually disrupted and finally closed off. Slowly the volcano stops growing, surface eruptions slow down and finally stop completely, and ultimately become extinct. As each volcano goes extinct, a new volcano starts to form on its southeastern flank. In fact the next Hawaiian volcano, called Loihi, has already started to form underwater southeast of the island of Hawaii. It is still a half-mile below the ocean's surface. Several thousand years more will be necessary before it grows tall enough to break into the sunlight. This movement of the Pacific Plate is also reflected in the age of the islands: Niihau, farthest to the northwest, is 5.5 million years old, next Kauai is 5.1 million years old, then Oahu is 3.7 to 2.6 million years old, and finally Maui is 1.3 to 0.9 million years old. On the Big Island of Hawaii, two shield volcanoes are still active; Mauna Loa and Kilauea. Between the two of them they have covered nearly 200,000 acres of land with their red-hot lava flows over the past 200 years or so. For many years Kilauea has been in a continually eruptive state. Molten lava from it's eruptions has been measured at temperatures of 1800°F and more! Where the lava flows entered the ocean, roughly 200 acres have been added to the Big Island. But as active as Kilauea and Mauna Loa are, their eruptions are comparatively safe and gentle, producing continuous small and liquid lava flows rather than dangerous bursts of fire and ash.


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