The Hawaiian Islands are the exposed peaks of a huge, mostly submarine, volcanic mountain chain formed by countless eruptions of fluid lava. The distinctive linear shape of this chain reflects the progressive northwestward movement of the Pacific Plate over a fixed and deeper "hot spot," which provides heat to partially melt the overriding plate, forming magma. The "Big Island" (Hawaii) still overlies the "hot spot" and taps the magma source to feed the frequent eruptions of Kilauea and Mauna Loa volcanoes and an active submarine volcano (Loihi) off Hawaii's south coast. Over a span of about 70 million years, the combined processes of magma formation, eruption, and continuous movement of the Pacific Plate over the stationary "hot spot" have left a prominent trail of volcanoes across the ocean floor called the Hawaiian Ridge-Emperor Seamounts Chain.

Eruptions of Hawaiian shield volcanoes are generally nonexplosive, in contrast to the explosive eruptions of composite volcanoes such as Mount St. Helens of the circum-Pacific "Ring of Fire." Because of their typically mild nature and accessibility, Hawaiian eruptions with their spectacular fountains, cascades, lakes, and rivers of incandescent lava, can be observed and monitored systematically and safely. The frequent eruptions of the active volcanoes of Hawaii provide an ideal natural laboratory for studying eruptive processes and products. Most of the volcano-monitoring techniques currently in use at other active volcanoes of the world were developed and/or refined by researchers at the Hawaiian Volcano Observatory, founded in 1912. More important from a human standpoint, the nearly continuous volcanic activity over the Hawaiian "hot spot" has created a string of beautiful islands - home to about a million people and a popular vacation spot for many more millions each year.

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