Plate tectonics and the Hawaiian "Hot Spot"

(1 ) Divergent or spreading--adjacent plates pull apart, such as at the Mid-Atlantic Ridge, which separates the North and South American Plates from the Eurasian and African Plates. This pulling apart causes "sea-floor spreading" as new material is added to the oceanic plates.

(2) Convergent--plates moving in opposite directions meet and one is dragged down (or subducted) beneath the other. Convergent plate boundaries are also called subduction zones and are typified by the Aleutian Trench, where the Pacific Plate is being subducted under the North American Plate.

(3) Transform fault--one plate slides horizontally past another. The best known example is the earthquake-prone San Andreas fault zone of California, which marks the boundary between the Pacific and North American Plates.

The great majority of the world's earthquakes and active volcanoes occur near the boundaries of the Earth's shifting plates. Why then are the Hawaiian volcanoes located near the middle of the Pacific Plate, more than 2,000 miles from the nearest plate boundary? In 1963, J. Tuzo Wilson, a Canadian geophysicist,

According to Wilson, the distinctive linear shape of the Hawaiian-Emperor Chain reflects the progressive movement of the Pacific Plate over a deep immobile hot spot. This hot spot partly melts the region just below the overriding Pacific Plate, producing small, isolated blobs of magma. Less dense than the surrounding solid rock, the magma rises buoyantly through structurally weak zones and ultimately erupts as lava onto the ocean floor to form volcanoes.

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 the trail of volcanoes across the ocean floor that we now call the Hawaiian-Emperor Chain. Scientists interpret the sharp bend in the chain, about 2,200 miles northwest of the Big Island, as indicating a change in the direction of plate motion that occurred about 43 million years ago, as suggested by the ages of the volcanoes bracketing the bend.

Part of the Big Island, the south-easternmost and youngest island, presently overlies the hot spot and still taps the magma source to feed its two currently active volcanoes, Kilauea and Mauna Loa. The active submarine volcano Loihi, off the Big Island's south coast, may mark the beginning of the zone of magma formation at the southeastern edge of the hot spot. The other Hawaiian islands have moved northwestward beyond the hot spot, were successively cut off from the sustaining magma source, and are no longer volcanically active.