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, provided an ingenious explanation within the framework of plate tectonics by proposing the "Hot Spot" hypothesis.

Wilson's hypothesis has come to be accepted widely, because it agrees well with much of the scientific data on the Pacific Ocean in general, and the Hawaiian Islands in particular.

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 southeasternmost 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.

The progressive northwesterly drift of the islands from their point of origin over the hot spot is well shown by the ages of the principal lava flows on the various Hawaiian Islands from northwest (oldest) to southeast (youngest), given in millions of years: Kauai, 5.6 to 3.8; Oahu, 3.4 to 2.2; Molokai, 1.8 to 1.3; Maui, 1.3 to 0.8; and Hawaii, less than 0.7 and still growing.

Even on the Big Island alone, the relative ages of its five volcanoes are compatible with the hot-spot theory. Kohala, at the northwestern corner of the island, is the oldest, having ceased eruptive activity about 60,000 years ago.

The second oldest is Mauna Kea, which last erupted about 3,000 years ago; next is Hualalai, which has had only one historic eruption (1800-1801), and lastly, both Mauna Loa and Kilauea have been vigorously and repeatedly active in historic times. Beacuase it is growing on the southeastern flank of Mauna Loa, Kilauea is believed to be younger than its huge neighbor.

The size of the Hawaiian hot spot is not know precisely, but it presumably is large enough to encompass the currently active volcanoes of Mauna Loa, Kilauea, Loihi, and, possibly, also Hualalai and Haleakala.

Some scientists have estimated the Hawaiian hot spot to be about 200 miles across, with much narrower vertical passageways that feed magma to the individual volcanoes.

From: Tilling, Heliker, and Wright, 1987, Eruptions of Hawaiian Volcanoes: Past, Present, and Future: Department of the Interior/U.S.Geological Survey Publication