feet), while the island of Kauai rises to only about 1,500 meters (5,000 feet). The older islands beyond Kauai have much lower elevations. An example is Necker Island, which is about 300 miles northwest of Kauai and is twice as old, at about 10 million years. The geological characteristics of the underwater part of Necker Island indicate that it once was more than 1,000 meters (3,000 feet high), but today the highest point on the island is less than 100 meters (300 feet) above sea level. Beyond Kure the tops of the seamounts get progressively lower. Suiko Seamount, once an island, is now 2 kilometers (more than a mile) underwater.

A final source of supporting evidence was the observation that island building is still underway. A new underwater volcano was discovered about 32 kilometers (20 miles) off the shoreline of the Big Island (see Figure 7). Known as the Loihi Seamount, it already rises more than 3,000 meters (9,000 feet) above the floor of the Pacific and is currently within about 1,000 meters (3,000 feet) of the ocean surface. Sometime in the next 100,000 years, it could rise above the waves to produce the newest addition to the Hawaiian islands.

Wilson’s hypothesis about the origins of the Hawaiian islands, now thoroughly tested and accepted, has contributed to a much broader understanding of geological processes known as the theory of plate tectonics. About the same time that Wilson made his hypothesis, he and other geologists were recognizing that the earth’s surface is broken into a number of “plates” of various sizes that can move about in relation to one another. Some plates are expanding outward from volcanic ridges that add new material to the plates’ edges. Some plates are losing material as their edges plunge into the earth’s interior at deep oceanic trenches. Wherever plates meet or slide against each other—at the San Andreas Fault in California, for example—earthquake activity can be particularly intense.

This theory of plate tectonics accounts for many previously unexplained geological features. For example, other hotspots beneath the

Figure 7

The Loihi Seamount off the coast of the Big Island is now above the hot spot beneath the Pacific Plate and is steadily growing. (Diagram adapted from drawing by Joel E. Robinson, U.S. Geological Survey.)



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