oceanic crust moved over the hot spot, each recently formed volcano was carried away from the hot spot toward the northwest, cutting off its source of lava. Meanwhile, a new island was forming so that over time a chain of islands was produced extending away from the hot spot. As the islands continued to move toward the northwest, away from the hot spot, they were eroded by the wind, rain, and waves and eventually sank below sea level to become seamounts.

Once this hypothesis was proposed, scientists began searching for evidence to test it. For example, the hypothesis predicts that the more northwesterly seamounts and islands should be older than the islands to the southeast. Geologists can measure the age of volcanic rocks by measuring the quantities of argon gas in those rocks. Immediately after lava cools, it contains no argon because the gas is expelled from the molten rock. But volcanic rocks also contain a radioactive

Figure 5

The Hawaiian islands formed as the Pacific Ocean floor moved over an underlying hot spot in the earth, shown here by a dotted circle. The present island of Kauai formed about 5 million years ago; Maui Nui, the landmass now represented by Maui and nearby islands, was in place more than a million years ago; and the Big Island continues to grow today. (Diagram adapted from Hampton L. Carson and David A. Clague, “Geology and Biogeography of the Hawaiian Islands,” in Warren L. Wagner and V.A. Funk, eds., Hawaiian Biogeography: Evolution on a Hot Spot Archipelago. Washington, DC: Smithsonian Institution Press, 1995.)