Figure 6

The maximum elevations of the Hawaiian islands gradually diminish from southeast to northwest, with the newest islands being the tallest. (Map reproduced by permission of Dynamic Graphics, Inc., Alameda, CA, USA, producer of EarthVision® software, ©1984-2003 Dynamic Graphics.)

form of the common element potassium. This potassium decays at a known and constant rate into argon, and the argon remains trapped in the rock. To determine the age of a volcanic rock, scientists can measure the amount of argon and the amount of radioactive potassium in the rock. The higher the ratio, the older the rock.

These measurements showed that the Big Island of Hawaii, at the southeastern end of the archipelago, is the youngest of the chain, with an estimated age of less than half a million years (Panel 1). The islands of Maui, Molokai, Lanai, and Kahoolawe, which once were joined in a landmass known as Maui Nui, are the next older. The islands of Oahu and Kauai have greater ages, with the latter being about five million years old. To the northwest, the volcanoes are progressively older, with Suiko Seamount in the northern part of the chain having an age of 65 million years.

This pattern is exactly what had been predicted by the hypothesis that the volcanoes were created by the movement of the crust over a source of heat. In fact, by comparing the ages of the volcanoes with their separations, geologists have concluded that the crust of the Pacific Ocean is moving at a rate of about 10 centimeters (4 inches) per year over the hot spot, or about one meter per decade.

Another source of supporting evidence involved the elevations of the islands and seamounts. As volcanic islands age, they gradually subside and erode. Thus, in Hawaii, the newest islands should be the tallest ones—which again is just what is found (see Figure 6). The highest peak on the Big Island is almost 4,250 meters (14,000



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