Why some ocean volcanoes grow tall.

Contrary to the jokes that fill birthday cards, some things do improve with age. Geophysicists have long known that the ocean floor gains strength as it gets older. A new study suggests that the mature seafloor also makes a better place for growing volcanoes.

The Pacific oceanic plate--a giant piece of Earth's outer shell--holds more than 50,000 undersea volcanoes, but oceanographers have analyzed only a tiny fraction of these in any detail. In the Aug. 8 Science, Paul Wessel of the University of Hawaii in Honolulu used satellite gravity measurements to gauge the sizes of 10,000 seamounts. Bigger volcanoes exert a greater tug on the satellites.

Wessel found that the biggest volcanoes sit on the oldest ocean crust--that exceeding 60 million years in age. Younger crust rarely has big volcanoes. Scientists uncovered hints of this relationship in the early 1980s, but they had data on only 59 seamounts at that time. The new study suggests that young crust is too warm and weak to support large volcanoes, whose weight buckles the plate.

Using the relationship between seamount size and seafloor age, Wessel estimated the age of seamounts in the Pacific. This analysis suggests that certain periods in Earth's history have produced bumper crops of seamounts.