New theory tells how Earth grows skin - research that explains formation of abyssal hills form underneath the Pacific Ocean - Brief Article

Science News, June 18, 1994

Forget the purple mountains' majesty and the fruited plains. in terms of sheer surface area, they can't compare to deep-sea ridges called abyssal hills, which cover 60 to 70 percent of Earth's face. But while these hills are the most pervasive geologic feature on the planet's surface, the secret of their formation has eluded scientists for 40 years.

Recent dives to the seafloor in the ALVIN submersible have now yielded an explanation of how the hills form in the Pacific Ocean, report Ken Macdonald and Russell Alexander of the University of California, Santa Barbara, and Paul J. Fox of the University of Rhode Island in Narragansett.

In January, the scientists studied a section of the East Pacific Rise 1,000 kilometers southwest of Acapulco, Mexico. This midocean ridge -- part of a worldwide system -- represents the border where two of Earth's surface plates spread apart. Moving about as fast as a fingernail grows, the plates slowly creep away in opposite directions, opening up cracks through which molten rock can rise to the surface and then harden to form new ocean floor.

Some theories about abyssal hills suggest that they develop through eruptions along the midocean ridges. Others argue that the hills form by fracturing of the crust. Evidence from the dives this year now suggests a combined theory, the scientists reported at a recent meeting of the American Geophysical Union (AGU) in Baltimore, Md.

As the newly formed crust moves away from the center of the ridge, it stretches, causing the rock to crack. Some blocks of crust drop to form valleys, while other parts remain elevated as steep-sided hills. Later, lava erupting from the midocean ridge pours over some of the fractures, coating portions of the hills with fresh volcanic rock. This two-step process explains how abyssal hills form in the Pacific and other spots where the plates spread apart at their quickest rate, according to the scientists. Other places, where the ridges spread more slowly, may create abyssal hills differently, Macdonald says.