LOS ANGELES - Thomas J. Ahrens, a Caltech geophysicist who pioneered the academic use of shock waves to study minerals under high temperatures and pressures such as those found at the center of the Earth, died Nov. 24 at his home in Pasadena. He was 74.

Today, physicists have several ways to look at materials under high pressures and temperatures. Diamond anvils can be used to exert very high pressures on minerals, and lasers are used to heat them to extreme temperatures. Modern computers can be used to calculate properties directly.

But when Ahrens started his work half a century ago, according to his Caltech colleague Paul Asimow, the only way to obtain the necessary conditions was by slamming two things together.

"When he started, it was the only way to obtain information about the deep Earth and to produce a sensible model of the deep Earth," Asimow said.

Government labs here and abroad were doing such studies for nuclear weapons testing, but Ahrens was among the first to do it in an academic setting to provide fundamental geological information about the Earth's core.

He started with $79.95 shotguns from Sears that shot special pellets at targets, but he soon graduated to a 30-foot steel tube loaded with a pound of gunpowder that could fire an 80-gram projectile, mimicking pressures at depths up to about 600 miles. Later, he developed a two-stage gun that could achieve firing speeds of nearly 18,000 mph.

Ahrens is survived by his wife, Earleen; two sons, Eric of Pittsburgh and Earl of Los Angeles; a daughter, Dawn of Los Angeles; and four grandchildren. - MCT

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