10/15/97

By Mitch Emmons, 844-5741 (emmonmb@mail.auburn.edu)

AUBURN GEOLOGIST TO PRESENT PAPER AT GSA ANNUAL MEETING

AUBURN Ð- A hundred million years of history affecting the formation of the Appalachian Mountains is the subject of a paper that will be presented by an Auburn University geologist at the annual meeting of the Geological Society of America on Oct. 20-23 at Salt Lake City.

The title of the paper by W.E. Hames, assistant professor in the Department of Geology, is, "How can one mineral from one mountain yield geologic ages that differ by 100 million years?"

"During uplift and erosion from deep within earth, rocks which are today at the tops of the highest mountains became cool before those exposed in adjacent valleys," Hames said. "This happens simply because earth's interior is hot, and rocks once deeply buried will cool as they are uncovered."

In some mountain ranges, the change in topography is sufficiently great, and the uncovering of rocks sufficiently slow, for many millions of years to pass between the cooling of rocks at the top and base of a mountain, he explained.

An argon-gas-based dating method (the 40Ar/39 method), developed in the mid-1960s, is used by scientists to determine exactly when rocks cooled during the formation of mountain chains, according to Hames.

"Argon-40 arises through the natural decay of potassium-40. Once formed in a mineral, the argon-40 can diffuse out, and this is particularly true for high-temperature conditions." Hames said.

"For a given type of mineral, larger crystals will tend to retain argon-40 at higher temperatures than will smaller ones. Similarly, the centers of large crystals will tend to retain argon-40 at higher temperatures than will the crystal edges. Recent 40Ar/39Ar dating studies show that cooling-age differences of many millions of years can exist between the center and edge of a single crystal."

Hames cites the formation Mt. Washington in New Hampshire to illustrate the magnitude of time that can elapse in a mountain's cooling history.

"We predicted that crystals of muscovite from the top of Mt. Washington, which has the highest elevation and topographic relief of the northeastern U.S., would have large age differences from their centers to their edges," Hames said. "Earlier study documented muscovite cooling ages of 305 to 275 million years ago from the top of Mt. Washington to the base, a difference of about 30 million years over an elevation change of about 5,000 feet. By using new techniques of laser dating, we show that ages up to about 400 million years occur in the cores of large crystals and as young as 270 million years along their edges.

"By utilizing the size-dependent retention of argon-40, we find that a 100 million year cooling history of an entire mountain range can be preserved in a single crystal," Hames said.

Hames and his coworkers dedicated their study to the memory of Craig Merrihue, a co-founder of the 40Ar/39Ar dating method and experienced mountaineer, who died in a climbing accident on Mt. Washington in 1965.

oct97:AU-mountains

CONTACT: Hames, 334/844-4881.