Most geologic dating depends on the decay of radioactive substances trapped when molten rock solidified. This is fine if what you want dated is bracketed by layers of lava, but that tends to be rare. Furthermore, a radiometric sample only gives you a span of time within which the date likely falls, so two rocks dated at 500,000 years ago may actually have solidified hundreds or thousands of years apart. Wouldn't it be nice to be able to pinpoint something occurring hundreds of thousands of years ago to within a few days?
As you now suspect, there is something that can do just
that—volcanic ash. A major eruption can spread ash over huge areas downwind, and
modern techniques often can separate ashes from different volcanos and different
eruptions. The Yellowstone region has been an eruptive center that has periodically
produced major ashfalls. Some of these, such as the so-called Pearlette type
"0" ash, reached into our region, laying down ash that ties thousands of
square miles to an event a few days long some 600,000 years ago.
Listen to the Audio (mp3 format) as recorded by KTEP, Public Radio for the Southwest.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.