An analysis of mineral grains from the bottom of the western
Grand Canyon indicates it was largely carved out by about 70
million years ago -- a time when dinosaurs may have even peeked
over the rim, says a study led by the University of Colorado
Boulder. The new research pushes back the conventionally
accepted date for the formation of the Grand Canyon in Arizona
by more than 60 million years, said CU-Boulder Assistant
Professor Rebecca Flowers. The team used a dating method that
exploits the radioactive decay of uranium and thorium atoms to
helium atoms in a phosphate mineral known as apatite, said
Flowers, a faculty member in CU-Boulder’s geological sciences
department. The helium atoms were locked in the mineral grains
as they cooled and moved closer to the surface during the
carving of the Grand Canyon, she said. Temperature variations at
shallow levels beneath the Earth’s surface are influenced by
topography, and the thermal history recorded by the apatite
grains allowed the team to infer how much time