Figure of western U.S showing the track of the Yellowstone hotspot as assumed by younging in a northeast direction of silicic calderas (yellow). Areas colored in shades of tan represent Neogene sedimentary deposits contemporaneous with the hotspot volcanism. Black dots are sampling locations of Yellowstone/Snake River Plain volcanic ash deposits. All radiometric ages for units in the interval including 4.49 Ma to 10.27 Ma are ages determined by Ar/Ar analysis at the Lamont-Doherty Earth Observatory Geochronology Laboratory and the at the Berkeley Geochronology Center. Many age determinations  are based on weighted averages of as many as 16 individual age determinations.  Click for detail of calderas.