By Michael Howell
Sometime around nine to ten million years ago the Bitterroot Valley was covered by a thick layer of volcanic ash. The ash in fact covered a large swath of western Montana and parts of southwest Saskatchewan, Canada. One layer of ash left by some ancient volcanic eruption is visible at a road cut on Ambrose Creek Road. Other layers have been exposed along Willoughby Creek. Although the age of these ash layers, called tephra beds, have been dated by various methods and identified as occurring during the Miocene period, the source of the ash, that is, the site of the volcanic event that produced it, has long been a matter of speculation. Now, thanks to recent improvements in scientific techniques (specifically the improvements in spatial resolution and sensitivity of laser ablation inductively-coupled plasma mass spectrometry), the source of these ash layers has been narrowed down to the volcanic events comprising the Yellowstone Hot Spot Tracks.
Speculation about the source of the ash layers included the Long Valley volcanic field in western California, the Cascade Arc stretching across Oregon and Washington and the Yellowstone Plateau. But in an article published this summer in Elsevier, the Journal of the International Union for Quaternary Research, J.A. Westgate and S.J. Preece from the Department of Geology at the University of Toronto, N.J.G. Pearce and W.T. Perkins from the Department of Geography and Earth Science at Aberystwyth University in Wales, and P.A. Shane from the School of Environment at the University of Auckland in New Zealand, claim to have definitively narrowed the field to the Yellowstone Hot Spot Tracks.
The study is based on examination of four sites in the Bitterroot Valley, two near Scoby, Montana and a few north of the border in Saskatchewan.
Although everyone recognizes Yellowstone Park as a hot spot for geothermal activity today, the activity in that area is really the most recent in a track of eruptive activity stretching over millions of years starting at the McDermitt Caldera on the border of Oregon and Utah (active about 16 million years ago), and moving northwest along the Snake River through Idaho and into Wyoming to the most recent blow-up in the Lava Creek area in Yellowstone Park only about 650,000 years ago.
Improvements in laser spectrometry, according to the article, have allowed scientists to convincingly hone in on the Yellowstone Plateau as the source of the ash by comparing lead isotope ratios in the glass contained in the ash layers. Taken together with the stratigraphic setting, the age, and paleomagnetic properties, the lead isotope ratio values and glass chemistry are convincing, according to the article’s authors, although they state further testing along the Yellowstone Hot Spot Tracks will be needed to absolutely nail things down.
The ash layer exposed along Ambrose Creek Road is perhaps slightly older than the layers exposed on the property of Bob and Judy Hoy on the north side of Willoughby Creek. The ash layer along Ambrose is estimated at 10 to 12 million years old. The three ash layers exposed at Willoughby are estimated to be from 9 to 10 million years old.