Some 16.3 million years ago, what is today Wallowa County was the site of one of the greatest volcanic eruptions known on Earth. Its products include the Bowlby stone that constructs the Wallowa County Courthouse, the EM&M building, and many other structures here. Altogether, volcanic vents that stretched from Maxwell Lake to Zumwalt and beyond produced about 9,500 cubic miles of basalt lava. (No, that’s not a typo.) The eruptions were part of hundreds of basalt lava flows that slathered the northwest in Columbia River and Steens basalts from 16.7 to about 12 million years ago.
The source of all this material was the Yellowstone hotspot. (Hotspots are a stationary source of magma or lava, that rises to the surface and erupts, certain a line of volcanoes as a plate moves over them.) Today, the rising heat from that hotspot drives the geysers at Yellowstone. The crater produced by its last catastrophic eruption is known as Yellowstone Lake. It remains a threatening active volcano, with a chamber of molten material stashed about 2 miles below the surface.
But new research by volcanologist Thomas Knott and colleagues of the University of Leicester in the UK and elsewhere, suggest that Yellowstone, our very own, though now distant volcanic hotspot, may be waning.
Eruptions attributed to Yellowstone include not only the basalts but a series of huge, flat-lying volcanoes known as calderas that emitted mostly ash. The first of these appeared at McDermitt, Nevada on the Oregon-Nevada border about 16.4 million years ago. Eruptions then migrated north, producing eruptions of basalt lavas – the Steens basalts and Columbia River basalts — across eastern Oregon. Many of those basalt eruptions, including the great Wapshilla Ridge lava flow, were centered here.
But after about 14 million yeara ago, eruptions returned to Idaho. They swept across southern Idaho, creating the broad, flat agricultural haven that we know today as the Snake River Plain. Yellowstone, at the western end of that sweep, is where the hotspot is stirring up trouble today.
We always thought that one of Yellowstone’s greatest eruptions after its basalt paroxysms here, occurred only about 1.5 million years ago just west of today’s national park. The rocks produced by that eruption are known as the Huckleberry tuff. That would mean the source of lavas has remained vigorous for at least 16 million years.
But Knott and colleagues mapped out two, previously unrecognized super-eruptions from the Twin Falls volcano, about 8.9 and 8.7 million years ago. Those eruptions, known as the McMullen Creek eruption, and Gray’s Landing dwarfed all other Yellowstone events on the Snake River Plain, Knott and colleagues said.
The first even, at 8.9 million years ago deposited glassy, molten ash almost to Challis, Idaho. The second event, Gray’s Landing, occurred 8.7 million years ago, and produced ash that covered about 9,000 square miles with a blanket of molten particles at temperatures estimated at 1700 degrees F. That single eruption produced about 700 cubic miles of ash so hot that much of it welded into glass. Mount St. Helens clocked in at 0.5 cubic miles. The largest eruption in the past 50 years, Mt. Pinatubo, vented about 1.2 cubic miles of material. They pale in comparison to Gray’s Landing’s 700 cubic miles.
Ever since the Gray’s Landing event, Knott says, the eruptions from the Yellowstone hotspot have been getting smaller, with the notable anomaly of the Huckleberry tuff.
“The size, frequency, and emplacement temperatures of Yellowstone hotspot super eruptions have been decreasing with time,” he said. “That may mean that the hotspot activity is waning.”
While that does not preclude future eruptions, it does suggest that any Yellowstone eruption will be smaller, producing only, say, a few hundred cubic miles of ash and lava. When that next eruption might happen is anyone’s guess. But it will be smaller than previous events. At times like these, any reason for a brighter outlook on the future is welcome.