The 30-mile-by-45-mile volcanic caldera that makes up most of Yellowstone National Park erupts with disturbing regularity — every 650,000 years or so.
It erupted 2 million years ago, then again 1.3 million years ago. Then about 642,000 years ago it exploded again, with 1,000 times the force of the 1980 Mt. St. Helens eruption. Even that cataclysm pales in comparison to the two-million-years-ago event, which ejected 1,500 cubic miles of rock into the atmosphere. That’s a cube of rock and dirt eleven-and-a-half miles on each side. Waist-high layers of tuff from that eruption have been found in Iowa, nearly a thousand miles away.
By way of comparison, Mt. St. Helens ejected 0.3 cubic kilometers of material. That’s a cube of material about two-thirds of a mile (or about 3,500 feet) on each side.
Let’s see, a quick calculation: an eruption every 650,000 years; last eruption 642,000 years ago; next eruption due — the day after tomorrow, QED.
Well, maybe not. Over geologic time, the periodicity of Yellowstone’s eruptions is slowing. In a 2002 paper, University of Utah geologists Michael Perkins and Barbara Nash found that their have probably been 142 huge eruptions over the last 16 million years. And 16 million years ago, there were 30 eruptions per million years. Those eruptions spread ash over three feet thick from the Great Plains to the Pacific Coast, an area of four million square miles.
But even though the volcano is not exploding at the moment, it is not particularly quiet. A 17-year-long University of Utah study of ground movements in Yellowstone has found that the power of the Yellowstone “hotspot” is much greater than previously thought.
Part of Yellowstone is sliding downhill toward the southwest at a brisk one-sixth of an inch a year. And over the 17 years between 1987 and 2004, the valley of Jackson Hole moved upward 1.7 inches and westward one-quarter of an inch.
More dramatically, the whole caldera “breathes” in an alarming fashion. “Conventional surveying of Yellowstone began in 1923. Measurements showed the caldera floor rose 40 inches during 1923-1984, and then fell 8 inches during 1985-1995,” says a release about the study.
This recent study used global positioning system equipment to take very precise measurements of the earth deformation. Measurements since 2004 show the caldera rising upward at a faster rate than ever observed before. When it bulges upward like this, it expends ten times more energy than it does when it produces earthquakes. It has raised the level of Yellowstone upward by one-third of a mile over the last two million years.
“The Yellowstone hotspot has had a much bigger effect over a larger area with more energy than ever expected,” says University of Utah geophysics Professor Robert Smith.
The hotspot is about 300 miles wide at its top end, which lies five miles below the surface, extending to a depth of 10 miles below the surface. The ground above it drags across the hotspot, being gradually deformed over time as the North American continental plate moves over it. The Snake River Plain was formed by this action.
Christine Puskas, one of the authors of the University of Utah study, says, “The North American plate has been moving the same direction for the past several million years.”
The action of the hotspot may be the reason that there have not been the expected catastrophic earthquakes of magnitude 7.5 in the Jackson Hole area. The Teton fault runs 40 miles north to south along the eastern edge of the Teton mountain range.
“The textbook model for a normal fault is not what’s happening at the Teton fault,” Smith says. “The mountains are going down relative to the valley going up. That’s a total surprise.” The Jackson Hole valley and the Tetons are slowly being squeezed together like the trash compactor that threatened Han Solo and Luke Skywalker in Star Wars.
One of the great things about geology is that, except for a few of its disciplines, it has virtually no practical application. It’s just interesting,. As a method of transportation, waiting for the Yellowstone hotspot to propel you toward retirement in Arizona is a failure.
Should these magnitude seven earthquakes hit, though, things would speed up considerably. Evidence indicates there have been thousands of these very powerful earthquakes over the last 13 million years. Vertical movement from a single quake been as much as six feet. The quakes have lifted the Tetons to their present height of 13,000 feet, nearly 7,000 feet above the valley floor.
The University of Utah study was published in the March 2, 2007 issue of the Journal of Geophysical Research – Solid Earth.
Dan Whipple is a guest columnist for New West writing from Broomfield, Colorado. Find his “Due West” columns at www.newwest.net/duewest.