All Old Faithful images on this page from photos, courtesy Ilya Bindeman
You wouldn't look at the Yellowstone Caldera and think "recycling." We neither. But it turns out that the giant volcanic hotspot in Wyoming has been reusing rock for at least two million years. And that recycling offers clues about when the titanic volcano will next erupt.
Like all volcanoes, Yellowstone's eruptions are powered by molten rock that collects in a chamber in Earth's crust. Eventually, buoyancy forces the magma out, where it becomes the familiar ash and lava. The chamber erupts, then collapses like a giant blister.
Using highly sensitive techniques to extract data from almost microscopic crystals, Ilya Bindeman and John Valley, both geologists at University of Wisconsin-Madison, found that the roof of the blister plunges deep into the hot magma below, where it is remelted -- recycled -- to make more magma.
The volcano at Yellowstone was 40 or 50 kilometers across -- so it had a lot of roof rock. The new understanding may clarify the workings of large, caldera-forming volcanoes, if the process is widespread. The abundant groundwater that powers Yellowstone's geysers was instrumental in the detective work that uncovered the natural recycling of rock -- and hence the schedule of Yellowstone's eruptions.
A volcano with a timetable
Now even the math-crippled gang at The Why Files can figure out that since the last eruption occurred 600,000 years ago, we're due for another in roughly 100,000 years. (Note to self: Buy batteries. Update calendar on Palm Pilot.)
Yellowstone's last big eruption spewed ash for hundreds of miles, and may have left a global blanket of air-borne ash that cooled Earth, perhaps causing widespread extinctions. The eruption -- about one thousand times larger than the blip that devastated Mt. St. Helens in 1980 -- paved the nearby landscape with ash deposits several hundred meters thick.
And that, as they say in the military, is not something you want happening on your watch. Timing matters.
When will the 'stone rock again?
Dates of formation of zircon, a tiny, heat-resistant crystal.
Evidence that rock bearing the crystals had been changed by hot water near the surface.
Anyone want a date?
When zircon crystals form, they contain uranium, but no lead. Gradually, the unstable uranium decays into lead. By measuring the relative amounts of each element, you can calculate when the zircon was formed.
The beauty of the technique is that zircon crystals form inside magma (at about 800 ° Celsius). After that, the crystals can survive 2,000 ° Celsius.
If you're working with batches of zircons, this dating technique is old science. The new date data came from the aptly named SHRIMP, which can take measurements from individual zircons, which are smaller than a grain of sand.
All wet -- 100,000 years ago
Isotopes are atoms of an element with varying numbers of neutrons. Although isotopes of a particular element are chemically identical, they can be distinguished with scientific instruments.
Oxygen in rainwater carries relatively little of the isotope with the atomic weight of 18 (vs. 16 for "garden-variety" oxygen). When volcanic rock is submerged in Yellowstone's hot groundwater for between 500 and 5,000 years, O-18 leaches into the rock, leaving an unmistakable sign of water.
By combining the year when the crystal formed (recall that this happens inside magma), with evidence for water, it's possible to conclude that magma created in such-and-such a year took a hot-water bath (and therefore was near the surface). Only after that did the rock remelt into new magma.
"This changes views on volcanism inside of Yellowstone Caldera, and similar calderas where we don't have fingerprinting help" from water to show that the magma was at the surface, says Bindeman.
The finding may alter our view of how much magma and heat are leaving the depths of the Earth, Bindeman adds, since we may be counting the recycled heat twice. "If a volcano is still eating its own roots, it is not new magma which is coming out of the mantle."
-- David Tenenbaum
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