POSTED 26 JULY 2001

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The water that erupts at Old Faithful also tells tales of volcanic history.

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
We'll get back to the technique shortly, but let's jump to conclusions. Examinations of crystals shows that, over the past 2 million years, the giant Yellowstone volcano erupted at 700,000-year intervals.

Map of United States shows large oval, extending from Oregon to Texas, where ash was deposited after the eruption of 600,000 years ago. (click on image to see the map in detail - 80K)

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.

Inside the Yellowstone Caldera at the Grand Canyon of Yellowstone are these altered lavas. These rocks contain intact zircon and quartz that give hints about the volcano's "eruption schedule."
Courtesy Ilya Bindeman

When will the 'stone rock again?
Those thick ash deposits provided the grist for the multi-stage analytical technique that extracted, almost literally, jewels of knowledge from grains of sand. (OK, they were crystals of quartz or zircon, not sand, but we just renewed our poetic license.) The research developed at the intersection of two lines of evidence:

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?
Zircon "remembers" its birth courtesy of a radioactive clock that, once started, continues ticking essentially forever.

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 SHRIMP gives accurate dates from tiny crystals. This is the chamber where samples are whacked with high-speed ions.
Photo by Brad Ito. Courtesy USGS/Stanford University School of EarthSciences

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
The second wing of the research comes from the volcanic rock in which the crystals are embedded. Like most rock, this stuff turns into mud and clay when exposed to Yellowstone's hot water. During the immersion, the zircon-bearing rocks acquire oxygen with the isotope fingerprint of surface water.

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