They started off looking for something totally different. For several sweaty weeks, Drew Coleman and Allen Glazner chipped rocks out of the ground in Yosemite National Park, hunting for the rare, dark remains that are bits of Earth’s melted mantle—chemically primitive stuff that hasn’t been tainted with a lot of other minerals and geological debris.
And when they lugged their heavy harvest back to the lab to determine just how old the rocks they’d gathered were, Coleman and Glazner thought they knew what they would find. But when the uranium-lead dating results on the rocks came through, they muttered, “Well, this can’t be right.”
Now, four years later, the strange results of their lab tests are still quaking controversy through the scientific world.
The rocks were pieces broken from a pluton—a city-sized blob of solid granite that gestates beneath volcanoes and eventually rises to the surface as the rocks above it erode. Yosemite’s Half Dome is one of them, and if Chapel Hill didn’t sit on top of its own resident pluton, Coleman and Glazner say, we’d be living in Chapel Valley.
Geologists have long thought that plutons were born of cooled magma deposits in only a few hundred thousand years—a geologic heartbeat. “Actually,” Coleman says, “turns out it’s more like several million years.”
Textbooks say that plutons are formed when magma accumulates in huge underground chambers, and any magma that leaks to the surface forms a volcano. Things can go two ways from there, the books say: either the magma nestles down to cool and eventually hardens into a granite pluton, or the roof of the chamber caves in and looses all hell into the atmosphere—a super eruption, in other words.
And this theory has led geologists to believe that every big, active volcano—such as Mounts Rainier, Fuji, and Pinatubo—broods over a huge chamber of red-hot magma. But geologists, for all their probing with seismic waves, have never been able to actually find one of these magma chambers; all they’ve found is what they assumed were the frozen remnants—the plutons.
Coleman and Glazner were expecting to find that their Yosemite rocks were all around the same age, having come from the same cooled magma chamber. And when they instead found that some were millions of years older than others, they went back for more samples. Sure enough, their results showed that the pluton underneath Yosemite was almost ten million years in the making.
Their results suggest that, contrary to popular belief, the vast majority of plutons were never completely molten. Rather, the earth’s consistent hiccups and growls send injections of molten magma into a pluton, heating and shifting the granite and causing it to grow little by little over long periods of time.
Imagine old-fashioned candle-making, where tapers are dipped over and over again into melted wax, the outer layers melting a little into the new ones, but then cooling and solidifying together. Plutons are just like that, only upside-down.
With each injection, groundwater around a pluton burns off as steam, which pulls more water up. And as the new water flows up through crust, it picks up ore metals such as copper, silver, and gold (plutons are the ultimate source of all gold, Glazner says). Some volcanic gurgles even disgorge diamonds from two hundred kilometers below. Then, as the water boils away, the metals create ore deposits concentrated in the pluton and, like tree rings, show us how old the pluton is and how it’s changed over time.
Coleman and Glazner’s new model shows that not all active volcanoes are potential super eruptions. And that’s good news, given that the next super eruption could mean the end of all human civilization. Geological events of that magnitude would spew out carbon dioxide and poisonous gases, devastate the global climate, and bury continent-sized areas beneath suffocating blankets of volcanic ash.
Earth’s most recent super eruption was 75,000 years ago. It came from the volcano Toba in Indonesia, and released about 2,800 times as much magma as the 1980 eruption of Mount St. Helens. It was a close call, Glazner says; there’s evidence that Toba’s super eruption created a bottleneck in human evolution, cutting the human population down to anywhere from 1,000 to 10,000 breeding pairs.
“There’s been nothing even remotely super-eruption-sized in recorded history, the past ten thousand years,” Glazner says. So it’s comforting (sort of) that Coleman and Glazner say that while super eruptions do happen, they’re the rare exception, not the rule.
Coleman and Glazner’s results created a hullabaloo in the geological community in 2003, and even now in 2007, not everyone’s convinced. “Dogma dies hard,” Glazner says. “A lot of the old guard don’t like this.”
But other geologists are getting similar pluton age results. It’s been a slow change, but scientists are beginning to reexamine dozens of other things we thought we knew about the earth’s skin and bones, and the way the earth matures. For example, how magma shoulders solid rock aside, and how the continents slide across the earth. And Coleman and Glazner are now watching their students rewrite the textbooks.
As an orientation for their fledgling geology students, Coleman and Glazner try to help them think in terms of Earth’s lifetime by taking them on a geologic time walk across campus.
Each of their steps on this walk, Glazner says to his freshmen, is ten million years. Then they begin their four-and-a-half-billion-year stroll, all the way from the steps of Wilson Library to Franklin Street.
Along the way, Glazner has taped down pictures of one-celled organisms, dinosaurs, and mammals, all according to when they showed up in time. There are only six and a half steps after the dinosaurs before they get to Franklin Street, he says, and human history is about as thick as one of the blue lines on the students’ notebook paper.
The introduction is important, Coleman says, because most of their students are still trying to wrap their heads around their own twenty or so years. “Ten million years,” he says. “What’s ten million years? Ten million years is nothing. That’s one step.”
Drew Coleman is an associate professor of geology and Allen Glazner is a professor of geology, both in the College of Arts and Sciences. They received funding from the National Science Foundation. John Gracely, a master’s student who graduated in 2007, dates and maps the distribution of pluton rocks in the field. Doctoral student Jesse Davis researches plutons’ histories of heating and cooling, and master’s student Breck Johnson uses Coleman and Glazner’s new model to determine the origins of huge potassium feldspar megacrysts in plutons. Glazner’s undergraduate class researched pluton chunks on the Carolina North property, and presented their findings at the Undergraduate Research Symposium this year.