The Oldest Rock in Alaska Calls Iditarod Home
Tom Buntzen keeps a rock in his office that's a little different from the others. It's gray, fits in his palm like a baseball, and has a few lichens still clinging to it. What separates it from others is that it could be the oldest rock in Alaska.
Buntzen, a geologist and operator of Pacific Rim Geological Consulting in Fairbanks, collected the rock near the ghost town of Iditarod in 1983, when he worked for the Alaska Division of Geological and Geophysical Surveys. Buntzen, Marti Miller, a research geologist with the U.S. Geological Survey, and others mapped the geology of the Iditarod quadrangle. They found a pocket of old rocks on a knoll they used as a helicopter pad. On the exposed bump, Buntzen and Miller noticed something weird. From the fabric and textures of the rock formation, they knew the rock was metamorphic, that sometime over its history this rock had been changed by the heat and pressure of being buried deep underground. The geologists did not expect to find such high-grade metamorphic rock in Iditarod.
They broke off a few samples with a hammer, then spent the next week flying to different spots within the Iditarod quadrangle. They were never to match the drama of their first find. Miller sent some rock samples to Don Turner, former head of the geochronology lab at the Geophysical Institute. In the lab, researchers can tell the age of rocks because some atoms of potassium within rocks decay over time to form argon gas, which becomes trapped in rock crystals. The older the rock, the more argon is trapped.
Turner heated the rocks to about 2,900 degrees Fahrenheit and released the argon, which he directed into an instrument called a mass spectrometer. The mass spectrometer allowed him to tell when a rock formed or when it was re-heated by forces within the earth.
Turner knew Miller and Buntzen had picked up a special rock. While most rocks in Alaska formed after 500 million years ago, the rocks from Iditarod were at least one billion years old. Further tests using a different technique on zircon crystals revealed the rocks were more than two billion years old. It didn't make sense. Why was an island of ancient rocks found in a sea of much younger rocks?
Miller pursued the question of what the rocks were doing near Iditarod, a town that gave its name to the 1,049-mile sled dog race from Anchorage to Nome. (Buntzen said the rock patch, less than 10 miles from the ghost town, is actually crossed by Iditarod sled dogs every other year, when the race follows its southern route). Earlier, a cluster of rocks about 150 miles southwest of Iditarod were discovered to be more than 2 billion years old. Both rock clusters likely belonged to a craton--ancient rocks that formed the core of a continent. The nearest craton to Iditarod is the Canadian shield, found today in Canada's plains and the Northwest Territories. Possibly, Miller said, the Iditarod rocks were broken away from Canada and carried very slowly by the movement of faults and Earth's plates to Alaska. Buntzen said another possibility is that the rocks might have originated in Siberia, in an area with similar rock about 600 miles northeast of Magadan.
The only thing for sure is that the Iditarod rocks are out of place, and Buntzen said geologists probably won't find anything older in the state. It's a neat feeling to hold something that old in your hands. Even Miller, a geologist for nearly 18 years, still feels a certain reverence for the oldest rock in Alaska. In her Anchorage office, she keeps a chunk of the Iditarod rock the size of her fist. "Every once in a while I have to reach over and pick it up," she said. "Just to touch it, to feel it."
