The San Francisco Volcanic Field in northern Arizona is full of spiky lava flows. Some seem as fresh as if they were laid down yesterday. But how old are they, really? Scientists are now dating these ancient volcanic eruptions with a method that involves a curious kind of clock—the cosmic rays emitted by stars.
Cosmic rays bombard the surface of the Earth constantly. These high-energy particles are expelled from exploding stars called supernovas. When those rays strike a fresh lava flow, they shatter the elements within the rock minerals.
Magnesium, for example, splits into a particular form of helium. The helium atoms are like sand piling up in the bottom of an hourglass. By counting them, scientists can find out when a volcano erupted and exposed that earthbound lava to the influence of distant stars.
A team of researchers from Arizona, Utah, and New Hampshire used this technique, called “cosmogenic dating,” at two cinder cones near Flagstaff. They took rock samples from Strawberry Crater and SP Crater, crushed them in a laboratory, and examined the helium atoms with a spectrometer. The results show the two volcanoes are 55,000 to 60,000 years old.
There are other methods for dating an eruption--scientists can measure when dust, laid down on top of new lava and then buried, was last exposed to light. They can study the decay of radioactive elements in rocks. Or, they can learn from the stories of indigenous people, passed down through generations.
But with new dating methods, they may one day even be able to plan for the next eruption.