Archaeologists long believed the volcano changed Sinagua culture. Yet because the date of the eruption remained elusive, they could not tell how fast those changes occurred. Ort, however, had a geological clock that let him make a better estimate. The Earth's magnetic north pole constantly drifts. Researchers have records of drift going back thousands of years made by measuring how certain minerals are magnetized to align with the magnetic poles as they settle in lake sediments. What Ort needed was a way to link these paleomagnetic measurements to the age of artifacts found in villages abandoned after the eruption. Ort...