Around the year 1085, northeast of where Flagstaff sits today, the ground split open along a fissure nearly six miles long. Lava shot hundreds of feet into the air in what geologists call a curtain of fire. Ash and cinders covered hundreds of square miles, lava filled nearby valleys, and by the time it ended, a new cinder cone stood roughly 1,000 feet above the landscape. And people were living right there. Archaeologists call them Sinagua, part of the ancestral Puebloan world of northern Arizona. Hopi and Zuni oral traditions still preserve memories connected to the eruption. The people closest to the volcano appear to have escaped before the worst of it. Pithouses buried by the eruption were found with very few belongings inside, suggesting people had warning and time to leave. But somebody went back. Archaeologists have found pieces of volcanic rock with corn impressions pressed into them near Sunset Crater. Researchers believe corn was deliberately placed near active volcanic...