Fifty-five pieces of lava with impressions of prehistoric corn have recently been recovered from NA 860, a small habitation site near Sunset Crater Volcano in northern Arizona. Archaeological, geological, and botanical information suggest that husked ears of corn were deliberately placed in the lava's path when the volcano erupted in the mid-to-late eleventh century A.D. Over 40 kg of basalt lava containing the hardened corn casts were then taken to NA 860 located 4 km away from the lava flow. At the site, the rocks underwent lithic reduction to expose the casts. We suggest that these "corn rocks" are indicative of ritual practices, perhaps serving as an offering made to appease the forces responsible for the eruption. Although both prehistoric and modern offerings are commonly associated with volcanoes in other parts of the world, this is the first evidence from the Southwest United States of possible ritual behavior related to volcanism.