Agathla Peak is an eroded volcanic plug consisting of volcanic breccia cut by dikes of an unusual igneous rock called minette. It is one of many such volcanic diatremes that are found in Navajo country of northeast Arizona and northwest New Mexico. Agathla Peak and Shiprock in New Mexico are the most prominent. These rocks are part of the Navajo Volcanic Field, in the southern Colorado Plateau. Ages of these minettes and associated more unusual igneous rocks cluster near 25 million years.