The Pleistocene-age San Francisco Mountain (Peaks) is the largest volcano in the Plio-Pleistocene - Holocene San Francisco volcanic field of northern Arizona. This picture of the eastern slopes of San Francisco Mountains was taken from the meadow immediately east of Sunset Crater. San Francisco Mountain is a composite volcano comprising layers of weak, unconsolidated of volcanic tephra (ash, lapilli and bombs) alternating with competent, dense lava flows. Volcanic activity in the San Francisco Peaks began about 1.3 million years ago and ended about 100,000 years ago; making it the longest lived volcanic center in the field. According to Dr. Richard Holm (Northern Arizona University Professor Emeritus), the eruptive volume of the Peaks is approximately 90 cubic kilometers. A typical cinder cone has an eruptive volume of about 1 cubic kilometer. Thus, this single composite volcano contains the eruptive equivalent of about 90 cinder cones.