12

Humphreys Peak

Coconino County, Arizona

12,633 ft

3,851 m

Morning light warms the northeastern flanks of San Francisco Mountain. Viewed from Bonito Park in Sunset Crater National Monument, Humphreys Peak, the highest point on San Francisco Mountain's eroded rim, is the distant summit second from the right.

Morning light warms the northeastern flanks of San Francisco Mountain. Viewed from Bonito Park in Sunset Crater National Monument, Humphreys Peak, the highest point on San Francisco Mountain’s eroded rim, is the distant summit second from the right.

 

Humphreys Peak is the highest of several summits perched along the eroded rim of San Francisco Mountain, which rises 5,600 feet (1,700 m) above the nearby city of Flagstaff. San Francisco Mountain is a dormant stratovolcano, towering over the lava domes, lava flows, and hundreds of cinder cones of the San Francisco volcanic field. Volcanic activity in this area began around 6 million years ago, as magma moved upward along faults and fractures and then broke through the ground surface in eruptions of lava and cinders. The volcanic cone of San Francisco Mountain began forming around 2.8 million years ago; andesite and dacite lava flows, interlayered with lahar and cinder deposits, eventually piled up to form a volcano whose summit may have reached an elevation of 16,000 feet (4,900 m). Eventually the volcanic cone was breached on its northeastern side, possibly by stream erosion or perhaps by a landslide-triggered lateral eruption similar to the May 18, 1980, eruption of Mount Saint Helens. Subsequent stream and glacial erosion widened and deepened the breach to form the scenic Inner Basin. The most recent eruption of San Francisco Mountain occurred around 220,000 years ago, when rhyolite extrusions built the rounded dome of Sugarloaf near the mouth of Inner Basin.

The eroded edge of a dark gray and reddish brown andesite lava flow overlies a lahar deposit near the 11,800-foot saddle on the western rim of San Francisco Mountain. The tilting of the rocks—down to the right—reflects their original dip as they flowed down the southwestern flank of the volcano during its cone-building eruptions.

The eroded edge of a dark gray and reddish brown andesite lava flow overlies a lahar deposit near the 11,800-foot saddle on the western rim of San Francisco Mountain. The tilting of the rocks—down to the right—reflects their original dip as they flowed down the southwestern flank of the volcano during its cone-building eruptions.

 

The base of San Francisco Mountain's eroded volcanic cone is evident when looking northward at the mountain from the Fort Valley area west of Flagstaff. Projecting the lower slopes upward suggests the summit of the volcano may have reached an elevation of about 16,000 feet (4,900 m).

The base of San Francisco Mountain’s eroded volcanic cone is evident when looking northward at the mountain from the Fort Valley area west of Flagstaff. Projecting the lower slopes upward suggests the summit of the volcano may have reached an elevation of about 16,000 feet (4,900 m).

  • LAT./LONG. 35.346°N / 111.678°W
  • Land Status/Administration
    Coconino National Forest
  • Physiographic Province
    Colorado Plateaus
  • Representative Rocks:
    • Class: Igneous
    • Type: Andesite
    • Age: Plio-Pleistocene
    • Landform: Eroded volcanic cone