Humphreys Peak
       7½’ quadrangle: Humphreys Peak, AZ
       Coconino County
       Kaibab National Forest

Volcanic Igneous
12,633 ft (3,851 m)

Bedrock: Humphreys Peak Olivine Andesite Pleistocene (~434,000 BP)

Olivine andesite, overlying hypersthene dacite, overlying ~916,000 BP hypersthene-pyroxene andesite, in a stratovolcano called San Francisco Mountain, which includes Agassiz and Fremont Peaks. The three rock units of Humphreys Peak were likely derived from the same magma by fractional crystallization. The mountain dominates the San Francisco Volcanic Field, which consists of more than 600 volcanic landforms, many of them basaltic cinder cones. They were erupted onto the Permian Kaibab Limestone during the Pleistocene over a large area north of Flagstaff.

For a discussion of all the volcanic highpoints, click here.

Surfical Geology: Many of the volcanoes in the volcanic field have changed little since their formation, due to the arid climate of the region. Sunset Crater erupted as recently as 1064 AD. Of course San Francisco Peaks has been altered more by erosion than the younger landforms, but the present rugged topography likely resulted from an explosive eruption that destroyed the upper part of the edifice. The Navajo call San Francisco Peaks "Doko’oosliid", or Abalone Shell Mountain, and it is one of the four sacred peaks surrounding their homeland.

Soil Series: No soil survey has been published for Coconino County.

Selected References:

  • Newhall, Christopher G., G.E. Ulrich and E.W. Wolfe, 1987, Geologic map of the southwestern part of the San Francisco Volcanic Field, north-central Arizona: U.S. Geological Survey Miscellaneous Field Map MF-1958.
  • Priest, Susan S. and others, 2001, The San Francisco Volcanic Field, Arizona: U.S. Geological Survey Fact Sheet 017-01. See web site:
  • Weinrich-Verbeek, K.J., 1979, The petrogenesis and trace-element geochemistry of intermediate lavas from Humphreys Peak, San Francisco Volcanic Field, Arizona: Tectonophysics v.61, no.1-3, p.103-129.

Other suggested sources of information:

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