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A geologic field guide to S P Mountain and its lava flow, San Francisco Volcanic Field, Arizona

August 19, 2021


We created this guide to introduce the user to the San Francisco Volcanic Field as a terrestrial analog site for planetary volcanic processes. For decades, the San Francisco Volcanic Field has been used to teach scientists to recognize the products of common types of volcanic eruptions and associated volcanic features. The volcanic processes and products observed in this volcanic field are like those observed on lunar and Martian surfaces. As a result, this region has been a favored location for training National Aeronautics and Space Administration astronauts and engineers since the Apollo missions.

Though the San Francisco Volcanic Field has more than 600 volcanic vents and flows, this guide will focus on S P Mountain (known locally as S P Crater, located ~30 miles north of Flagstaff, Arizona), one of the best preserved and most accessible of the volcanic cones and lava flows. S P Mountain presents both major types of basaltic eruptions—explosive and effusive—as well as some commonly associated tectonic landforms.

We assume that the user has a basic understanding of geologic concepts and terminology. For more specialized terminology, we include tables showing the classification scheme for lava compositions, styles of eruptions, and tephra sizes (tables 1, 2, and 3). If a further introduction or refresher in volcanological terminology is desired, we suggest reviewing such terms on the U.S. Geological Survey Volcano Science Center’s online glossary (

One term requires clarification at the start of this guide—the term cinder. The terms cinder and cinder cone are widely used to describe the material and edifice produced by lava fountains. However, the term comes from the mining and construction industries and has no clear or formal definition. The international committees in geology and volcanology have chosen the term tephra to be the general term to describe pyroclasts (material ejected through a volcanic explosion or from a volcanic vent). Therefore, in this guide, we use the term tephra rather than cinder.

This guide is outlined as follows:

  • A brief tour of volcanism across the solar system
  • A brief geologic history of the Colorado Plateau and San Francisco Volcanic Field
  • Background on distributed volcanism and S P Mountain
  • Driving directions and field stops
  • Questions for discussion
Each field stop includes a brief description, learning goals, tasks, and a summary of key points. At the end of the field guide are discussion points and questions that will ask the user to consider what they have observed and learned and how such knowledge can be used to better our understanding of geologic processes on other planetary bodies.
Upon the completion of this field guide, we expect the user to:
  • Have a basic understanding of the volcanic processes relevant to S P Mountain and its lava flow.
  • Be able to identify different volcanic textures that are associated with tephra cones.
  • Be aware of the different observations one can make at different scales (for example, observing lava flow morphology from aerial or satellite imagery versus tephra characteristics in the field).
Publication Year 2021
Title A geologic field guide to S P Mountain and its lava flow, San Francisco Volcanic Field, Arizona
DOI 10.3133/ofr20211072
Authors Amber L. Gullikson, M. Elise Rumpf, Lauren A. Edgar, Laszlo P. Keszthelyi, James A. Skinner, Lisa Thompson
Publication Type Report
Publication Subtype USGS Numbered Series
Series Title Open-File Report
Series Number 2021-1072
Index ID ofr20211072
Record Source USGS Publications Warehouse
USGS Organization Astrogeology Science Center; Volcano Science Center