Geologic field-trip guide to Mount Shasta Volcano, northern California

Scientific Investigations Report 2017-5022- K3
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Abstract

The southern part of the Cascades Arc formed in two distinct, extended periods of activity: “High Cascades” volcanoes erupted during about the past 6 million years and were built on a wider platform of Tertiary volcanoes and shallow plutons as old as about 30 Ma, generally called the “Western Cascades.” For the most part, the Shasta segment (for example, Hildreth, 2007; segment 4 of Guffanti and Weaver, 1988) of the arc forms a distinct, fairly narrow axis of short-lived small- to moderate-sized High Cascades volcanoes that erupted lavas, mainly of basaltic-andesite or low-silica-andesite compositions. Western Cascades rocks crop out only sparsely in the Shasta segment; almost all of the following descriptions are of High Cascades features except for a few unusual localities where older, Western Cascades rocks are exposed to view along the route of the field trip.

The High Cascades arc axis in this segment of the arc is mainly a relatively narrow band of either monogenetic or short-lived shield volcanoes. The belt generally averages about 15 km wide and traverses the length of the Shasta segment, roughly 100 km between about the Klamath River drainage on the north, near the Oregon-California border, and the McCloud River drainage on the south (fig. 1). Superposed across this axis are two major long-lived stratovolcanoes and the large rear-arc Medicine Lake volcano. One of the stratovolcanoes, the Rainbow Mountain volcano of about 1.5–0.8 Ma, straddles the arc near the midpoint of the Shasta segment. The other, Mount Shasta itself, which ranges from about 700 ka to 0 ka, lies distinctly west of the High Cascades axis. It is notable that Mount Shasta and Medicine Lake volcanoes, although volcanologically and petrologically quite different, span about the same range of ages and bracket the High Cascades axis on the west and east, respectively.

The field trip begins near the southern end of the Shasta segment, where the Lassen Volcanic Center field trip leaves off, in a field of high-alumina olivine tholeiite lavas (HAOTs, referred to elsewhere in this guide as low-potassium olivine tholeiites, LKOTs). It proceeds around the southern, western, and northern flanks of Mount Shasta and onto a part of the arc axis. The stops feature elements of the Mount Shasta area in an approximately chronological order, from oldest to youngest.

Suggested Citation

Christiansen, R.L., Calvert, A.T., and Grove T.L., 2017, Geologic field-trip guide to Mount Shasta volcano, northern California: U.S. Geological Survey Scientific Investigations Report 2017-5022-K3, 33 p., https://doi.org/10.3133/sir20175022K3.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Preface
  • Contributing Authors
  • Introduction
  • Tectonic Setting
  • Regional Volcanism
  • Eruptive History of Mount Shasta
  • Parental Magmas and Petrologic Evolution of the Mount Shasta Suite
  • Glacial Geology
  • Volcano-Related Hazards
  • Road Log
  • References Cited

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Geologic field-trip guide to Mount Shasta Volcano, northern California
Series title Scientific Investigations Report
Series number 2017-5022
Chapter K3
DOI 10.3133/sir20175022K3
Year Published 2017
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Volcano Science Center
Description ix, 33 p.
Country United States
State California
Other Geospatial Mount Shasta
Online Only (Y/N) Y