Volcanology and Petrology of Interbedded Andesitic Lava Flows and Volcaniclastic Rocks from Washburn Volcano, YNP

Field and Laboratory Project - Volcanology and Petrology of Interbedded Andesitic Lava Flows and Volcaniclastic Rocks from Washburn Volcano, Yellowstone National Park

Todd Feeley, Montana State University

Intended Audience: This field trip was part of the 2003 Teaching Petrology in the 21st Century workshop. It is designed for use by undergraduate or graduate courses in petrology.


Yellowstone National Park, Mount Washburn, Lamar River Formation. We will examine outcrops exposed along the Mt. Washburn Trail, which begins at Dunraven Pass (8850') along the Grand Loop Road, north of Canyon Junction. The trail climbs 1,393 feet slightly over 3 miles to the summit of Mt. Washburn (10243').


What follows is an example of a three part exercise for undergraduate petrology students involving volcanic and shallow intrusive rocks in the Washburn Range, Yellowstone National Park. We will loosely follow Part 1, although Parts 2 (petrology) and 3 (geochemistry) are also included. The exercise is largely based on a recent study by Feeley et al. (2002), although on this trip we will only examine stratigrahpically high rocks on Mount Washburn proper; stratigraphically lower rocks to the southwest beneath Dunraven and Hedges Peaks are off-road and off-trail


We anticipate that hikers will start from Dunraven Pass at 8:00 AM. Hikers should return to the vans at 12:00 PM, from there we will go to a picnic area for lunch. All hikers must therefore turn around and head for the vans no later than 10:45. Because of these strict time limitations, there will probably not be sufficient time to examine the rocks in as much detail as one would hope. The trip is thus largely a self guided tour. It will, nevertheless, give you an opportunity to examine several rock types associated with calc-alkaline composite cones and provide a spectacular view of the Yellowstone Caldera, weather permitting.


  1. To study intraflow characteristics of andesite lava flows and flow breccias;
  2. to study sedimentological characteristics of volcanic debris flows and related sedimentary deposits;
  3. to distinguish lava flows from dikes and sills;
  4. to interpret the volcanic feature represented by Mount Washburn; and
  5. to interpret the petrology and geochemistry of calc-alkaline volcanic rocks in order to discover their origins.

Notes and Tips:

Because of National Park regulations, no sample collection is allowed. Bear in mind that the Washburn trail is the most heavily used backcountry trail in Yellowstone, and rock picks are quite disturbing for some guests. It is therefore probably best to leave these in the vans. The hike to the summit is not particularly physically demanding, although it is at altitude: participants should thus consider their physical condition. At the very least, bring water, snacks, sun screen, and rain gear.

Assessment and Evaluation:

The goals of this exercise have been met upon satisfactory completion of the following final products:

  1. Sketches, descriptions, and interpretations (e.g., lava flow, dike, sill, debris flow, stream flow, flow breccia) of features of rock units at stations listed below and illustrated on the accompanying map (Fig. 1). The descriptions should include thickness of individual units. In addition, for dense, nonfragmental units (e.g., lava flows and tabular intrusions), include features such as chilled zones, distributions of vesicles, and a hand specimen petrographic description. For volcaniclastic units include clast sizes and composition, matrix texture, depositional structures. Answers to specific questions for stations.
  2. Petrographic descriptions of rock units (note: we will complete this back in laboratory on thin sections cut from previously collected Washburn samples).
  3. Interpretation of geochemical data for lava flows from Washburn volcano (note: we will complete this as a class project back at the university).

Materials and Handouts: