Field Experience in Volcanology, Summer 2010
Yellowstone and the Snake River Plain
Geology students from GWU participated in the initial offering of our new Field Experience in Volcanology course in July 2010, travelling to the Yellowstone Plateau volcanic field and neighboring Snake River plain for seven days of field-based learning about volcanic rocks and processes. Students spent most of their time in and around Yellowstone National Park, but also examined Precambrian basement rocks in the Gallitan River valley and Holocene basalts at Craters of the Moon National Monument. The purpose of the trip was to obtain practical experience in the field geology of the Yellowstone area, the site of three major eruptions during the past 2 million years, and to examine field evidence for past geologic activity including repeated eruptions of flood basalts, magmatic arc development, and Laramide orogenesis. This page presents some of the highlights of the trip.
Photo #1A: Students examining structurally complex, moderate- to high-grade metamorphic rocks in the Gallitan River valley north of Yellowstone. These lithologies, which include amphibolite, schist, and migmatite, were strongly deformed under ductile conditions, as indicated by the large boudins of amphibolite visible in the photo.
Photo #1B: Granitic pegmatite that crosscuts the metamorphic lithologies contains very coarse-grained crystals of pink alkali feldspar, the growth of which likely post-dated metamorphism and ductile deformation.
Photo #2: Various facies of lithic lapilli tuffs associated with eruption of the 0.6 Ma Lava Creek Tuff were investigated up close after climbing steep talus slopes. These deposits are located just outside of the main caldera which is represented by the hummocky surface covered by post-caldera rhyolite lava flows in the background.
Photo #3A: The prominent face of the rhyolite flow at Obsidian Cliff was visited in order to provide opportunities for students to observe the textures characteristic of far-travelled, silicic lava flows. This locality, which is noteworthy from both geological and archeological perspectives, provided Native Americans with obsidian since pre-historic times.
Photo #3B: Features that are visible in the rhyolitic lava flow at Obsidian Cliff include pristine volcanic glass (black), flow banding (marked by trails of small, gray and pink spherulites), coarse pink spherulites, and lithophysae (large pink cavities) formed by degassing in the viscous lava.
Photo #4: Students inspecting the steep front face of a large lava flow of the Plateau Rhyolite that extended beyond the main caldera. Such voluminous flows represent the fourth, but perhaps not the last, major eruptive episode at Yellowstone. The pockmarked surfaces result from numerous lithophysae that formed during cooling and degassing of the viscous lava.
Photo #5: No visit to Yellowstone would be complete without viewing an eruption of the Old Faithful geyser! The Mallard Lake resurgent lava dome forms the high ground in the background and is partially responsible for channelling groundwater into the subterranean plumbing system of this famous hydrothermal feature.
Photo #6A: Snow is rarely completely absent from high elevations in July in the Yellowstone region due to the continual, thermally induced uplift of the area that results from the influence of the underlying mantle hotspot. This snowfield was located on the flank of Mount Washburn, an eroded Eocene stratovolcano that sits astride the Pliocene-Pleistocene Yellowstone volcanic field. Groundwater generated largely by snow melt in the Yellowstone area is partially responsible for making agriculture possible far to the southwest in much drier parts of Idaho.
Photo #6B: What do students do when they come unexpectedly upon a mountain snowfield in early July? Make snowballs, of course!
Photo #7A: Spatter cones developed along a fracture zone in Craters of the Moon National Monument are part of the terminal stage in magmatic activity associated with hotspot volcanism in the Yellowstone-Snake River Plain region. According to recent models, such voluminous basaltic lavas might someday cover the entire Yellowstone region.
Photo #7B: Students standing at the oxidized summit of a Holocene scoria cone in Craters of the Moon National Monument. An older, partly vegetated cone is located in the background to the left; lava fields of the Snake River plain are on the right.
Photo #8A: Concentrations of petrified trees and transported logs of Eocene age are a major source of geologic interest in the Yellowstone region. This pair of in situ stumps, called the “Two Towers” by Lord of the Rings fans in the group, occur in lahar deposits of the Lamar River Formation on Specimen Ridge in the northeast portion of the National Park.
Photo #8B: An important feature of the Lamar River deposits is the presence of a fine-grained layer of volcaniclastic deposits and soil interspersed between thick mudslide deposits that contain the petrified trees at Specimen Ridge. The occurrence of such deposits indicates that mudslides which travelled down the slopes of Mount Washburn in the Eocene occurred during infrequent intervals, between which soils were generated in sufficient thickness to support forest vegetation such as the giant upright petrified tree in the photograph.
Photo #8C: Students celebrating after a steep climb up Specimen Ridge. Multiple terraces of the Lamar River valley are visible in the near background; mountains of the Absaroka Range and Bearthooth Plateau are in the distance.
Photo #8D: What would be your first impulse upon seeing an in situ petrified stump of a 30 million year old tree for the first time?
Photo #9A: Earthquake Lake was created when a powerful temblor shook the region in August of 1959. The lake was formed when an enormous landslide, faintly visible in the center distance, blocked the local river drainage. Such events are a testament to the ongoing tectonic activity of the Yellowstone region.
Photo #9B: The weathered scarp of the 1959 Hebgen Lake earthquake is still visible. Here, the women are standing on the upthrown block whereas the guys are standing on the downthrown block, separated by a normal fault.
Photo #10: The Yellowstone area is a splendid natural laboratory for geology and the natural sciences. As always, however, when visiting a new area, it is important to meet the year-round residents to obtain their perspectives……