SNAKE RIVER VOLCANICS - IDAHO
FIELD TRIP STOPS – VOLCANICS IN SOUTHEASTERN IDAHO.
LOCATION: Southeastern Idaho, between about 20 to 50 miles northeast of Idaho Falls.
GEOLOGIC FEATURES: Basaltic Lava; Rhyolitic Tuff; Caldera; Mesa Falls Eruption
DESCRIPTION: The Snake River Plain runs from northwest Wyoming to the Idaho/Oregon border and is composed of volcanics ranging in age from 14 myo in the west to the current volcanic activity in northwest Wyoming. The progression in age represents the movement of the North American Plate westward away from the Yellowstone Hot Spot.
Volcanics in this part of southeast Idaho are as young as Pleistocene (e,g, Menan Buttes – 10,000 ya) and as old as 2.1 my, with the volcanics of the Mesa Falls Eruption being 1.3 myo. Subsequent younger lava flows have found their way into the Mesa Falls area only to be eroded further by the action of the Henry’s Fork River.
The 1.3 myo Henry’s Fork Caldera (18 x 23 mi), seen in the Mesa Falls area, is the second most recent caldera-forming eruption associated with the Yellowstone Hot Spot. It is thought to have ejected 280 cubic km of material. The Huckleberry Ridge Tuffs (2.1 mya) represent the third most recent eruption while the Lava Creek Tuff (640 kya) was the most recent eruption over the Yellowstone Hot Spot.
(1) Given the distance from Yellowstone N.P. to the volcanics at the Idaho/Oregon/Nevada confluence (400 miles; 14 myo),
calculate the rate of movement of the North American Plate over the Yellowstone Hot Spot.
(2) How does a Caldera differ from a Crater.
(3) Define a Tuff.
(4) CHALLENGE: Name the last three Supereruptions occurring at Yellowstone. Construct a Table that provides the Age and amount of Material Ejected from each.
-Repanshek, Kurt. A Refreshing Yellowstone National Park Side Trip: Mesa Falls. National Parks Traveler. Accessed on 4/24/20: https://www.nationalparkstraveler.org/2016/06/refreshing-yellowstone-national-park-side-trip-mesa-falls
Figure 1 - The Menan Buttes are two of the worlds largest Volcanic Tuff Cones. These cones form when rising magma mixes with phreatic water or a shallow body of water to produce ash that eventually is compacted to form Tuff that is typically well-bedded. The North Butte (seen here) is a National Natural Landmark that formed in the Late Plesitocene (about 10,000 ya).
Figure 2 - Lava flows are plentiful in the Snake River Plain. Here, lava is well-exposed by erosion of Fall River.
Figure 3 - Volcanics are not always Basaltic, but can be Rhyolitic (more silica-rich). Here are Rhyolitic Tuffs and Welded Tuffs in which ash is thick and hot enough to coalesce into a particularly resistant rock.
Figure 4 - Located within the Mesa Falls Caldera we see Lower Mesa Falls created by the Henry's Fork River having eroded through the relatively soft Mesa Falls Tuff plunges 65 ft. over a resistant bed of basaltic lava.
Figure 5 - After falling over the Lower Mesa Falls, the Henry's Fork immediately bends to the south (left).
Figure 6- Still within the caldera, the river continues to flow south following the base of the resistant igneous layer.