BRIEF ANALYSIS OF THE SNAKE RIVER PLAIN WITH EMPHASIS ON THE CRATERS OF THE MOON

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BRIEF ANALYSIS OF THE SNAKE RIVER PLAIN WITH EMPHASIS ON THE CRATERS OF THE MOON

INTRODUCTION

The Snake River Plain (SRP) is an arc that extends 400 miles from east to west across southern Idaho (Figure 1). It is 50 miles wide at its narrowest point to the east and 15 miles wide at the Yellowstone Plateau. There are many different theories for the origin of the SRP including a depression, a down warp, a graben, and a rift. The east and west portions of the SRP are uniform in topography; however, there are major structural and geophysical differences. The geological and geophysical anomalies vary from the east section of the SRP to the center section and on to the western section.




The western section of the SRP is a fault-bound basin filled by interbedded volcanic rocks and lakebed sediments of Tertiary and Quaternary age. Well-drilling has produced core samples showing 11,150 feet of alternating sediment and volcanic rocks before terminating into granite. Geologists speculate the granite could be a southern extension of the Idaho Batholith (Link and Phoenix, 14). The SRP consists mainly of basaltic lava less than 700 Ka with most of the vents east of Twin Falls in the eastern Snake River Plain. Much of the central SRP is covered by volcanics, including the Craters of the Moon National Monument. This paper will briefly describe the SRP and its topography, with the main focus concentrating on the Craters of the Moon National Monument. It will discuss the sources of the flows, types of lava extruded, stratigraphy, reoccurrence intervals, and when the next eruption is expected.

Figure 1. Picture of the Snake River Plains extending from east to west across the southern portion of Idaho (from Digital Atlas, 000).

THE SNAKE RIVER PLAIN

According to Alt and Hyndman (15), the SRP is made up of layers of rhyolite covered by a basalt crust. The eastern SRP and the western SRP are very different stratigraphically, even though they look very similar on the surface. The SRP is the track of a hot spot, a plume of magma that has worked its way to the surface through the continental plate. The initial lava erupted is rhyolite, which is very viscous and therefore leads to a violent eruption. The chemical and mineral composition of the magma changes with each eruption and thus the viscosity decreases, changing the eruption characteristics to the more effusive basaltic magma. This is why the layers are rhyolite covered by basalt. Traveling from east to west along the arc of the SRP to the western tip cinder cones and extinct rhyolite volcanoes can be seen all along the way, such as Big Southern Butte, Middle Butte, East Butte, and the Menan Buttes (Figure ).

The three buttes that can be seen when traveling west to Arco on Highway 0 are East Butte, Middle Butte, and Big Southern Butte (Link and Phoenix, 14). All three are rhyolite domes. Big Southern Butte has been dated at about 00 Ka, East Butte at about 600 Ka, and Middle Butte has not been dated because the rhyolite did not break through the basalt crust. The stratigraphy of the central and eastern portions of the SRP are Quaternary and Tertiary sediments (QTs) up to 00 feet (ft) thick, tertiary basalt (Tb) up to 1000 ft thick, quaternary basalt (Qb) and quaternary tertiary basalt up to 6000 ft thick with some thin layers of quaternary wind blown (Qw) and alluvial (Qa) deposits up to 40 ft thick (Figure ). Upon reaching the eastern end of the SRP at the Island Park and Yellowstone Park areas, the calderas become noticeable to an aware traveler. The steep climb that eventually flattens out is one indicator, and the noticeable bowl-shape of the two calderas is another. The Island Park Caldera, thought to be the largest symmetrical caldera in the world, was created by alternating eruptions of rhyolite and basalt from vents along the caldera floor. It is an elliptical collapse structure 18 to miles in diameter located in the center of a rhyolite shield. The Yellowstone plateau is underlain by Quaternary rhyolite covered by wind blown loess.

Figure . Cross-section of the Snake River Plain (from Digital Atlas, 000).

CRATERS OF THE MOON

Craters of the Moon National Monument is the product of basaltic volcanic activity dated between 15 Ka and 1 Ka that quit erupting about Ka. It is located approximately 18 miles southwest of Arco on Highway 0/6, and covers approximately 1600 square kilometers (Figure 4). It is very intriguing to look at with its aa lava (pronounced “ah-ah”), pahoehoe lava (pronounced “pah-hoy-hoy”), and lava bombs. Pahoehoe lava is a relatively smooth, ropy lava that was less viscous when extruded. This low viscosity is what gives this lava the smooth, pleated texture seen at the Craters lava field. Lava bombs come in three categories spindle bombs; ribbon bombs; and breadcrust that ranges in length from ½ inch to ft, and form as airborne blobs of molten lava cool and harden as they fall to the earth. Nearly every type of lava feature associated with basaltic volcanism can be found at Craters of the Moon. There are cinder cones, lava tubes, spatter cones, and tree molds to be seen, allowing the geologist a wide variety of volcanics to study. Lava tubes are formed when the lava flows fluidly along the ground and the outer layer cools and hardens, but the molten lava inside continues to flow, eventually draining out leaving a hollow tube or cave in its wake. The Craters of the Moon lava flows originated from the Great Rift, which is a series of north-northwest trending fractures extending 50 miles from the north margin of the eastern SRP Plain south to the Snake River (Hughes et al., 1).

Figure 4. Craters of the Moon Geologic map (modified from Digital Atlas, 000).

CONCLUSION

The SRP is an anomaly that has intrigued geologist for decades. With its complex lava flows, varied stratigraphy, and series of caldera complexes, it will keep geologists busy for decades yet to come. There is, however, an inherent danger in this area due to the possibility of future eruptions of the Yellowstone Caldera (~700 Ka recurrence interval) and the very real possibility of the Craters of the Moon erupting again in the near future (~ Ka-.5 Ka recurrence interval).

REFERENCES

Alt, D., and Hyndman, D. W., 15, Northwest Exposures A Geologic Story of the Northwest Missoula, Mountain Press Publishing Company, 44 p.

Digital Atlas, 000, available URL http//imnh.isu.edu/digitalatlas/, accessed May 8, 00.

Hughes, S. S., Smith, R. P., Hackett, W. R., and Anderson, S. R., 1, Mafic Volcanism and Environmental Geology of the Eastern Snake River Plain, Idaho, in Hughes, S. S. et al, ed., Guidebook to the Geology of Eastern Idaho Idaho State Museum of Natural History, Pocatello, Idaho, Guidebook, p. 14-168.

Link, P. K., and Phoenix, E. C., 14, Rocks, Rails, and Trails Pocatello, Idaho State University, 18 p.

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