the Snake River Plain
The origin of the Snake River
Plain is attributed to several geologic processes.
The Snake River Plain is divided
into Upper (eastern Snake River Plain) and Middle (western Snake River Plain)
segments on the basis of geology and hydrology. The eastern Snake River
Plain is underlain by volcanic rock (primarily basalt with lesser amounts of rhyolite)
and relatively thin layers or lenses of sedimentary material that thin towards
the center of the basin. Prolific amounts of water can be withdrawn from the aquifer
associated with the eastern Snake River Plain. The repetitive sequence of
successive basalt flows that make up the aquifer provide favorable hydrologic
conditions for water to easily move through the system. Transmissivity values
(a measure of the ability of water to move through a volume of material) associated
with the eastern Snake River Plain aquifer are generally one to two orders of
magnitude greater than those determined for the aquifers in the western Snake
River Plain (Lindholm, 1996).
- Migration of the North
American continent over a region of high heat flow (plume or hot spot) in
the earth's upper mantle resulting in large volumes of volcanic material being
erupted. The age of the volcanic events generally progresses from oldest,
about 13 million years ago, to the youngest episodes currently ongoing,
traversing from west to east across southern Idaho.
- The volcanic complex
identified at Yellowstone National Park is interpreted to represent volcanic
events that occurred at various locations along the Snake River Plain as the
volcanic activity progressed eastward to its present location. The most
recent major eruption in Yellowstone occurred approximately 600,000 years
ago (Christiansen, 1982).
- In between explosive
rhyolitic eruptions, basaltic volcanism flooded the Snake River Plain, burying
the older volcanic structures.
- The western Snake River
Plain was down-dropped, lowering the basin relative to the surrounding mountains.
Subsequently, the western part of the basin was filled with a thick sedimentary
section overlying the older basalts (Whitehead, 1986).
The western Snake River
Plain is underlain by a section of sedimentary material that may be in excess
of 5000 feet thick in the central portion of the basin. The generally fine grained
nature of the sedimentary material does not allow water to move as freely as
observed in the eastern Snake Plain (Newton, 1978).
Information supplied by Idaho
Water Resource Research Institute, University of Idaho December 1998
Authors: Dr. Gary Johnson, Donna Cosgrove, and Mark Lovell.
Graphics: Sherry Laney and Mark Lovell
All State of Idaho images and graphics created with GIS files obtained through
Idaho Department of Water Resources Public Domain GIS unless otherwise noted.