|
Situated along
the northwestern edge of the eastern Snake River Plain at an
average elevation of about 1,500 m, the INEEL is bounded on the
west and northwest by the Lost River and Lemhi Ranges and the
mouths of the Big Lost River and Little Lost River Valleys and on
the north by the mouth of the Birch Creek Valley and the southern
tip of the Beaverhead Mountains of the Bitterroot Range (Shaded Relief Map). Saddle Mountain,
on the southern end of the Lemhi Range, rises over 1,500 m above
the Plain to an elevation of 3,147 m. The eastern and southern edges of the INEEL are continuous
with the sagebrush rangelands of the Snake River Plain, but are
punctuated by the Plain’s predominant topographic features, Big
Southern, Middle, and East Buttes.
The latter two, also referred to as the Twin Buttes, are
within the INEEL boundary; Big Southern Butte, elevation 2,300 m, is within 4 km of the southern
boundary.
These buttes
are the most conspicuous among the many reminders of the volcanic
origin of the Snake River Plain. Many smaller buttes and cinder cones dot the landscape, and
lava outcrops and lava tubes are common features of the rolling
and broken terrain of the southern two-thirds of the INEEL. The Plain is thought to have been formed by the
southwestward migration of the North American tectonic plate over
a stationary “hot spot” or plume in the earth’s mantle. This migration has formed a linear volcanic province
extending from southwestern Idaho to Yellowstone National Park
over the past 17 million years (Pierce and Morgan 1992). Explosive volcanic activity in the area encompassing the INEEL
occurred between 4 and 7 million years ago (Pierce and Morgan
1992), resulting in silicic lava flows and pyroclastic deposits
that are at least 2,500 m thick (Hackett and Smith 1992). These rhyolite rocks underlie the more recent “fractured
and rubbly basalt lava flows,” which are interbedded with
sediments forming a highly permeable aquifer (Link and Phoenix
1994). Depths up to 1,100 m for these interbedded basalt flows have been documented at
the INEEL (Hackett and Smith 1992). The most recent basalt flow at the INEEL, the Cerro Grande
flow, occurred about 13,000 years ago (Kuntz et al. 1994). It extends only for a few km north of the southern boundary
and can be readily identified on the Landsat image on the back
cover. The Hell’s Half Acre flow to the east of INEEL is 5,200 years old (Kuntz et
al. 1994). At nearby Craters of the Moon National Monument, basalt was extruded as
recently as 2,100 years ago. The extensive basalt plains of the southern two-thirds of
the INEEL (Shaded Relief Map)
are between 200,000 and 730,000 years old
(Hackett and Smith 1992). Basalts
on the northern part of the INEEL attain ages of a million years
or more.
The three prominent
buttes are also relatively young. All are extruded rhyolite domes formed by the squeezing up of
viscous rhyolite through layers of basalt (Link and Phoenix 1994). Big Southern Butte is about 300,000 years old, and the age
of East Butte is roughly 600,000 years. It has not been possible to date Middle Butte because the
rhyolitic core did not break through the basalt cap. Its distinct profile results from exposure of only tilted
basalt, whereas the more readily weathered rhyolite is exposed on
Big Southern and East Buttes (Link and Phoenix 1994).
|
