ECOS Guide to the Ecology of the Northern Rockies
 
   
 

 

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Idaho Batholith


Idaho Batholith

The Idaho Batholith is a collection of granitic plutons covering about 15,400 square miles in central Idaho and western Montana.  It is exposed in a north-south elongate geometry, about 200 miles long and about 75 miles wide.  The northern portion of the batholith is exposed in the Bitterroot Mountains.  This portion was emplaced around 70 to 80 million years ago.

The batholith’s composition varies from west to east.  To the west, the rocks are tonalites and quartz diorites; to the east, they are granodiorites and granites. The boundary between the two composition types also coincides with the margin of the North American continent before the California terranes were accreted.  Research suggests that the tonalites on the west side originated by the melting of oceanic rocks near the subduction zone off the west coast of North America, and that the granodiorites and granites on the east side originated by melting of Belt Supergroup or pre-Belt continental crustal rocks.  The sedimentary rocks probably were melted by hot mafic magmas rising from the same subduction zone that produced the tonalites.

The Idaho Batholith is a distinct and easily recognizable lithology.  Granitic rocks have a salt-and-pepper appearance.  The dark minerals are biotite mica and hornblende, and light minerals are plagioclase and quartz. The minerals are relatively coarse, up to an inch or more in diameter, and can be identified without a hand lens.  Other minerals are present in small proportions and are typically too small to be recognized with the human eye.

Many exposures of the Idaho Batholith in Montana are dissected by fractures, giving the outcrop a blocky character.  Some exposures of the granite have weathered to a distinctive light gray or light tan.

 

 
The ECOS program is sponsored by the University of Montana's Division of Biological Sciences, and the College of Forestry and Conservation. Carol Brewer Program Director, Division of Biological Sciences. Paul Alaback Program Co-Director, College of Forestry and Conservation.
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Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the National Science Foundation.