EPOD - a service of USRA

The Earth Science Picture of the Day (EPOD) highlights the diverse processes and phenomena which shape our planet and our lives. EPOD will collect and archive photos, imagery, graphics, and artwork with short explanatory
captions and links exemplifying features within the Earth system. The
community is invited to contribute digital imagery, short captions and
relevant links.


Archive - Gneiss Outcrop in Montana

February 23, 2020

Gneissbarnes (2)

Each Sunday we present a notable item from our archives. This EPOD was
originally published February 25, 2014.

Photographer: Lindsey Barnes
Summary Author: Rod Benson
This outcropping of gneiss is located in a cirque called
Beehive Basin, about 35 mi (56 km) south-southwest of Bozeman,
Montana. Gneiss is a metamorphic rock that forms when
sedimentary rock (such as shale) or igneous rock (such as
granite) is changed by heat and or pressure and in some cases even
by chemically active fluids.
Gneiss makes up most of the lower portion of Earth's continental
crust -- very old material referred to as basement rock. In
most places, basement rock is covered with younger sedimentary rocks.
However, drill down far enough anywhere on a continent and you’ll
usually strike gneiss. In fact, the rock that makes up the mountains in
southern Montana, including the gneiss in the photo, are among the
oldest in the state -- formed during the Archean Eon over 2.5
billion years ago. Here, blocks of crust were forced upward and many of
the younger rock formations eroded away, exposing the basement rock.
Today this gneiss makes up many of the mountains in and around Montana,
including the Beartooths, the Tobacco Roots and a significant
portion of the mountains in between. Photo taken on January 18, 2014.
* Beehive Basin, Montana Coordinates: 45.331608, -111.395211

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More...

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* This Dynamic Earth
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* USGS/NPS Geologic Glossary
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