Xenoliths in Southern Idaho - Ancient Underground Drama Revealed

January 05, 2017


Photographer: Rob Sheridan
Summary Author: Rob Sheridan

About 2.5 billion years ago, course-grained, iron-containing granitic rock formed deep underground in what is now referred to as the Green Creek Complex in southern Idaho. The Green Creek Complex is one of the most ancient exposed rock formations in the western U.S. Fast forward about 2.47 billion years (30 million years ago) for more recent buried drama, when this formation was fractured and intruded by the Almo Pluton -- the light colored bands pictured above that fill fractures in the slightly darker-colored Green Creek Complex rock. This molten granitic rock carried fragments of the walls of its magma chamber along for the intrusive ride.

Such fragments are called xenoliths, non-igneous rock fragments embedded within igneous rock. In the case of the Almo Pluton’s magma chamber walls, these distinctly different fragments were composed of a dark biotite schist or black mica -- pointed out here by the intrepid climbers. In more recent millennia, uplift and erosion exposed the ancient Green Creek Complex rock and the younger intruding Almo Pluton rock along with it xenoliths. Weather, lichens and moss have further eroded and fractured the formation, which now forms the heart of the City of Rocks National Reserve in Southern Idaho. Photo taken on November 11, 2016.

Photo Details: Camera Maker: NIKON; Camera Model: COOLPIX AW120; Focal Length: 4.3mm (35mm equivalent: 24mm); Aperture: ƒ/4.1; Exposure Time: 0.0007 s (1/1500); ISO equiv: 125;    
Software: COOLPIX AW120V1.0.