Xenoliths of the Boars Tusk EarthCache
Xenoliths of the Boars Tusk
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Travel out to this site is over a minimum of 15 miles of dirt road. Some is graded, but the last 3 miles or so are over a 2-track road through the bottom of the valley. Parking is at N 41 57.710 W 109 11.765.
The Boars Tusk is located in the Leucite Hills Volcanic Field north of Rock Springs. It is a volcanic neck which rises several hundred feet above the Killpecker Valley floor. It is one of the Leucite Hills lamproites, and is composed chiefly of agglomerate and tuff. The volcanic neck consists of two spires. The southern spire has an outer zone of dense, non-vesicular lava with abundant native rock xenoliths.
Xenoliths (from xeno meaning “foreign”, and lith meaning “rock”) are pieces of native rock that were picked up by the magma when it passed through the native rock layers to the surface. Pieces of this rock broke off into the magma, and while most melted, some remained to be encased in the magma when it cooled. After erosion, these are now visible in the exposed lava. Most of the xenoliths present in the Boars Tusk are composed of shale, siltstone, and breccia from the Green River and Wasatch Formations. In addition to the sedimentary xenoliths, some granitic xenoliths are also present.
The coordinates will take you to a large boulder on the southeast side of the Boars Tusk. This boulder has numerous visible xenoliths.
To receive credit for this Earthcache:
1.) Count the xenoliths which are greater than 3” in diameter that you can see on the south face of the large boulder at the site of the coordinates. What color are most of the xenoliths you can see?
After you have emailed me the answers to these questions, post at least 1 picture of you with your GPS receiver with the Boars Tusk in the background from this site.
The above information was compiled from the following sources:
Hausel, W.D. 2006. Geology and Geochemistry of the Leucite Hills Volcanic Field. Wyoming State Geological Survey Report of Investigations No. 56.
Mirnejad, H. and K. Bell. 2006. Origin and Source Evolution of the Leucite Hills Lamproites: Evidence from Sr-Nd-Pb-O Isotopic Compositions. Journal of Petrology 2006 47(12): 2463-2489. Published online on September 20, 2006. (visit link)
Wikipedia: (visit link)
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