Once you arrive at ground zero, you will definitely be "at fault!" But seriously, you will actually be standing on a rock outcrop that is part of an ancient fault line, the extensive Brevard Fault Zone. On the surface, the area of damaged rock of this zone is 0.6 – 1.2 miles wide and some 360 miles long. (Goldberg, Steven A. and Butler, J. Robert p. 8-9)
Geologic map of Asheville area, showing surface rock types and fault lines.
In the world of geology, a fault is a linear break in a mass of rock, with movement along the break. Enormous amounts of pressure have caused the rock to deform and then rupture along the fault line. Normally the pressure has been applied along a line of quite some length – miles or even hundreds of miles. The zone of rupture may be from a few inches wide to several miles wide and may extend far below the surface of the earth. The movement of the rock along the fault line may be horizontal, vertical, or a combination, but will always involve heat and pressure. (Leet, L. Don and Judson, Sheldon p. 445-450)
So, in what way are these rocks "faulty?" Or, what effect did this heat and pressure have on the rock itself, besides causing it to break? The original rocks in the fault area were slowly crushed and transformed into new types of rock by the large amounts of heat and pressure present. Given an original rock with a particular chemical makeup, the new minerals that are formed depend on the amount of heat and pressure present.
With increasing temperature, chemical bonds in minerals are able to break and reform in new configurations. As temperature changes, original minerals within a rock become unstable and transform through chemical reactions into new minerals (within the limits of the available elements).
Every mineral is also stable over a certain range of pressures. If pressure increases or decreases due to rock movement, an existing mineral may become unstable and transform into a new mineral (once again, within the limits of the available elements). (Tennis)
The end result is a predictable type of rock in a fault zone, according to the chemical composition of the original rock that was present. For example, if clay-based rocks were present before faulting, the resulting rocks will be schist and phyllonite; if quartz sandstone was present, metasandstone or metaquartzite results; if limestone, then marble.
At this location along the Brevard Fault, the original rocks were clay-based, along with some layers of quartz sandstone. As a result, both phyllonite and metasandstone are visible in this outcrop. (Geologic Map of North Carolina; Hurlbut p. 499-501)
Phyllonite is a slightly shiny or silky gray-green rock that occurs in thin, platy layers, sometimes with a wavy appearance. Here it weathers to a rusty or yellow brown. (Geologic Map of North Carolina; Ralph)
Phyllonite sample
Metasandstone occurs as dirty white or light yellow-orange blocky layers with no visible grains. Some layering (a characteristic of its sedimentary parent rock) is visible, but not to the extent of the phyllonite. (Alden)
Metasandstone outcrop
To log this earthcache:
1. Locate the outcrops of both phyllonite and metasandstone on the upstream west side of the bridge. The phyllonite extends underneath the bridge. The metasandstone is at the base of the bridge wingwall.
Estimate the percentages of each, considering the entire visible outcrop from the ditchline to the far side of the bridge.
In an email to rkclimber4Jesus@bellsouth.net (or through my geocaching.com profile page), tell me what percentage of the outcrop you think is phyllonite and what percentage is metasandstone.
2. Tell me the original types of rocks from which both the phyllonite and the metasandstone were formed.
DO NOT POST YOUR ANSWERS OR SPOILER PHOTOS IN YOUR LOG! If you do, I will have to delete your log.
Don’t miss the additional earthcache and traditional cache nearby! And have a gneiss day!
Works Cited:
Alden, Andrew. "http://geology.about.com/od/rocks/ig/metrockindex/rocpicquartzite.htm." geology.about.com. N.p., n.d. Web. 23 Mar 2014.
Geologic Map of North Carolina. Map. Raleigh: North Carolina Geological Survey, 1985. Print.
Goldberg, Steven A. and Butler, J. Robert. Geologic Cross-section Through [sic] Part of the Southern Appalachian Orogen. Washington, DC: American Geophysical Union, 1989. p. 8-9. Web.
Hurlbut, Cornelius S. Dana’s Manual of Mineralogy. 18th Edition. New York, London, Sydney, Toronto: John Wiley & Sons, 1971. p. 499-501. Print.
Leet, L. Don and Judson, Sheldon. Physical Geology. 4th Edition. Englewood Cliffs, New Jersey: Prentice-Hall, 1971. p. 445-450. Print.
Ralph, Jolyon. "Definition of phyllonite." http://www.mindat.org/. Mindat.org. Web. 23 Mar 2014.
Tennis, Clarence F., ed. "http://www.geosci.ipfw.edu/PhysSys/Unit_4/metam.html." http://www.geosci.ipfw.edu/. Indiana University - Purdue University Fort Wayne, 27 02 2014. Web. 23 Mar 2014.