Elk Knob Geology EarthCache

This 1.9 mile (each way) hike will take you to a gorgeous view and through some incredible geology! Don't forget your camera and a water bottle! NC State Parks kindly remind you to follow all state park rules and regulations - PLEASE stay on the trail. NC State Parks has also requested that you leave a note on your dash stating that you are earthcaching.
 

Regional Geology

The rocks at Elk Knob belong to a group called the Ashe Metamorphic Suite (not what Asheville, NC was named after, unfortunately) that extends throughout the Blue Ridge Mountains in NC and VA. This suite of rocks is primarily composed of amphibolite (metamorphosed basalt) interlayered with schist (metamorphized clay- and silt-rich sediments) and locally occurring ultramafic rocks (igneous rocks that make up the earth’s crust). Below are examples of amphibolite and schist:

Amphibolite:

Schist:

The Ashe Metamorphic Suite lies completely within the Blue Ridge Thrust sheet overlying the Gossan Lean fault, as shown by the map below:

Elk Knob Features

The base and peak of Elk Knob is composed primarily of amphibolite, with a middle mixed rock unit. As you hike up and down the knob, look for the change in rock type! The dominate rock in the mixed middle layer is schist, which is typical of the Ashe suite. It is much lighter in color than the amphibolite, and has a shiny, silvery appearance due to its high mica content. Below is a diagram of Elk Knob from above and in cross-section. Amphibolite is shown in grey-green, and the mixed layer in red:

Formation of the Ashe Metamorphic Suite

The Ashe Metamorpic Suite is interpreted as metamorphosed oceanic crust (amphibolite) and associated sediments (schist of the mixed rock unit). These rocks formed the floor of the ancient Iapetus Ocean, and were likely generated at a mid-ocean spreading center or back-arc basin, and could be 450-735 million years old, but the origin and age is still being debated.

Building the Appalachians

The rocks at Elk Knob were metamorphosed and deformed during the earliest phase of Appalachian mountain building, known as the Taconic orogeny. This involved the docking of a volcanic arc on the Laurentian margin during the Ordovician (~450 mya) As the lithospheric crust thickened in the collision zone, the rocks at the surface were buried up to 20 km where the heat and pressure metamorphosed the ocean floor basalt and sediments into the amphibolite and schist you are observing today.

Today’s Appalachians: The result of erosion

The forces of nature, especially winds, water, and ice, have created thelandscape you view today from Elk Knob. Over millions of years, these forces exposed the once deeply buried amphibolite and schist, which now form the peak of Elk Knob.

To receive credit for this Earthcache, please use the Geocaching app or website to message me the answers to the following questions:

1. 1.Using the diagram of Elk Knob, or your GPS, estimate the elevations (in feet) where the boundaries between the amphibolite and mixed layers occur. Were you able to observe the change in geology on your hike?
2. 2.What do you think is the most significant cause of erosion and weathering on Elk Knob? What evidence did you find of this on your hike?
3. 3.What is one difference between schist and amphibolite? Both types are found along the trail, feel them and make observations during your hike. What are the textures of the two different rocks like?
4. 4.(Optional) Post a picture with you and your GPS at the top in your log!

You did it! Feel free to log after you have sent me your answers – I will contact you if any are incorrect.

Sources:

http://www.ncgeology.com/Elk_Knob_geology/pages/geologic_setting.html

http://www.scotese.com/

https://en.wikipedia.org/wiki/Ultramafic_rock