Geologic Setting and History

Prologue

 

The geologic story of the rock types at Elk Knob and their geologic history was determined through many years of research by many investigators in the Blue Ridge Mountains of western North Carolina.  For a scientific review of the geologic interpretation of the rocks found at and within the area adjacent to Elk Knob State Park, the reader is referred to the 1997 and 2005 Carolina Society Guidebooks and references within as a guide to the general geology of the region.

 

Geologic Setting

The rocks at Elk Knob belong to a group of rocks recognized throughout the Blue Ridge Mountains in North Carolina and Virginia as the Ashe Metamorphic Suite. This suite of rocks is dominantly composed of amphibolite (metamorphosed basalt) that is interlayered with schist (metamorphosed clay- and silt-rich sediments) and locally occurring ultramafic rocks (Abbott and Raymond, 1984). The Ashe Metamorphic Suite lies wholly within the Blue Ridge Thrust sheet overlying the Gossan Lean fault (see map below; click here to learn more about the regional geology) (Rankin et al., 1973; Hatcher et al., 2006).  
Amphibolite is a dark-colored metamorphic rock composed of black hornblende and white-gray plagioclase feldspar.   Schist is a metamorphic rock with a high mica content. Elk Knob schist is dominantly composed of muscovite (white mica) &/or biotite (black mica), plus quartz and feldspar. Garnet, kyanite, staurolite &/or other trace minerals may be present.
 

 

GEOLOGIC MAP OF NORTHEAST TENNESSEE AND NORTHWEST NORTH CAROLINA

Regional geologic map of northeast Tennessee and northwest North Carolina, modified from Hatcher et al., 2006. Click here to learn more about the regional geology surrounding Elk Knob.

 

Below is a geologic map and cross section of Elk Knob that shows the base and top of the mountain are composed of amphibolite bodies (green), separated from each other by a middle mixed rock unit (brown) that wraps around the entire circumference of the mountain. This means a hiker cannot reach the mountain’s peak without hiking through the mixed rock layer. The dominant rock in the mixed rock layer is schists typical of the Ashe Metamorphic suite, and can be distinguished from the amphibolite because it is much lighter in color and has a shiny, silvery white appearance due to its high mica content. See if you can note the change in rock type on your next Elk Knob summit!

Elk Knob amphibolite is a dark metamorphic rock dominantly composed of black hornblende and white plagioclase feldspar, in addition to other trace minerals too small to identify in hand sample. Garnet is present in the upper amphibolite body and locally in the lower amphibolite body at Elk Knob. The presence of garnet in some amphibolite while absent in other amphibolite, reflects differences in original bulk rock composition (Wilson and Raymond, 2011). Green epidote layers, veins and boudins are common.

The mixed rock unit is dominantly composed of schist and locally contains minor amounts of interlayered amphibolite. Schist is a metamorphic rock composed of primarily mica, with varying amounts of quartz, feldspar, and/or other common metamorphic minerals. The schist at Elk Knob varies from a white muscovite mica schist to a darker gray and black, biotite mica schist and semischist (less mica), and may locally contain staurolite, kyanite, and/or garnet.

cross section location

 

Origin of Elk Knob Rocks

Rocks at Elk Knob, and the Ashe Metamorphic Suite in general, are interpreted as metamorphosed oceanic crust (now amphibolite) and associated sediments (now schist of the mixed rock unit). These rocks formed the floor of the ancient Iapetus Ocean, which opened as a supercontinent called Rodinia rifted apart to form two diverging landmasses, Laurentia (ancient North America) and Gondwana (composed of southern hemisphere continents). It is uncertain whether the ocean crust was generated at a mid-ocean spreading center or volcanic back-arc basin in the Iapetus Ocean, although many studies favor the later (Abbott and Raymond, 1984; Misra and Conte, 1991; Raymond et al., 2003a; Raymond et al., 2003b; Swanson et al., 2005; Swanson and Raymond, 2010). If the Ashe Metamorphic Suite originated as a mid-ocean spreading center, it could be as old as NeoProterozoic (i.e., ~735–565 million years old). Conversely, if originated in a back-arc rift environment, the Ashe Metamorphic suite maybe as young as Ordovician (~450 million years old), a time when volcanic arc terranes outlined the Laurentia margin.

The Iapetus Ocean formed as the supercontinent Rodinia rifted apart separating the two ancient landmass, Laurentia and Gondwana. Drafted after Scotese, Paleomap Project available on the world-wide web @ http://www.scotese.com/.

 

Building the Appalachian Mountains

The Iapetus Ocean eventually changed from an opening to a closing ocean basin. The Ashe Metamorphic Suite was caught in this closing basin as the gap narrowed between Laurentia and Gondawana and intervening terrains collided with the Laurentian margin. This led to formation of the Appalachian Mountains over ~200 million years time and culminated with the collision of Laurentia and Gondwana to form the better-known supercontinent Pangea.
The final building of the Appalachian Mountains occured as Laurentia and Gondwana collided to form the supercontinent Pangea. Drafted after Scotese, Paleomap Project available on the world-wide web @ http://www.scotese.com/.

 

 

 

 

 

 

 

 

 

Rocks at Elk Knob were metamorphosed and deformed during the earliest phase of Appalachian mountain building, known as the Taconic orogeny. This mountain-building event involved docking of a volcanic arc on the Laurentian margin during the Ordovician (~450 Ma). As the lithospheric crust thickened in the collision zone, the rocks at the Earth’s surface were buried as much as 20 km and “cooked” by the earth’s internal heat and pressure to metamorphose the ocean floor basalt and sediments to the amphibolite and schist we see today at Elk Knob. Peak pressure-temperature estimates of ~7kb and 700ºC for the Ashe Metamorphic Suite indicate amphibolite facies of metamorphism and agrees with observed mineral assemblages (Raymond and Abbott, 1984) Eclogite facies assemblages do occur locally, indicating that the Ashe Metamorphic Suite may have experienced higher-grade metamorphism (Willard and Adams, 1994; Adams et al, 1995; Adams and Trupe, 1997; Abbott and Greenwood, 2001; Page et al., 2003).

Deep in the earth’s crust, the amphibolite and schist were also complexly deformed. Detailed geologic mapping and preliminary structural analysis by Wilson and Raymond (2011) reveals these rocks experienced at least four-fold events and ductile faulting of the middle schist unit that likely occurred during or post peak metamorphism. Late brittle faults cut earlier formed structures.

 

Weathering and Erosion to Land’s Surface Today

All supercontinents eventually break-up, so just like Rodinia, Pangea also rifted apart. This gave birth to the Atlantic Ocean and set broken continental blocks on a global trajectory to form our present-day continental configuration. Nature’s forces including wind, water, and ice began to weather and erode the Appalachian Mountains. Given millions of years of geologic time, nature’s erosive forces exposed the once deeply buried amphibolite and schist, which now form the high peak of Elk Knob State Park.
  Many hikers stop to catch their breathe and take photos by this sign that marks the highest point on top of Elk Knob.