Geology of Grandfather Mountain

Landis Wofford (USA)

Like all mountains, the Blue Ridge Mountains of western North Carolina and Eastern Tennessee are the result of the action of plate tectonics. The crust of our planet is composed of five primary plates, or huge pieces of rock that move very slowly over deeper layers of hot, pliable rock.

Some of the plates are composed of heavy oceanic crust, while others are made of lighter continental crust. At the middle of each oceanic plate, a large crack pours lava out onto the ocean floor. This causes oceanic plates to expand by an inch or two every year. When oceanic crust is forced against continental crust, the oceanic crust is pushed underneath the continental crust. When continental crust is forced against continental crust, huge mountains usually are formed.

The Appalachian Mountains were formed in the remote past, some 200Ma, by collision of two continental crusts. During such mountain building, huge sheets of rock are pushed over each other. A rock layer called the Blue Ridge Thrust Sheet was moved over 60 miles to cover what is now Grandfather Mountain.

These mountains were once ten times higher than they are today. Over hundreds of millions of years, erosion has carried away most of the rocks to form thick layers of sediment across the Piedmont, Coastal Plain, and in the Atlantic Ocean.

Grandfather Mountain is the tallest mountain in the Blue Ridge and is now a popular tourist destination resort. It is also the only private park in the world designated by the United Nations as an International Biosphere Reserve. At Grandfather Mountain, this erosion referred to in the paragraph above has worn away the Blue Ridge Thrust Sheet from over the top of the underlying older rock, allowing us to study them. Geologists call this a ‘window in time’.

A window of this type is like an old pair of blue jeans through which the knee can be seen. The knee is the older rock and the denim is the younger rock layer. Erosion over the knee has created a means by which the older material can be studied.

Rocks on Grandfather Mountain are generally metamorphic. Pressure, heat and the introduction of new substances cause metamorphism during mountain building. Changes include the development of new minerals, making a rock that has been deeply buried and compressed to look very different from the original rock.

The park has a Nature Museum and there, and also at Split Rock, it is possible to see metamorphosed conglomerate. Around the upper parking lot, the rocks are phyllite or meta-siltstone. The Swinging Bridge (a 228-foot suspension bridge spanning an 80-foot chasm at more than one mile in elevation) is built on granular pebble conglomerate rocks. The geologic cross-section at the top of the page is scanned from Map I-709-A, Geologic Map of the West Half of the Winston-Salem Quadrangle, North Carolina, Virginia and Tennessee, by D.W.Rankin, G.H. Espenshade and R.B. Neuman, published by the United States Geological Survey in 1972. If you want to know more about Grandfather Mountain and the park, see: http://www.grandfather.com/index.php.

You can see from the illustration of the Blue Ridge Belt that the entire Tablerock Thrust Sheet has been eroded away. Rocks of the Blue Ridge Thrust Sheet are much younger than rocks within the Grandfather Mountain window. So are the rocks east of the Brevard Fault Zone. Here is a very simple diagram of what happened.