Monday, July 30, 2018

Granite “Pavement” Exposure – Mitchell Mill State Natural Area - Rolesville, NC


Rock Type:  Granite

Geologic terrane or major geologic element:  Rolesville batholith

Age:  Late Paleozoic – approximately 300 million years old

Location:  Google Maps Link

USGS 7.5-minute Quadrangle:  Rolesville

Site Access:  Heading south on Highway NC96, there is a space to pull off the road on the right.  Park here and follow the path by the big blocks of rock.  You will see an extensive exposure of granite.  From this exposure, follow the trail from the back corner, near more blocks, and you will come to an even larger expanse of granite.  This site is Mitchell Mill State Natural Area, part of the NC State Parks system.

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Technical Information:  Speer, J. A., 1994, Nature of the Rolesville batholith, North Carolina (pages 57-62 in Carolina Geological Society Field Trip Guide, 1994); See also description for Stop 11 on pages 105-107.
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Introduction
The Rolesville batholith is the largest body of granite in the southern Appalachian region.  It measures about 25 x 70 miles, and occupies the eastern third of Wake County, and about two-thirds of Franklin County.  At this site there is a huge flat exposure of granite that is a part of the batholith.

The rock body
A pluton is a three-dimensional body of igneous rock.  Granite plutons range in size from a vein-like dike just a few inches wide up to the size of a batholith, which by definition covers an area greater than 100 square kilometers.

Granite occurs in bodies called plutons. Plutons usually look like pink blobs on geological maps (pink is the most common color used to denote granite). In three dimensions, plutons may be shaped like mushrooms or inverted teardrops, so they may extend to considerable depth beneath the surface. In the eastern Piedmont, almost all of the granitic plutons are between 280 and 320 million years old – Late Paleozoic in age. The largest such body is the Rolesville batholith, which is in reality 10 or 20 separate plutons that intruded the same region over a period of geologic time, cutting across each other and coalescing to form the batholith (a mega-pluton).

The rock itself
Granite is composed of two types of feldspar – orthoclase and Na-plagioclase, plus quartz and biotite mica. Sometimes it may contain hornblende or muscovite mica or garnet. It always also contains some tiny minerals in small amounts. Granite is the most common type of rock that is quarried for crushed stone, used in making concrete and asphalt, among other things.

Origin
The granite magmas that intruded this region during the late Paleozoic were generated in the lower crust of the earth, and moved upward until they stopped, cooled, and crystallized. The magmas formed as a result of the tremendous collision between continental plates that raised the Appalachian Mountains. Many of these magmas ascended along fault zones that were active at that time, such as the Nutbush Creek fault that runs across Western Boulevard just down the road from Jordan Hall. There is no evidence that any of these granitic magmas reached the surface – if they had, we would expect to see some remnants of volcanic rock of the same composition and age.

Surface features
First, we will drive to the northeast corner of the county and visit Mitchell’s Millpond State Natural Area, located where the Little River crosses Highway 96.  This 93-acre site is a Registered Heritage Area, and contains some of the best examples of native plant communities that grow in such a “granitic flatrock” environment.  It is also an excellent spot to examine a very large exposure of granite belonging to the Rolesville batholith.  Here we will see medium-grained biotite granite, with a variety of special features, including pegmatite dikes and quartz veins that cut the granite.  We will see how such rock weathers by fracturing along joints and exfoliation surfaces.  We will also see the effects of erosion by running water, including numerous potholes.  While there, we can talk about ground-water resources in the area, and we may touch upon the idea of burying high-level radioactive waste in such a body of granite, as was proposed by the U.S. Department of Energy some years back.
Medium-to-coarse grained, massive to weakly foliated biotite monzogranite: the Rolesville batholith is the largest single pluton in the southern Appalachians, covering over 772 miles² in outcrop scale (a batholith is a continuous area of plutonic rocks larger than 40 miles²). This igneous intrusive rock (pluton) has a high percentage of quartz, feldspar, and biotite - different combinations of these minerals, and others, classifies this large exposure as granite with many distinct granitic facies varying relatively in texture and mineralogy. These differences in compositions are believed to be caused by multiple sequential magma pulses that differed slightly in composition. http://www.carolinageologicalsociety.org/CGS/1990s_files/gb%201994.pdf (pp. 5, 57-61)