"The Rock"- An Igneous Pluton EarthCache

Located in Mt. Airy in Surry County, the North Carolina Granite Company is the largest open-face granite quarry in the world! Many of the city’s homes, churches and businesses are made from the black speckled and white granite. The granite pluton being quarried in Mt Airy is roughly circular in shape with a diameter of five miles. It underlies the entire City of Mt. Airy and the surrounding area.

Granite, in the broad sense of the term, is a massive rock with a salt-and-pepper appearance due to random distribution of light and dark minerals. The mineral grains are coarse enough to be individually visible to the naked eye. Granite is a plutonic igneous rock. There are two types of igneous rock—plutonic and volcanic. Both types result from the cooling and solidification of molten rock, or magma.

Magma originates deep within the Earth and rises toward the Earth’s surface at temperatures of about 1,000 °C if granitic in composition and of as high as 1,200 °C if basaltic—by comparison, steel melts at about 1,430 °C. Magma that cools and solidifies within the Earth’s crust forms plutonic rock (named for Pluto, the Roman god of the underworld). Eventually, the granite reaches the earth’s surface where a pluton becomes a batholith. Granite quarries are only formed on extremely large batholiths.

Geologists say several steps are required to form granites. The process starts with partial melting of continental sedimentary and metamorphic rocks 12–25 miles down in the earth’s crust (a process called generation). This must be followed by the collection of the melt (called segregation), then transportation of the now less dense, buoyant magma upwards (ascent), and finally the intrusion of the magma to form a body in the upper crust (emplacement). There, as little as 1–3 miles below the earth’s surface, the granite mass fully crystallizes and cools. Subsequent erosion exposes it at the earth surface.

The three phases of pluton creation along the Eastern Blue Ridge corresponds with established geologic ages for three major mountain building events known as orogenic pulses that occurred along the western flank of the southern Appalachians.

Tectonics (Continental Plate Movement and Earthquakes), deformation by faulting and rifting, and metamorphism from extreme heating and cooling all played a role in creation of this giant igneous pluton. The magma composition of plutons are related to plate the tectonic setting and history of the pluton as it raised through the earth’s crust. This pluton is believed to have been formed with the Late Paleozoic Era collision of the African and North America continents. Heat from partially-melted oceanic crust melted the granitic continental crust. The pluton was formed during the cooling process and slowly pushed it’s way up to the surface.

Understanding what magma is and why it is thought to exist underground can help explain the pluton development process. The molten material which flows from volcanoes is known as lava and cools to form volcanic rocks. So lavas must be molten rocks; that is, they were originally rocks that melted deep inside the earth underneath volcanoes. When deep inside the earth, these molten rock materials are called magmas because they are slightly different in composition and physical properties due to the steam and gases they have dissolved in them that erupt separately from the lavas through volcanoes.

Geologists say before volcanic eruptions there are warning “rumbles” inside volcanoes. These are earthquakes generated by the magmas moving up into the volcanoes. Such earthquakes have allowed geologists to reconstruct how magmas first “pond” below volcanoes in reservoirs known as magma chambers before their final passage upward through volcanoes to erupt as lavas. If the magma cools when it “ponds” in the magma chamber, rather than rising further to erupt at the earth’s surface, then it crystallizes as an intrusion. Subsequent erosion of all the overlying rock layers eventually exposes such intrusions at the earth’s surface.

This scenario has been confirmed by mining operations that have excavated into granite intrusions that must have formed under volcanoes. The remnants of such volcanoes overlie the granite intrusions, and their volcanic rocks are the same compositions as the granite intrusions (the former magma chambers).

Individual bodies of granitic rock, particularly large ones, generally vary in mineral makeup and commonly overlap the boundaries between specific rock classifications. Bodies of granitic rock may also overlap each other’s compositional ranges, and so composition is only one factor in the recognition of separate rock bodies. The chief distinguishing property may be the presence or absence of specific minerals. Or it may be the general physical appearance defined by the texture of the rock—the size, shape, and arrangement of the minerals.

Younger magma shoots thin sheets into cracks in the older rocks. Some of the younger plutons contain inclusions, or fragments of older rock, which were embedded in the younger rock while it was still molten. Where dark inclusions or other oriented structures are present, the contact between two rock bodies may truncate structures in the older body, while similar features in the younger rock may parallel the contact.

These granite intrusions of North Carolina and The Blue Ridge Piedmont represent rocks formed from the earth's crust. They are literally basement rocks that have risen to the earth's surface. There are hundreds of studies underway on the geochemical composition of these giant rocks that have emerged from below and each day geologists learn more about their origin and the geologic history of the region.

Not every state has an official rock. Granite was designated in 1979 as the official Rock of the State of North Carolina. It was in honor of the state's first commercial natural stone business, but more importantly in recognition of the many beautiful structures built from Mount Airy “White” as it's called that dot the earth. Tests on the Mt. Airy granite show that a cubic foot weighs 165 pounds, and its crushing strength is 29,233 pounds per square inch.

North Carolina Granite Quarry produces what is known as “Dimension Stone” which is defined as natural rock material quarried for the purpose of obtaining blocks or slabs that meet specifications as to size (width, length, and thickness) and shape. Although a variety of igneous, metamorphic, and sedimentary rocks are used as dimension stone, the principal rock types are granite, limestone, marble, sandstone, and slate. Dimension stone is used extensively in home improvements, historic preservation, exterior and interior paneling for buildings and construction applications.

Most dimension stone was sold or used as rough blocks for building and construction, flagstone, curbing, ashlars and partially squared pieces, and monumental and memorial stones.

Beginning in late 2000, the company began a new quarrying process. The process involves water jet drilling in combination with horizontal wire sawing. The new process allows quarrying faces to be developed throughout the property, extends reserves, and provides a more efficient quarrying process. Technology at the quarry has changed dramatically over the past two centuries. Explosives once loosened the rock for removal. In the 19th Century, long drills and wedges were used in order to break the granite into large blocks which can weigh as much as 2 tons.

A public observation point along the adjacent perimeter road of the quarry gives visitors a panoramic view of the rock and the technology utilized by its workers. The observation deck can be reached by car and is open without restriction each day that the quarry is in operation, including weekends, during the daylight hours.

Take US 52 to SR 103 to the observation area. No permission required for the visit. Travel to the coordinates listed above and provide the following information.

At the coordinates provided above for the upper observation area:

1.Estimate the thickness of a granite slab perched against the fence line along the observation area.

a/10" b/15" c/20"

2. Estimate the distance across the giant pluton from the observation deck to the tool buildings across the width of the rock.

3. Shoot an elevation for the observation platform at the highest exposed level of the rock.

4. Travel back down the entry road that accesses the observation points. Stop at the lower observation area and take a photo of your GPS and the dramatic length of exposed rock at the gravel turn around area by the church. Post it with your log. The coordinates for the lower observation turn around point are 36 30.466 and 80 35.422.

5. Shoot an elevation here at the lower surface level of this igneous pluton.

“The Rock” has been harvested for more than 265 years. And it will be for the next several thousand years! The great grand children of your great grand children's great grand children will be able to design and build a building that will match exactly one you might build today with Mount Airy White Dimension Stone. Funny, what once was thought of as a white elephant when the land the quarry stands on was purchased for a farm by Robert Gilmer in 1849 is really something rare on the earth and famous around the globe.

In the Southern Appalachians, the history of the last billion years is recorded in the rocks. By reading the rocks, geologists can trace their incredible history of earthquakes and volcanoes in what is present day Carolinas. What an amazing thought to think of ancient continents colliding and mountains forming while volcanoes erupted and faults formed from earthquakes. Visit the rock and be a geologic sleuth yourself and take a peak at what geologic forces have created here.