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Your Billion-Year-Old Metagranite Rocks!

If you have found this web page, you probably have received a piece of Corbin Metagranite, affectionately known as the beautiful blue-quartz-bearing billion-year-old basement rock from the Blue Ridge in Bartow County. It wonderfully exemplifies how rocks tell stories.

The rock is mentioned and pictured on page 197 of Roadside Geology of Georgia by Pamela Gore and Bill Witherspoon, a book for the general reader interested in the science behind Georgia’s rocks and scenery. You can follow book events and subscribe to an e-newsletter at this link. Here is a tale of your rock, beginning at the tail end of its nickname. Page numbers in parentheses guide you to further reading in the book.

Bartow County

Cartersville’s county lies at the outer growing edge of metropolitan Atlanta. Your rock is provided courtesy of Vulcan Materials, which quarries the rock at its Bartow County quarry (p.197). Vulcan operates many quarries around the city and elsewhere, including one in Norcross that at 600 feet deep is nearly as deep as Stone Mountain is tall (p. 292). The rock is crushed to various sizes. Most pavement contains the product of crushed stone quarries, and nearly all commercial buildings rest on a base of crushed stone.

Blue Ridge

The name of a prominent ridge crossed by settlers in Virginia also extends to an entire geologic and physiographic province, representing the most rugged part of the Appalachians and the highest elevations in Georgia (p. 6). It and the adjacent Piedmont Province are formed exclusively of metamorphic and intrusive igneous rocks. These are rocks that formed many miles below the surface, from other rocks changed by heat and pressure, or from magma that cooled underground.

Just two miles west of the Bartow Quarry, a long-dead major fault called the “Blue Ridge thrust front” (p. 185) separates the Blue Ridge and Piedmont from the Valley and Ridge Province, which exposes exclusively sedimentary rocks, laid down in ancient seas from about 540 to 300 million years ago. Seismic profiling in the 1970’s – a way of looking into the Earth developed in the oil industry – revealed that sedimentary rocks continuous with those of the Valley and Ridge extend southeastward beneath the Blue Ridge and Piedmont rocks all the way to the Coastal Plain, a distance of more than 100 miles. Geologists have concluded that the Blue Ridge and Piedmont rocks were transported this distance to the northwest on overthrust faults, in the aftermath of a great collision between North America and Africa. This happened about 300 million years ago, as part of the assembly of the supercontinent, Pangaea (page 5).

Billion-year-old

The Corbin Metagranite is among the oldest rocks in Georgia, at 1.1 billion years old, according to radiometric dating of its minor mineral zircon (see below). This is three times the age of metamorphism of the rocks of Atlanta and the cooling of Stone Mountain Granite (p. 13 table), but only one-fourth the age of the Earth and Solar system as determined from meteorites, and one-twelfth the age of the observable universe. The crystals in your rock were solidifying from magma at about the time that the first multicellular life – similar to today’s seaweed and jellyfish – was able to emerge. Earth’s initial atmosphere and oceans contained little if any free oxygen, but by one billion years ago, photosynthesis by microbes such as cyanobacteria had released sufficient oxygen to support early plants and animals.

Basement

In his “Blowing in the Wind,” folksinger Bob Dylan intoned, “How many years can a mountain exist before it is washed to the sea?” The geological answer is that it varies, but between 200 and 500 million years of erosion elapsed after the Corbin rock crystallized as granite deep underground, probably beneath a mountain range, before sand, clay and gravel settled out of water onto it and neighboring rocks. Deeply eroded crystalline rock that lies underneath a younger succession of sediments is called basement (p. 181, 306). Rocks about a billion years old are found as basement to younger rocks of the Blue Ridge and Piedmont along the whole length of the Appalachians, and have also been recovered from wells drilled through the sedimentary rock layers that lie to the west.

Beautiful blue-quartz-bearing

Most specimens have blocky, shiny white to gray crystals of feldspar, the most common mineral in granite and in the Earth’s crust. The darker part of the rock appears blue, especially in strong sunlight or when immersed in water. This is quartz, the second most common mineral in the crust. Due to its resistance to chemical weathering, quartz is common in sand, gravel, and the stones you dig up in your yard.

Blue quartz is not common, though, except in rocks around a billion years old. Within the quartz are tiny specks of certain titanium minerals that scatter light, giving a blue appearance. These minerals grew especially inside quartz formed around a billion years ago, presumably because of some chemical difference typical of magma at that time. This is a reminder that the chemistry of Earth’s crust has evolved over time, partly in response to changes in the atmosphere, such as the addition of free oxygen by photosynthesizing bacteria.

Look for blue quartz not only in this granite, but also in Blue Ridge granite of similar age from Georgia to Virginia. It is also found as pebbles within other rocks in the northwestern Blue Ridge, from Allatoona Dam (p. 198) through Fort Mountain (p. 213) to the Great Smokies in Tennessee and beyond. The pebbles were eroded, more than 600 million years ago, from vast expanses of blue-quartz-bearing granite like your rock.

Dating your beautiful rock

Your rock is also (slightly) radioactive! Not to worry, but tiny crystals of a mineral called zircon, scattered through your rock, each contain a trace amount of uranium. This is true of most granite, and many other igneous and metamorphic rocks. A radioactive substance ejects particles and energy from the nuclei of its atoms. In so doing it changes at a predictable rate into another substance, and if that substance is radioactive, a whole “decay chain” of different elements (including radon*) is produced until reaching a non-radioactive, stable substance, which in the case of uranium is lead.

From laboratory measurements, the half-life of each radioactive substance (time it takes for half of the substance to decay) is precisely known. This allows scientists to date zircons (not romantically!). Zircons contain two isotopes of uranium (same number of protons, different number of neutrons): ²³⁵U and ²³⁸U. Half of a sample of ²³⁵U will change to ²⁰⁷Pb in 704 million years. Because a brand-new zircon has no lead, a zircon with equal amounts of ²³⁵U and ²⁰⁷Pb is 704 million years old. In a 1.1 billion-year-old zircon, the ratio between these two isotopes is about 1:3.

²³⁸U decays to ²⁰⁶Pb with a half-life of 4.47 billion years. In a 1.1 billion-year-old zircon, the ratio between these two isotopes is about 9:1. By analyzing lots of zircons for both kinds of uranium and lead, scientists can be very confident of the age of a rock. But there is more to the zircon story because of a tool called an ion microprobe, a popular type of which goes by the acronym SHRIMP.

Your rock is Amazonian

Zircons are amazing survivors. Geologists have long known that beach sands and rocks such as sandstone contain “detrital zircons,” which, when dated, give the cooling age of the granite from which they were eroded. Inspection of some zircons in granite showed that these were also detrital – that is, they had formed during the cooling of an earlier body of granite, been uplifted to the surface, eroded and deposited as sediment, then buried once more. They then survived the melting of the surrounding rock into magma, and as new granite cooled, they retained the appearance of an eroded, rather than freshly crystallized, zircon (p. 182).

The development of ion microprobes, such as the Sensitive High Resolution Ion Microprobe (SHRIMP), in the 1970’s, has opened up a whole new zircon world. A SHRIMP can measure isotopes in a crystal for a point less than 0.005 mm across. Ion microprobe analyses of zircons showed that many are concentrically zoned, with the oldest part of the zircon at the center, and layers of distinct younger ages built outward. Each layer, remarkably, records a cycle of high temperature formation deep underground, which had to be followed by uplift, erosion, transportation, sedimentation, and deep burial, prior to high temperature formation of the next outer layer.

From many analyses it has become apparent that the zircons of the Blue Ridge and Piedmont have a different early history from those found in well cores of the basement of the sedimentary rocks to the west. Moreover, zircons in most rocks from the Blue Ridge have an early history that closely resembles zircons collected from granite in the Amazon rain forest of South America. The suggested explanation is as follows. An ancient continent called Amazonia collided with ancestral North America not long before your granite formed. Where the two continents “welded” together, some rocks melted into magma, then cooled to make your rock. Several hundred million years later, ancestral South America rifted away from ancestral North America at a different location, leaving a sizable piece of Amazonia behind. ¡Olé!

Radon – stay safe!

*One step along the decay chain of uranium-238 (as well as thorium) is radioactive radon gas, which is the second leading cause of lung cancer. This is why people who live in the Blue Ridge and Piedmont should consider testing their basement for radon.