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Backyard Geology

Scientists can tell a lot about the geologic history of Spirit and Opportunity's landing sites by examining the rocks they find. The same applies to your backyard. The rocks you see around you offer a record of ancient happenings in your patch of planet Earth. Over the next several months, Athena Student Interns from across the United States will present a glimpse of the geology that is native to their regions.

What's in YOUR backyard?

Durham Academy Upper School – Durham, NC

Backyard Geology of the Triangle (Raleigh-Durham-Chapel Hill) Area of North Carolina
(modified from A Geological Field Guide to the Triangle Area by Samuel Fuerst and Sheila Wilkerson, 1991)

The ASIP team that hails from Durham Academy lives in the Piedmont region of North Carolina. The Piedmont belt is not confined only to NC, but extends southeastward to Alabama and northeastward to Maryland. The rock types that can be found in the Piedmont vary greatly, but can be satisfactorily described and explained within the context of Plate Tectonics. The bulk of the rocks are thought to have formed when Europe and Africa were moving toward the Americas during the time periods known as the late Precambrian Eon and the Paleozoic era (about 700 million years ago until about 245 million years ago). During this time, a process called subduction was occurring between the converging land masses.

This subduction resulted in the formation of a number of volcanically active island masses flanked by marine sedimentary basins. This chain of islands must have resembled the existing volcanic islands (Japan, the Philippines, Indonesia) and adjacent sea-filled basins of the western Pacific Ocean. The western Pacific is undergoing active subduction as the Pacific Plate and other smaller plates are being pushed beneath the eastern boundaries of the Eurasian and Indian Plates. By the end of the Paleozoic era, the western edge of Europe and Africa had collided with the American continents squeezing the volcanic islands between them forming the super continent known to us as Pangaea.

The portion of the Piedmont in the Durham, NC area includes two of these Piedmont belts; the Carolina Slate Belt, consisting mainly of metamorphosed igneous rocks and the Raleigh Belt, which is a highly metamorphosed sequence of sedimentary rocks. During the convergence of continental masses, rocks of the Piedmont were subjected to intense deformation and metamorphism. The effects of these processes can be seen in the orientations and mineralogies of the rocks. In general, rocks throughout the Piedmont have been folded such that they are now standing up on edge. Metamorphic minerals commonly found in Slate Belt rocks include chlorite and epidote. The green color of these minerals indicates that the metamorphic conditions were within “greenschist” grade pressures and temperatures. In the Raleigh Belt, minerals such as kyanite can be found indicating higher pressure and temperature conditions known as “amphibolite” grade. Many volcanic belts contain deposits of metallic minerals and the Carolina Slate Belt is no exception. Close to Durham, many metallic resources including gold, copper, and tungsten, have been mined in the past. Rocks of the Raleigh Belt contain significant amounts of graphite. Graphite is carbon and may represent the metamorphosed remains of ancient life. During convergence of continental masses, tectonic processes caused Piedmont rocks to be intensely deformed by folding and faulting.

The Durham area also contains 300 million year old rocks of the Durham Triassic Basin. The Durham Basin was formed by the rifting (pulling apart) of Piedmont Rocks during the Mesozoic Era when Pangaea began to break apart and the Atlantic Ocean began to open.

The Durham Basin never was invaded by the sea. Instead it filled with sedimentary deposits that formed in lakes, rivers, swamps and alluvial fans as the climate alternated from dry to humid. None of the sediments are very far from their parent Piedmont rock sources and so have “arkosic” composition. Arkosic rocks tend to be reddish in color, a characteristic pervasive in the sedimentary rocks of the Durham Basin. Coal can be found in the Basin as well as Mesozoic aged petrified wood. Recently, three new species of dinosaur were found in a quarry in the Basin just south of Durham.

Today, the Durham area experiences the effects of a humid climate. Frequent rainfall subjects the ancient rocks to significant mechanical and chemical weathering. Abundant vegetation grows on thick soils. Rock outcrops are hard to find, and when found, often are significantly weathered into a material known as saprolite (rotten rock).

Thus, the geology of the region is often difficult to see and study. Inference is an important part of geologic studies in this area.

Rocks of the Carolina Slate Belt

This is Few’s Ford of the Eno River, a typical Piedmont stream. In many places along the Eno, small rapids occur where volcanic rocks trend across the river. Here a volcanic breccia can be seen.

Closeup of volcanic breccia at Few’s Ford. Breccia is a name given to any rock with angular fragments. The fragments here are up to 3 inches across. This rock may have been formed when volcanic material was transported in a submarine flow called a turbidite.

Volcanic rock from Duke Quarry. This rock contains volcanic clasts (originally pumice?) that have been elongated by heat and pressure. The clasts are parallel to the metamorphic foliation. Foliation in metamorphic rocks is caused when mineral grains line up perpendicular to the pressures exerted during tectonic activity. This rock is now a phyllite, a term applied to fine-grained, foliated metamorphic rock. This phyllite was quarried and used to construct many of the buildings on the campus of Duke University in Durham.

Piedmont Minerals Quarry in Hillsborough, NC. The mineral being mined here is pyrophyllite which is used to make various heat-resistant materials. Pyrophyllite is a metamorphic mineral rich in aluminum. Its occurrence here indicates that hot spring (hydrothermal) activity altered the volcanic rocks found here. These hot springs were active over 500 million years ago.

This rock has been interpreted to represent an ancient nuee' ardente (or pyroclastic flow). This rock is classified as a crystal-lithic tuff, a rock type deposited by a hot, fast-moving gaseous volcanic flow produced by violent eruptions. The rock probably once contained abundant pumice. Nuee' ardentes have been the cause of major disasters during human history, such as the destruction of Pompeii by Mt Vesuvius.

This is a metamorphosed vesicular basalt. Vesicles are formed by volcanic gas escaping lava as it solidifies into rock. The vesicles here are the dark spots (2 to 3 cm in diameter) that are now filled with minerals. Basalts are mafic (iron-magnesium rich) volcanic rocks that are formed in the early stages of island arc development. The rock now contains much chlorite and epidote, green metamorphic minerals indicative of greenschist facies pressures and temperatures, hence metamorphic basalts are often called “Greenstones”.

The Forest Theater on the campus of the University of North Carolina at Chapel Hill. The theater is constructed of Chapel Hill Granite, which is actually a granodiorite. Piedmont rocks contain many such intrusive igneous rocks. In Chapel Hill, there are many stone walls composed of this rock. Many sidewalks in the town consist of “Chapel Hill gravel” a weathering product of the Chapel Hill Granite.

Rocks of the Durham Triassic Basin

Triassic-aged sandstone. This rock was deposited in a fluvial (river/stream) environment. The original bedding can be seen. This rock has been weathered to a material called saprolite (“rotten rock”) It appears to be rock, but crumbles easily in the hand. Because of the subtropical climate of North Carolina, many outcrops consist of saprolite, further complicating interpretation.

Fanglomerate. Fanglomerate is a term applied to sedimentary breccia with large angular fragments. Some fragments in this outcrop are up to 1 foot in diameter (quarter for scale). This rock formed in an alluvial fan very close to the border fault of the rift basin. The large fragments are composed of Slate Belt material, which outcrops just west of the fault.

Diabase. This dark, mafic igneous rock is of Mesozoic age and occurs throughout the Piedmont within tabular dikes. These dikes were presumably feeders for volcanic activity that signaled the rifting of Pangaea and the formation of the Atlantic Ocean.

Rocks of the Raleigh Belt

Falls’s Lake Gneiss. Gneiss is a foliated metamorphic rock composed of light and dark bands. The parent rock of gneisses is often hard to determine due to the extremely high pressure and temperatures that from them. This gneiss has been interpreted to have formed from metamorphism of a granite.

Garnet-kyanite-biotite-muscovite-quartz schist. A schist is a coarse-grained foliated metamorphic rock. The round reddish crystals are garnet. Its presence, along with that of kyanite, indicates high pressure and temperatures. The parent rock for this schist was probably a clay-rich sandstone.

Fall’s Lake Mélange. This rock unit forms a thin band between the Slate Belt and the Raleigh Belt. This is a brecciated rock rich in a green mineral called olivine. Olivine and other minerals found here are not common at the Earth’s surface, but instead, occur beneath the crust within the upper mantle. Presence of this mélange has been interpreted as representing material squeezed up between Piedmont terrains during continental collision.