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.
