They bore witness to the rifting of a supercontinent
Deborah Painter (USA)
They looked ordinary. The cobbles and pebbles, in the streambeds and along the banks of the shallow streams in Pittsylvania and Franklin Counties, Virginia, USA, seemed like any other quartzite and gneiss specimens one sees used as building stones. They were recorded by the ecology teams working the field this past November 2021 only in terms of size and the habitat value they offered for fish and invertebrates that use the oxygenated waters created by the water flowing over them, and to hide and rest in low-flow areas created by them. However, they weren’t ordinary. They, like all rocks, tell a story, and the story they told is a very, very old one.
The people who live in the geological region known as the Piedmont Province tread every day atop soil covering some of the oldest rocks in Virginia. The bedrock here in Franklin and Pittsylvania Counties is seen exposed only in some areas and not others, since it is buried under thick soil layers. Often, the only places one can see the rocks are in the valleys of streams where erosion has removed much of this soil.
When doing quality assurance on stream habitat assessments completed by the field crews, I could not help but pay attention to the rocks and cobbles in the streams in the hilly countryside of this south-central area of Virginia, such as Teels Creek (Fig. 3).
These rocks are varied, but primarily metamorphic, and include:
- mica schists (Fig. 5),
- granite gneisses (Fig. 7),
- biotite gneisses
- meta-argillites, and
- quartzite (Fig. 8).
Schist originated as either an igneous rock or as shale, a sedimentary rock that has been metamorphosed and forms bands or ‘foliation’. As the name implies, granite gneiss is the igneous rock granite that has been metamorphosed. The minerals are realigned by pressure and heat into bands. Meta-argillites are metamorphosed argillites (that is, mudstone, which is a sedimentary rock). They do not display foliation and, in that aspect, are distinguished from slate and schist.
A number of other metamorphic rocks, such as quartzite, also do not have minerals aligned into bands. Quartzite is a non-foliated rock that was once pure or nearly pure quartz sandstone, a sedimentary rock. Sand grains are not flat or elongated and will not align into bands when metamorphosed.
Limestone, also a sedimentary rock, is composed of mineral grains that are not flat or elongated. Limestone that is metamorphosed is known as marble and marble is occasionally found in Pittsylvania County. The heat to which these rocks are subjected while metamorphosed is not intense enough to melt the rocks; if so, they would become magma and when they cooled, they would be igneous rocks. Leatherwood granite, an igneous rock, is relatively common in the area. It has unusual, rounded inclusions of orthoclase with a rim of oligoclase (known as “rapakivi” texture). Charnockite is another granitoid rock found in Franklin and Pittsylvania.
Minerals that make up these metamorphic and igneous rocks include:
- feldspar,
- kyanite (Fig. 9),
- apatite,
- garnet,
- magnetite,
- quartz,
- muscovite and biotite micas (Fig. 10), and
- staurolite.
Both garnet and mica are abundant in the schists. A specimen of mica is depicted in Fig. 10.
Steatite or ‘soapstone’, a non-foliated metamorphic rock that formed as a result of heat and pressure applied to the mineral talc, occurs in various localised areas in these counties. One of the world’s best known soapstone mines was once located in Pittsylvania County.
Staurolite is a silicate mineral that is found in areas of regional metamorphism. It is pale brown in colour and often occurs as twin cruciform crystals known as fairy crosses. Fairy Stone State Park in Bassett, Virginia is a public park where these crystals can be collected. It is not distant from one of our stream sites, Dinner Creek.
Exposed igneous and metamorphic rocks in Pittsylvania and Franklin Counties average 825 million to 540 million years in age and date from the late Neoproterozoic rifting of Rodinia to the first period of the Palaeozoic Era, the Cambrian, when the new ocean thus created had already existed for many millions of years. These are the Goochland terranes. A ‘terrane’ is defined as a fault-bounded area of rock with a distinct history and stratigraphy. The Goochland terranes are named for Goochland County, which lies to the west of the city of Richmond.
Similar age terranes are found in Africa, India and South America, helping us piece together in our minds the former continent. (Fig. 11).
What is some of the evidence for this continent?`
Palaeomagnetism is one good piece of evidence. The study of magnetic minerals in rocks has helped geophysicists and geologists piece together the history of horizontal movement of plates. Magnetisation of minerals occurs within rocky material during exposures to heat and pressure. Both chemical changes and exposure to elevated pressure and temperature occur during metamorphism. Freedom of movement allows the minerals to become magnetised along the Earth’s existing magnetic lines of force at the time. Fortunately, magnetite and other ferromagnetic materials are abundant in many metamorphic rocks. The application of palaeomagnetism in studying tectonics is too large in scope for me to attempt to cover. I recommend Academic Press’ Paleomagnetism: Continents and Oceans by Michael W McElhinny and Phillip L McFadden, for a very readable book on this subject.
The complete cyclic opening and closing of an ocean basin due to plate tectonics is known as a ‘Wilson Cycle’. Virginia has experienced two complete Wilson Cycles and I am referring in this article to the earlier one. Rodinia is less well known than Pangaea, which is part of a later Wilson Cycle. In fact, Pangaea was actually the most recent supercontinent.
The well-known science fiction author, Edgar Rice Burroughs, once published a novel with the title The Land That Time Forgot. Rodinia was truly a land that time forgot or, at the very least, one that science was unaware of for a long time, since it was first described by science in 1970, then being called ‘Pangaea 1’. Later, it was renamed Rodinia, derived from the Russian word ‘to give birth’.
Ancient plate tectonics can be determined in part by similar terranes in widely separated geographic regions, and by the location and depth of metamorphic rock and the magnetic alignment of their crystals, when the rock has been reheated. There is a beginning, middle and end for each ocean basin. John Tuzo Wilson, the eponymous Canadian geologist who first described this cycle in 1968, observed that plates carry portions of the seafloor. He saw examples of the various stages of a cycle in our seas and oceans of today that he used to make predictions for the past and the future.
There are six stages of an ocean basin’s lifecycle:
- the embryonic,
- juvenile,
- mature,
- declining,
- terminal, and
- suturing.
The basins start in the embryonic cycle as rifts, then widen in their juvenile and mature stages, and in the latter half of the cycle begin to shrink. When this occurs, they experience uplift. In suturing, only scars of the old basin remain.
The continent of Rodinia began its existence in the first period of the Neoproterozoic Era, 1.1 billion years BP (Before Present). The continent formed from sections of the supercontinent Columbia. Rodinia began rifting apart 825 million years ago, during the first period of the Neoproterozoic known as the Tonian. It ceased to exist as a distinct continent 750 million years ago, during the late Tonian. Four stages of separation occurred between 825 and 550 million years ago. No fossil record has been found of life occurring on land in those ancient times. Rather, all life is thought to have been confined to the seas. Single celled algae and sponge-like animals bore witness to the widening of the new ocean basin.
Callan Bentley, Assistant Professor of Geology at the Piedmont Community College in Charlottesville, Virginia, describes the rift zone that was located here in the Piedmont Region as something comparable to what is happening right now in the Afar Triangle region of Ethiopia. This is an analogue to the ‘embryonic stage’. In the Afar Triangle, we see lakes, flood basalts, hot springs and large amounts of sediment from the surrounding uplands eroding into the basin.
About the author
Deborah Painter is an ecologist and general environmental scientist. She lives in the United States.
References
Bentley, Callan. The Grenville Orogeny and rifting of Rodinia, Virginia: https://www.youtube.com/watch?v=g6itZWD8bQc&t=303s
Bailey, Chuck. The College of William and Mary’s The Geology of Virginia Piedmont page: http://geology.blogs.wm.edu/piedmont/
Conley, J. F., J. D. Marr, Jr., and C. R. Berquist, Jr. November 7, 1981. Stratigraphic Relationships Between Rocks of the Blue Ridge Anticlinorium and the Smith River Allochthon in the Southwestern Virginia Piedmont, Thirteenth Annual Virginia Geologic Field Conference. Virginia Division of Mineral Resources, Charlottesville, Virginia.
Interactive Geology Map of Virginia, Virginia Department of Mines, Minerals and Energy: https://energy.virginia.gov/webmaps/GeologyMineralResources/
McElhinny, M.W., and P.L. McFadden. 2000. Paleomagnetism: Continents and Oceans.Academic Press San Diego.
United States Geological Survey. Leatherwood granite: https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=VACAlw%3B0
Varet, J. 2017. Geology of Afar (East Africa). Germany: Springer International Publishing.