Appalachian a priori data


 
A priori data and Introduction
 
The locality in which you are has not always always looked the way it does now and, in fact, may never have. One has a difficult time imagining that the rocks underneath your feet may have at one time been deep in the earth and red hot with molten magma, or had once formed a broad expanse of tidal flat baking in a tropical sun, or were located within a deep sea or at another time, were part of a mountain chain. The geologist is driven by curiosity to determine, "What was it like right here 10, 100, 1000 million years ago...and how do I know that?"
   
The state of Virginia and the surrounding Mid-Atlantic states have a long and fascinating geologic history. The oldest rocks in the Blue Ridge are as much as 1.8 billion years old. At approximately 1.1 billion years before present (b.p..) the Blue Ridge was comprised of molten rock deep inside a mountain range and connected directly to Africa. At 600 million years b.p. Virginia and Africa rifted apart and an ocean basin, the Protoatlantic, opened along the east coast. At this time the land on which Richmond, Washington, D.C., Virginia Beach, and Norfolk now lie had not yet appeared.
 
To create the now geologically quiet eastern half of Virginia over the past 600 million years, the following events occurred in sequence: a first rifting event (opening of the Protoatlantic), formation of a geologically quiet continental margin, a first orogeny (mountain building; Taconic), a second orogeny (Acadian), a third orogeny (Alleghenian), which brought Africa back to North America and closed the Protoatlantic ocean, a second rifting event (opening of the Atlantic ocean), and creation of the present continental margin.
 
In that span of time, life populating the state has ranged from a time when only bacterial cells existed, to shallow seas filled with invertebrate life, to flood plains covered with tropical jungles of strange plants, to dinosaurs tromping across the state, to mammoths, mastodons and giant ground sloths, and eventually humans.
 
 
Contributed by Lynn Fichter
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Theoretical models
 
Geologic  histories  are imbedded within and draw heavily upon a number of theoretical models in geology, including plate tectonics, rifting and mountain building models, and the Wilson Cycle(see below). We have attempted to provide both simple and more detailed descriptions of those models. Understanding the processes that form rocks requires an in depth background in geology but we realize many readers do not have this a priori knowledge.  To this end we have included pages that summarize igneous, sedimentary and metamorphic rocks classifications and interpretations. Researchers in the scientific community and historians alike have spent the last few hundred years trying to understand the earth and how it works. Over a million and a half geologic papers have been written and the rate of publication is increasing. Clearly, as with everything else, our ideas have changed in that time.

One of the most significant theories that has changed the way we think about the earth is the theory of plate tectonics. This builds from the concept of continental drift (Wegener, 1915). In 1961 and 1962 it was accepted that internal mantle convection drove the process of sea floor spreading. This profoundly changed the way we look at, not only the earth as a whole, but at all it component pieces. Prior to plate tectonic theory we had few models that focused on the world globally. Mountain building models, for example, were contained within the geosynclinal theory and were essentially ideas of mountain formation with no viable mechanism to explain how mountains formed. The Wilson Cycle is another significant model.  

The question we address here is, “what premises and theoretical models lie behind the geologic history of Virginia?”  We begin with three premises that exist as core truths about the earth and how it works.

  • Premise One: No rock is accidental

No rock is accidental. No idea in geology is more profound than this; it runs from the center to the whole of geology and influences every sub-discipline of the field. Genuine understanding of the science of geology begins with one's ability to understand and explain why no rock is accidental. It is true because of premise two.

  • Premise Two: Minerals and rocks are stable only under the conditions at which they form.    

Everything strives to come into equilibrium under existing conditions. Igneous rocks evolve into sedimentary rocks when exposed at the earth's surface, sedimentary rocks evolve into metamorphic rocks when they are buried deep in the earth, and metamorphic rocks melt to become igneous rocks again when they are hot enough. This concept is contained within the rock cycle. On earth, conditions are always changing, leading to premise three.

  • Premise Three: Nothing in geology makes sense except in terms of tectonics.
Tectonics is concerned with deformation in the earth and the forces that produce deformation. But why does this deformation take place?
 
The earth is an open system.That is, it dissipates (uses up) energy. The energy comes from the Earth's molten interior (and from the sun), and has kept this planet tectonically active for 4.5 billion years. But it is this energy from the interior that has driven the Earth's physical/chemical evolution, and been ultimately responsible for all the rocks, continents, mountains, etc.Several models, some nested inside others, are implicit in the geologic history of Virginia. The chart below shows how the models behind the history are related to each other.

The Models
 
Several models, some nested inside others, are implicit in the geologic history of Virginia. The chart below shows how the models behind the history are related to each other.

 

Tectonic models of the earth can be arranged in a hierarchy. The plate Tectonic Theory describes how the earth as a whole works, but directly under that are various subdivisions. These include the six tectonic regimes as seen in the chart above, structural geology, and the tectonic rock cycle, among other subjects.

The six tectonic regimes are the individual pieces and components that make up the earth's rigid outer layer (lithosphere), and interact within plate tectonic theory. Each is characterized by specific processes that generate specific kinds of rocks and structural features. The six regimes are not isolated, but interact with each other in complex ways. The Wilson Cycle is the simplest model of those interactions, and a pivotal model behind the Virginia history presented here. Structural geology is a core discipline in geology that describes the way rocks are deformed. Since there are few parts of the earth that have not been deformed, a structural analysis is usually the first thing done in a region under study. The Tectonic Rock Cycle is a theory of how earth processes, and specific rocks and tectonic features, are related to each other.
     
We need familiarity with a little of each of these subdivisions to understand Virginia history. The six tectonic regimes are the basic components of all earth models and are introduced briefly below.

Plate Tectonic Theory (Link to Synopsis)
 
Plate tectonics is the theory that the earth's lithosphere (outer rigid shell) is composed of several dozen "plates", or pieces, that float on a ductile mantle, like slabs of ice on a pond. The plates and their relationships are described by the 6 tectonic regimes (below).
 
At its simplest, plate tectonic theory involves processes describing plates rifting into pieces diverging apart and new ocean basins being born, followed by motion reversal, convergence back together, subduction of the oceanic crust, plate collision, and mountain building. This cycle of opening and closing ocean basins is the Wilson Cycle.
 
Plate tectonics is one of the great unifying theories in geology. Virtually every part of the earth's crust, every kind of rock and every kind of geology can be related to the plate tectonic conditions that existed at the time they formed. Nothing in geology makes sense except in terms of plate tectonic theory.

Plate Tectonic theory is rather complex, and we have developed it just enough for someone to read and understand the geologic history of Virginia. A quick and dirty summary of plate tectonic theory is presented in this Plate Tectonic Link; note that there is text below the cross section. Also, see discussion of the tectonic regimes below.

The Six Tectonic Regimes
 
The six tectonic regimes are the individual components that interact with each other and form the basis of the plate tectonic theory. They can be grouped into three divisions. Below we briefly describe some important features and facts of each.

Continents and Ocean Basins - divisions that compose the plates. 
 
These include those portions of the earth most of us live on. Geologically they are blocks of relatively light weight rock that "float" above sea level on the earth's hot plastic interior. Typically these float no more than a few hundred feet above current sea level, but since sea level rises and falls over time, sometimes sea level is high enough that most Cratons are below sea level. Modern Cratons include, for example, the interior of North America east of the Rocky Mountains and west of the Appalachian mountains (e.g. the mid west), and central and western Australia.
    
Continents are typically sharply separated and distinct from ocean basins, the other major component of the earth's outer shell, on many criteria. The most distinctive is composition. Continents are composed of Felsic and Intermediate igneous rocks, like Granite and Diorite, or their metamorphic equivalents, schist and Gneiss. They are about 40 km thick.
 
Continents are typically distinguished from from ocean basins, the other major component of the earth's outer shell, based on several critical criteria. The most significant distinction between the two is composition. Continents are composed of Felsic and Intermediate igneous rocks, like Granite and Diorite, or their metamorphic equivalents, schist and gneiss. The average thickness of the existing continents is approximately 40 km.
 
When the earth formed, continents did not exist but were created by processes at convergent plate boundaries (subduction zones). Once the continents formed, they became permanent structures and have grown over time. Individual continents split apart and rejoin via the Wilson Cycle. The oldest continental fragment is the Slave province in northwest Canada, which has been dated at 3.9 billion years, but continents are still growing today all around the world.

2. Ocean Basins - not everything that is below sea level is ocean basin, but ocean basins are always below sea level. When you see a photograph of the earth from space, most of the area underwater is ocean basin. The major exceptions are rims of continental shelf around the edges of the continents. Ocean basins compose the largest surface area on earth. As opposed to the light felsic composition of the continents, ocean basins are composed of relatively heavy Mafic rocks like Basalt and Gabbro. These rocks "float" on the earth's hot plastic interior about 5.2 kilometers below sea level.
    
Technically, ocean basins are composed of the Ophiolite Suite, a multilayered Sequence of rocks. It is generated at Divergent plate boundaries all around the earth, but also disappears down subduction zones at Convergent plate boundaries. Unlike continents, ocean basins form and disappear quickly; the oldest we have is only about 200 million years old (compared to the oldest continent at 3.9 billion.)

Plate Boundaries
 
There are three ways plates meet and interact; this interaction occurs at plate boundaries. Plate boundaries tend to be long, and linear or gently curved areas that are very unstable. Typically they are only a couple of hundred miles wide, but may be thousands of miles long. Most of what is interesting about the earth happens at these boundaries. The descriptions below are terse. We rely more on illustrations than descriptions. More detailed discussions of the processes that occur at the boundaries are explored in the Wilson Cycle and the Tectonic Rock Cycle.
 
3. Divergent Plate Boundaries - where plates move apart and new crust (Ophiolite Suite) is created. As long as a divergent plate boundary is active, an ocean will grow wider.

4. Convergent Plate Boundaries - where plates move together and crust is destroyed by descending into subduction zones. Large, explosive volcanoes (composite type) are always associated with convergent boundaries.

5. Transform Plate Boundaries - where plates slide past one another. Very interesting geology occurs along transform boundaries, as all the faulting along the San Andreas fault system in California attests to, but they play a small role in most of the models developed here.

Hot Spots -
These are isolated, roughly circular, plumes of hot plastic rock that rise from deep in the mantle to the earth's surface.
 
6. Hot spots -  As these plumes from the mantle rise, they heat the lithosphere, the outer rigid layer of the earth, causing it to swell upward into a dome about 1000 km in diameter and about 4 km high. Volcanic activity is commonly associated with the hot spot.
    
The earth has about 120 hot spots today, and they punch their way to the surface just about randomly, under continents, under ocean basins, and on plate boundaries. Although most hot spots are isolated, when they get together they can create a new divergent plate boundary, and ocean basin.

The Wilson Cycle (Single Page version, Multiple Page version)

The simplest model for Virginia's geologic history is the Wilson Cycle, the opening and closing of an ocean basin. Virginia has experienced parts of at least 2 Wilson Cycles during the past billion years and its entire history can be understood in terms of rifting events that tear a supercontinent apart to form a new ocean basin, and subduction and collision events that close the ocean basin again and create new continental crust. This cycle is implicit in the histories to follow, but the Cycle is not dealt with directly in the history. A basic understanding of this cycle will help to understand the history:
    
Two Wilson cycle models are available, a Single Page version, and a Multiple Page version. The single page version is sufficient for anyone to understand the geological evolution of Virginia. The Multiple Page version is divided into nine arbitrary stages. It requires a good understanding of igneous, sedimentary, and metamorphic rocks, as well as structural geology. Note that both Wilson Cycle models are simple, ideal models. The earth has many continents, which migrate across its spherical surface in very complex ways. Just about any scenario you can think of, and any exception you can imagine is quite possible - and has probably happened during some point in the earth's history.

The Tectonic Rock Cycle
 
The earth formed about 4.5 billion years ago and at that time was composed largely of a single mafic/ultramafic igneous parent rock, just as the moon is today. But while the moon died (geologically) shortly after its origin and became a closed system, the earth has remained geologically active (an open system). The result is that the earth has evolved over time.The most crucial fact to understand regarding the tectonic rock cycle is that each and every other kind of rock on the earth has evolved from that parent rock. The evolution has taken place through countless Wilson Cycles over the past 4 billion years. Through that time, the earth, that was very simple in the beginning,, has become more and more complex. The rich diversity of rocks form because plate tectonics has created a great diversity of tectonic environments and each environment generates a specific suite of rocks unique to itself.
    
The Tectonic Rock Cycle, is the most theoretically abstract and sophisticated model of how rocks and the earth evolve together. It is not for the novice, but its principles lie behind every part of the geological evolution of Virginia.
 
Contributed by Lynn Fichter 
Monday, December 01, 2014
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