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The Future

Past, Present, Future, and Time Warps
     When thinking about geological events it is normal to experience a time warp. A year, a decade, a lifetime have meaning we can understand personally. Yet it is possible that in a human lifetime nothing geologically significant will happen in an area, that is, nothing that will end up recorded in the rock record.
     A time warp also exists if we try to compare events close to ourselves with events from the distant past. It is like looking at a distant star through a telescope. In the telescope it is a single object, compact, neat, and understandable, not the enormous, raging, overpowering gas ball filled with complex violent processes we would see if close up.
      Geological events in the past are similar. They almost seem to belong to a mythical past where time and space and distance are tied up in a neat package with a distinct beginning, middle, and end. The same events if seen closer in time seem stretched out and slowed down.
     This stretching and slowing occurs, in part, because, standing on a beach during a once-in-a-hundred-years storm, feeling the wind raging, seeing the waves crashing, and seeing the destructive erosion as beach houses get swept into the sea, we have a sense that something important and powerful is happening. In the rock record, on the other hand, the results of such a storm may ultimately be preserved only as a thin sandstone bed, or nothing at all, and it is hard for us to comprehend that all that fury eventually comes to so little.
     Current events also get in a time warp because the processes are incomplete - we are living in one geologically instantaneous moment of a still incomplete cycle which may require tens, or hundreds, of millions of years to complete. The world we live in just seems too solid and permanent, or changes too slowly, to think that it can someday be a basin under deep, cold, dark, stagnant water, or a towering, erupting volcano. After all, what is going to happen to our house, and our town, and our friends and neighbors when this happens? Our needs, and wants, and mortality intrudes on our thinking in ways it does not when thinking about the distant past. So how can we understand the future?

Predicting the Future
     In the past few stages of our geologic history we saw that in the recent geological past the east coast of North America underwent a rifting event (Stage L), and then became a stable, quiet divergent continental margin (Stage M). It is conceivable that for the indefinite future nothing new or important will happen, that the present mountains will erode down, and the east coast will just sit here and do nothing, except perhaps accumulate a little more sediment. It is perhaps unsettling that the future could be so bleak because it is like death, the remains of the body lying in the earth, undisturbed, forever.
     Is there reason to believe that the future will be more interesting? Yes there is. Think about the Mid-Atlantic history told so far. One of its most distinctive features is cyclicity - the repetition at regular intervals of the same geologic events. The geology of Virginia, extending back beyond 1.1 billion years, and likely back to 1.8 billion years, contains the record of an ocean basin (unnamed), and orogeny (Grenville), a rifting (Protoatlantic), two interim orogenies (Taconic and Acadian), a closing of the Protoatlantic (Alleghenian orogeny), and another rifting event (opening of the Atlantic).
      In a like manner, when looking at rocks from around the world, back to the oldest rocks at 3.96 billion years, we see cycles of rifting and mountain building events. It is reasonable to believe that cycles of rifting and mountain building will continue into the future. There will come a time, of course, when all geological activity will stop because the Earth's internal heat driving the geological processes will be all used up. But for perhaps the next 5 billion years the Earth's cycles will continue, and that is longer than the earth has been around so far.
     What can be expected in Virginia's future? We begin by noting that nowhere in the world today is there an ocean basin much older than 200 million years, even though ocean basins in general must have existed since before the Earth's first continents formed 4.0 billion years ago. Thus, an ocean basin cannot survive forever, in fact does not survive for long. Therefore, we conclude that the Atlantic ocean will not survive forever either. At some time in the future the Atlantic ocean must begin to close and disappear.
     Now, by this time in the history this should not surprise us, for at its simplest the Earth's history, and the Mid-Atlantic's history is one of the opening and closing of ocean basins taking place in Wilson Cycles. What we have along the present-day Atlantic coast is the stable divergent continental margin of the opening half of a Wilson Cycle that began with the Triassic rifting event. At some point in the future, the spreading ocean is going to reverse its direction, and the Wilson cycle will enter its closing phase.
     How will the Atlantic ocean close? It will require a decoupling and the creation of a subduction zone to consume the Atlantic ocean crust. When this occurs, Europe/Africa, which are now drifting away from North and South America, will reverse directions and move back toward North and South America again.
     Where will the continents come back together, and how, and when? This is much harder to predict than just knowing that they will eventually come back together. There is no way of knowing where or when a decoupling will happen, until it happens; decouplings can form anywhere in an ocean basin. And, there is no way of knowing how continents will move over the earth's surface; history shows us that almost anything is possible.
     But if we ask the questions systematically there are some reasonable possibilities. Let's assume that the decoupling is going to begin now, and follow out some scenarios for the immediate future.

Scenario One.
A decoupling occurs under the edge of Europe and/or Africa. These regions then immediately experience volcanic (cordilleran) mountain building. But in the eastern U.S. nothing happens, except that in time the last remains of the Allegheny mountains are eroded down to a peneplain. And there it will sit, doing nothing, for maybe 150-200 million years.
     A subduction zone on the eastern side of the Atlantic still consumes oceanic crust, however, and the Atlantic ocean basin will shrink as it subducts. Finally Africa and/or Europe, with its cordilleran mountain building, will collide with the east coast, override its edge, and produces a major mountain range perhaps the size of the Himalayas. With this, Virginia's rocks will get shuffled, again, and future geologists will have a very difficult time sorting it out. This scenario is likely to produce a major thrust faulted system superposed over the present thrust faults system created during the Alleghenian orogeny.

Scenario Two.
A decoupling occurs under the edge of North America. This results immediately in volcanic (cordilleran) mountain building in Virginia - which will quickly come to look like the string of Cascade volcanic mountains in Washington, Oregon, and northern California. These volcanoes could be active for perhaps 150-200 million years, the time necessary for the Atlantic ocean to close. During this time vast batholiths of diorite and granite will penetrate throughout the rocks of Virginia pushing them aside and metamorphosing them. The coast line will also change dramatically. The broad coastal plain and continental shelf will be gone, replaced by mountains rising abruptly out of the sea, and a deep trench just off the coast.
     Finally, eastern North America will collide with Africa (or Europe), ride up over its edge, build Himalayan size mountains, and assemble another supercontinent. In this case Africa (or Europe) would be the foreland, and experience the major, Alleghanian-type thrust faulting.
     Also in this scenario, since North America and the Mid-Atlantic region would be the hinterland there is a good likelihood that most of the Mid-Atlantic and Virginia would be destroyed in the ensuing erosion and spread across the foreland continent, just as parts of Africa in the Alleghanian orogeny were eroded and spread across the North American continent. There is no event in Virginia's known past comparable to this.

Scenario Three.
A decoupling occurs within the ocean basin, creating a volcanic island arc. Now, the subduction zone could dip to the west, in which case the remnant ocean basin will be to the east. In time the volcanic arc will collide with Europe or Africa and create a mountain. In this scenario Virginia does not change, at least for a long while.
     But, the subduction could dip east (as in the cross section above) in which case the remnant ocean basin will be on the east between the volcanic arc and Virginia. It time the volcanic arc will collide with Virginia producing a Swiss Alps-type mountain range along its edge, and a foreland basin in the Blue Ridge, Shenandoah valley region. This would be similar to the Taconic and Acadian orogenies.

     These three scenarios illustrate the rich possibilities for Virginia's future. In many respects the situation we are in now is similar to Virginia during the Cambrian. Then Virginia was a divergent continental margin with a future that could have seemed endlessly boring. Eventually it proved to not be boring, and it will not end up being boring this time.
     The three scenarios described above are the simplest possibilities. Looking at the history of Virginia so far we have seen events much richer and more complicated than imagined at first, and why should the future be any different? We encourage you to use your imagination. The Earth's and Virginia's geology is rich with details and possibilities and it is all here for us to study and enjoy. Savor the anticipation of what Virginia might yet be.

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Last Update: 9/13/00

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