To those of us who love them, maps are wondrous things; and just looking at a map with eyes and mind wide open can allow us to see patterns previously unnoticed.
Looking at a map of North America reveals the ancient, eroded Appalachians in the east and the younger and higher mountains of the western third of the continent. In between is the deceptively flat and seemingly stable heartland, its smooth contours broken only by scattered ranges even more ancient than the Appalachians, now so worn down that they are really little more than hills. Dominating the heartland is the vast Mississippi River system with its three main branches, the Ohio, the Mississippi itself, and the Missouri, all draining the continent into the Gulf of Mexico in the south from the higher ground in the north. But, in the far northeast is a most peculiar feature, the broad St. Lawrence Valley. This river’s flow is far less than that of the Mississippi, but (rather like the Rio de la Plata in South America) its lower reaches are abnormally wide—far wider than the Mississippi. Since rivers and lakes, for obvious reasons, follow the low ground, it is possible to trace the areas of low elevation along the St. Lawrence, then along Lake Ontario and Lake Erie, the Wabash River Valley, the lower Ohio River, and on down the Mississippi. But the lower regions of the St. Lawrence look as if some great force was trying to split the continent apart.
From all of this, a somewhat ominous pattern emerges. Most well informed observers know of the series of powerful earthquakes near the Mississippi and New Madrid, Missouri, in 1811 and 1812, and the countless smaller quakes that have continued to shake that region to this day. Geologists have discovered in this area something they call the New Madrid Seismic Zone and, just to the southwest, the Reelfoot Rift. In fact, there are a number of ancient rift zones, or long faults, in the heartland; and there have been earthquakes almost everywhere in America, including Arkansas, and, especially recently, a good many in Oklahoma. There have been earthquakes in Indiana and even on the flat Gulf Coast Plain areas of Texas, Louisiana, and Alabama where sediment is miles deep. In fact, geologists speak of the St. Lawrence Seismic Zone and the Wabash Seismic Zone; and some consider the New Madrid area to be an ancient rift where, long ago, seismic forces tried to split the continent. Some believe that the rifting began some 750 million years BP (before the present) during the Neoproterozoic Era, when North America was part of the continent of Rodinia. Others believe that the rifting may date to the time when the Appalachians formed. Most believe that it is a failed rift…but is it?
Retired Texaco geologist Jack M. Reed finds the conventional view very suspicious. In fact, Reed believes that the entire Gulf of Mexico is an active seismic zone and, possibly, a separate, south-moving crustal plate. He thinks that there may very well be one continuous rift from the mouth of the St. Lawrence to the Mouth of the Mississippi and that the seismic activity in the Gulf has helped to create this rift. He points to such tectonically active formations as the Monroe Uplift, the La Salle Arch, and the Sabine Lake area on land, and, in the Gulf of Mexico, the Desoto Canyon, and a domed uplift nearby, the Cretaceous Shelf Edge, the Suwannee Strait, and the West Florida Escarpment. The Suwannee Strait is a depression extending across northwestern Florida and parts of Georgia, which Reed suspects is connected to the active seismic zone just inland from Charleston, South Carolina—a city that experienced a powerful earthquake in the nineteenth century.
Geologists and geophysicists currently tend to believe in the theory of plate tectonics, and if we examine this theory and then, in light of it, take another look at the overall structure of North America, we can at least begin to understand what is happening. Although the exact mechanism driving plate movement is poorly understood, and despite the fact that almost anything anyone says about it will be an oversimplification, the theory is supported by massive evidence and explains a good deal about the earth. Basically, geologists believe that the crust and the solid upper mantle of the planet (the lithosphere) are broken up into a number of plates and their boundaries are either divergent: along spreading zones (most of them under the ocean) where magma oozes up from the lower mantle; or convergent: with one plate subducting under another; or transform: where the plates rub against one another horizontally. An example of a transform boundary is the San Andreas Fault which would be an ordinary strike-slip fault were it not considered to be the boundary between the North American Plate and the Pacific Plate. The plates of the lithosphere move over the partly melted layer of the mantle known as the asthenosphere.
Not all geologists agree on the number of plates or their exact boundaries; sometimes it is difficult to tell if a break in the crust is truly a plate boundary or simply a massive strike-slip fault. Also, the boundary is often not a simple line but a wide zone. It was at first assumed that sea floor spreading explained the movements, with magma welling up along the entire length of a rift zone due to convection currents. However, although there are hot spots where convection currents drive magma plumes up through the crust (examples are Iceland, the Big Island of Hawaii, and Yellowstone, among others), it is a bit difficult to explain how a convection current would form a long, narrow line. In the case of Iceland, incidentally, the hot spot is also on a spreading zone. Perhaps the plates are not only pushed at spreading zones, but also pulled by cooler, denser basalt sea floor sinking under the lighter granitic rocks of continents. Tidal forces and the earth’s rotation may also play a role, and perhaps even asteroid and comet impacts have had something to do with it. But the movements, for billions of years, have split continents, moved them apart, and brought them back together, combining and recombining our planet’s surface features.
The original nucleus of North America was the North American or Laurentian craton, whose highest portions today make up the Canadian, or Laurentian, shield. It was formed by eruptions of sialic (containing relatively high amounts of silica and aluminum) igneous rock called plutons which were less dense than the underlying mafic (less silica and more iron) rock of the mantle. This formation is related to similar formations in Greenland. Through an imperfectly understood process, much of it was apparently uplifted to heights dwarfing most modern mountains, for, today, after several billion years, there are still very high mountains in Greenland and in northeastern Canada’s Arctic islands. This is why the Mississippi flows from north to south and why there are still ranges of very high hills in parts of central Canada, Minnesota, and Michigan. Most of the highlands of eastern Canada, including those around the St. Lawrence Valley, are part of this original craton.
Over a billion years ago, a volcanic plateau formed, and was later eroded and mostly covered by sediments, forming today’s Ozark Mountains (really just hills) of southern Missouri and northern Arkansas. Another early plate collision formed the Wichita Mountains of southern Oklahoma, perhaps 500 million years ago; and, about 300 million years BP the South American plate is believed to have subducted under North America forming the Ouachita Mountains of Arkansas and eastern Oklahoma and the Hill Country of central Texas; the eroded mountains in between are mostly covered by sediment. This Ouachita orogeny also produced the northwest to southeast trending Southern Oklahoma Rift.
Some 480 million years BP North America collided with what are now Europe and Africa, crumpling the crust and forcing it up to form the Appalachians, probably about as high then as the Himalayas are today, and forming the continent of Pangea. Later, around 130 million years BP and along roughly the same boundary, the continents separated along what is now the Mid-Atlantic Ridge, forming the Atlantic Ocean. Geophysicist J. Tuzo Wilson noted the tendency for the crust to break, reform, and break again in the same areas; this is referred to as the Wilson Tectonic Cycle. Some contemporary geophysicists, like Tony Lowry and Marta Perez-Gussinye, suspect that this happens because these regions have more quartz in the rocks that weakens the crust. For a long time, the immense North American Plate moved roughly west colliding with the Pacific Plate, which mostly subducted under our continent. New mountains were folded up, volcanoes erupted, and “terranes” or islands and fragments of continents accreted to the western edge of North America. All of this gave us the Rockies, the Sierra Nevada and Cascades, the Coast Ranges, and all the other high western peaks.
Now the North American plate, or at least its western edge, appears to be moving to the southwest, while the eastern Pacific Plate moves to the northeast, making the plate boundary, the San Andreas Fault, a strike slip transform fault. But things are not quite that simple. Roughly parallel to the dreaded San Andreas are innumerable other faults, like the Hayward Fault and the Great Valley Fault along the eastern margin of the Coast Ranges. The Sierra Nevada (rather like the Tetons of Wyoming or the Sawtooth Mountains of Idaho) is a huge block of the crust, broken and thrust upward by a fault on its east side and tilting up more gently on the west. Another look at the map reveals the Gulf of California, or Sea of Cortez, where an extension of the East Pacific Rise (a seafloor spreading zone) is literally splitting Baja California away from the rest of the continent. The splitting does not stop at the water’s edge, but extends, under sediments, under California’s Imperial Valley. There are volcanoes all along the Sea of Cortez and cinder cones and geologically recent lava flows throughout southeastern California. Then there is Death Valley, resembling the Dead Sea in the Middle East or Lake Baikal in Siberia, only without the water. Death Valley, at its deepest point, is 282 feet below sea level with deep sediment under that. Like any sea floor spreading zone, it is bounded by mountains: the Amargosa Range to the east and the Panamint Range to the west. North and west of Death Valley, along the eastern Sierra fault zone, are the Long Valley Caldera,site of an ancient mega-eruption, and Mono Lake, Mono craters, and the Mammoth Mountain volcano. North of Death Valley the faulting and low ground extend along the Walker Lane that goes as far as Oregon. So it is not clear that the San Andreas is really the true plate boundary; and, eventually, the Sea of Cortez may extend further and further north.
And there is at least one other major rift zone that looks like more than just a regular earthquake fault. The Rio Grande rift zone extends from Colorado to El Paso and is bounded by steep faulted and folded mountains to the east, like the Sangre de Christo, the Sandias, the San Andreas Mountains, the Organ Mountains, and the Franklin Mountains. On the west are volcanoes and geologically recent lava flows, like the San Juan Mountains and the Jemez Mountains. There is no way to know if this rift will grow larger or not.
Returning to the heartland and its apparent rift zone, the big problem in predicting what may happen is the fact that we do not fully understand the forces acting on the North American plate, which, rather than simply moving to the west, may now be rotating in a counterclockwise direction…or, at least, its western portion is. Clearly, if a plate is being pushed in one direction and/or pulled in another, every portion of it, dragging over the asthenosphere, will be affected. If the forces are acting to pull the continent apart along the ancient rift zone in its center, rest assured that it will, indeed, come apart. Remember that high mountains rise today where once there was ocean floor, and level plains and ranges of hills are all that remain of past mountains. The only constant, as they say, is change.
And if change comes to the heartland, will it be imperceptibly slow or catastrophically rapid? Geology, early on, was dominated by Bible-inspired catastrophists; later, uniformitarianism became the ruling paradigm. Our understanding of asteroid and comet impacts, mega-volcanoes, and mega-tsunamis has moved most geologists more toward the middle, and they tend to believe that generally slow processes are occasionally interrupted by truly awesome events. Yet it would seem that, even if there are massive earthquakes along, say, the New Madrid seismic zone, or even if the land subsides, or the St. Lawrence River becomes even wider, it is unlikely that the entire continent could split open at once or even in a few centuries. And more active areas like Africa’s Afar Triangle or the Sea of Cortez here in North America would seem more likely candidates for sudden change.
But massive earthquakes have become much more frequent in recent decades, and, as the earth shakes its way toward 12/21/2012, we should probably not be surprised at anything.