by John McPhee
Van Schmus said that he and several others were going to date xenoliths in volcanic necks in the Four Corners area, where Colorado, Utah, Arizona, and New Mexico all touch. That is where Shiprock is, and Shiprock (like Devil’s Tower, in Wyoming) is a volcanic neck. Also called a volcanic chimney, a volcanic neck is a conduit through which magma rises toward eruption on the surface of the earth. After everything freezes and epochs pass and erosion tears down the volcano and surrounding land, the volcanic neck may be left standing high because the frozen magma it is made of is so much tougher and more durable than the rock that once lay around it. Differential erosion. A xenolith, in this instance, is a rock from around the margins of the neck that fell into the magma when it was soft. Like a bit of chocolate in a cookie, it is a foreign body with its own age and its own history, distinct from the stuff it went into. The xenoliths in volcanic necks come from all depths of the crust and upper mantle, and the Four Corners project is meant to yield a crustal profile that is the equivalent of a drilled hole thirty miles deep. It is a project that summarizes in itself the new revelation of Precambrian scenes—a revelation made feasible by techniques and technology that were not available ten years ago. Advances in metamorphic petrology—specifically, in understanding the temperatures and pressures at which minerals form—will enable the team to determine the depths from which the xenoliths derive. Advances in trace-element geochemistry will allow them to associate the xenoliths with their original geologic environments. Trace-element indicators will suggest, for example, whether the environment was oceanic or continental. Samarium/neodymium dating will yield crustal histories. Refinements in uranium/lead analyses will allow information to be gathered from units as small as a hundred-thousandth of a gram of zircon.
In what would become Kansas, Nebraska, Colorado, and thereabout, the last of the island-arc dockings occurred around 1700—seventeen hundred million years ago. This final collision further cooked and deformed the rock now visible in the walls of Fremont County’s Arkansas River gorge. The coast of the continent, in 1700, ran from southwestern Texas through Oklahoma, Missouri, Illinois, and Michigan. South of Denver and Cañon City, it was somewhere in southern New Mexico. By 1650, the continental margin had evidently developed into something much like the modern west coast of South America, which is paralleled most of the way by the Peru-Chile Trench, the boundary between the South American and the Nazca tectonic plates. The Peru-Chile Trench is as much as twentysix thousand feet deep, a number that suggests the grandeur of the collision that is taking place as South America, moving west, rides over the Nazca Plate, which consists of nearly ten million square miles of ocean crust. Below the trench, the subduction zone dips to the east and under South America. Down there, the descending crust significantly melts, and the resulting magma has risen through the rock of the edge of South America to emerge as Volcán San Pedro, Volcan Llullaillaco, Volcán El Potro, del Toro, Domuyo, Mercedario, Aconcagua—the Andes. The vertical difference between Aconcagua and the bottom of the trench not far away is forty-nine thousand two hundred and eighty-eight feet. That was the style (the altitudes are unknown) of the mountains of Kansas 1.65 billion years ago, and of the North American coast, running through southern Oklahoma—an Andean margin, with ocean crust subducting and melting beneath it, making Mercedarios and Aconcaguas.
Of the various forms of geophysical data—seismic reflections, gravity anomalies, and so forth—the most useful in the illumination of the Precambrian have been measurements of varying magnetic fields. Data are collected mainly by air, and their effect is to strip off the Phanerozoic cover and show you the Precambrian as if nothing else were there. By 1980, magneticists had reached the point where they felt they could correctly identify rock types from the rocks’ magnetic signatures. Strong magnetic fields showed up on their maps in varied intensities of red, and weak ones in shades of blue and green. The granite family was essentially blue and green, but granites containing magnetite were red. The Midcontinent Rift showed up almost luridly, because of the strong signal of its ironrich basaltic rock. Where drill holes existed, identifications were compared with and supported by drill cores. In 1982, the publication of Isidore Zietz’s Composite Magnetic Anomaly Map of the United States represented what Van Schmus describes as a “major breakthrough” in Precambrian geology. An accomplishment of, among other things, advances in computer programming, it assessed, related, and blended data that had previously been limited to regions and states. To be sure, it was not perfect. Data assembled by one state’s geological survey could be thin and sketchy, while the state next door had been gone over with a fine magnetic comb. The results of such contrasts were apparent geologic schisms along certain state lines, known to sarcastic Precambrian geologists as boundary faults.
Van Schmus would like to see the boundary faults disappear as a result of a dream federal project flying magnetometers over the entire country between the Appalachians and the Rockies with onekilometre spacing. “In the past decade, we’ve made a tremendous first-order advance in the understanding of the basement,” he said. “To take it to the next level of refinement is to take it to more orders of magnitude in effort and cost. The price of one fighter-bomber would advance our knowledge of the basement by an order of magnitude. The full survey would require three thousand trips across the country the short way, at ten dollars a mile. That would come to fifty million dollars, maybe a hundred million before you’re through. What would you learn? You don’t know before you see it, but you would understand the structure of the continent.”
Across the Composite Magnetic Anomaly Map of the United States from one side to the other of the Proterozoic continent runs a series of red bull’s-eyes that have an average date of fourteen hundred and fifty million years before the present. These are the so-called 1450 plutons, an enigmatic event in the behavior of the earth, unprecedented, unrepeated, and unexplained. Something partially melted the whole accretionary belt and the continent was stitched with twenty-five hundred miles of granite plutons. They were granites that tended to be rich in magnetite, hence the red signature. Alternatively known as the 1450 batholiths, they included the Sherman granite of the Laramie Range, the Silver Plume granites of Colorado, the St. Francois Mountains of southern Missouri, and, in Wisconsin, the Wolf River batholith. They cooked and altered yet again the rock that now flanks the deep gorge at Cañon City. The basement of Illinois—very much of a piece with the 1800-to-1650 accretionary complex that lay under the states to the west —became covered with rhyolite lavas of the 1450 events.
Plutons and batholiths are almost by definition a stage and consequence of great orogenies—the building of mountains. Leon Silver, a geologist at Caltech, has called the 1450 plutons the “Anorogenic Perforation of North America.” That is the title of a scientific paper by Silver and others calling attention to the signal mystery of these huge bodies of igneous rock: they entered the earth unaccompanied by mountains. The common wisdom about the 1450 phenomenon has been that it happened in the pressure release of an extensional regime. That is, a supercontinent was coming apart, stretching, thinning, breaking elsewhere to form oceans, and while the event pulled, extended, stretched, and thinned crust that is now North American it brought the melting heat of the mantle closer to the surface, producing anorogenically not only the 1450 plutons but also vast terranes of granite and rhyolite that filled in and covered the upper crust along the eastern and southern margin of the continent. It is even possible that a large piece of North America was torn away in the continental stretching and is now somewhere in the world, its origin in North America as yet unrecognized.
In a very different scenario for the 1450 plutons and related events, some theorists hypothesize a large stable continent in which heat flow is inhibited because there is no active volcanism or rifting. Temperatures under the continent gradually build up with heat from the mantle, and the lower crust undergoes partial melting. As a result, the plutons form and rise. The one explanation seems as logical as the other, and
the cause of the 1450s is not understood. The train of plutons is not the track of a geophysical hot spot, like the volcanic chains of a Tristan da Cunha, a Reunion, or an Hawaii. The 1450s did not intrude the old shield. Some geophysicists have proposed that while the lower crust of the whole continent probably became plenty hot, only the post-1800 accretionary part of North America —the midcontinent south of the shield—was fertile enough to give rise to granites, because everything granitoid had long since been distilled out of the old Archean cratons. Geologists have also mentioned the possibility that the “new” crust—the accreted crust—was enriched in uranium and thorium and potassium, and therefore radioactively melted itself. Van Schmus does not have a favorite guess. He will only shake his head and say, “There is nowhere else in the world where a string of plutons of essentially the same age goes across four thousand kilometres.”
Gravity anomalies are another scope into very deep time. Whereas magnetic anomalies are measured with airplanes, gravity anomalies are measured by people in cars. They stop at every section corner with a gravity meter and take a reading. When missiles mattered a good deal more than they do now, gravity readings did, too, because data on varying gravity fields were essential in the calculations of ballistic trajectories. Even so, collection of gravity data has been nationally and globally spotty. Every square mile of a planet is a lot of section corners. “They haven’t really developed a good airborne gravity meter yet,” Van Schmus remarked. “When they do, it will be nice, because you can fly remote areas like the Brazilian Shield and get a good gravity map.” Magnetic maps include more detail, but they do not show what is in the crust in a complete and three-dimensional way, as gravity maps do. In Van Schmus’s words, “The magnetic field is sampling the shallow crust, the upper few thousand feet. The gravity field is averaging, basically, the whole lithosphere. When you look at a gravity map you are seeing deeper features of the crust.” A gravity meter measures the densities of the rocks below it. A gravity low will be a response to granites or a sedimentary basin—light continental rock. A gravity high is an indication of the densest material: ocean crust right out of the mantle, or even the mantle itself. The Midcontinent Rift, with its basaltic rocks analogous to an ocean spreading center, registers on the meters as a strong gravity high. For interpreting basement geology in the United States, gravity maps are most useful between the Appalachians and the Rockies, because the confusions that are caused by rugged topography do not interfere.
In many ways, the geophysical data from gravity meters and magnetometers are only as good as the nearest tangible rock, the control point that gives credence to the interpretation of numbers. If Precambrian Kansas or Precambrian Nebraska happens to be your target, the nearest rock may be two or three thousand feet below the surface, and you will depend on the drill cores of wells. They anchor the insights. In combination with magnetic maps, the petrology, chemistry, and radiometric dating done on rock fragments from wells has provided the most powerful current method of looking great distances back into time, with gravity maps as a supplement.
Van Schmus has said of the magnetic signature, “It only works if you’ve got some rock to test it with.” When oil companies go particularly deep, academic geologists have a way of appearing and asking for chips. Since oil does not form in Precambrian rock (nothing there to make it of), most oil drilling stops when the Precambrian level is reached. In a few places, though, where faults were thought to have caused oil and gas to go down into the Precambrian, drill holes have followed. Texaco went twelve thousand feet into Kansas—far into the dry sediments of the center of the Midcontinent Rift. In the heart of Iowa and on the flank of the rift, Amoco drilled seventeen thousand feet on the strength of a show of hydrocarbons in similar rock near Lake Superior—producing no oil but substantial amounts of data. Elsewhere, drilling rigs have gone several hundred feet into the Precambrian because the operators did not know where they were, or, thinking they were hitting a Precambrian wedge thrust over younger rock, tried to go through it. When rig operators hit the Precambrian, university geologists sometimes pay them to continue—a smiling face between industry and academia known as piggyback drilling. For a hundred and fifty dollars an hour, or so, the drillers keep going until the bit wears out. For five hundred to a thousand dollars, geologists go away with several tubs full of rock chips. Companies sometimes drill for an extra hour “just to be nice.”
Nebraska wells have confirmed that the Nebraska basement is much the same as Colorado’s—accreted island arcs, and so forth. Van Schmus remarked, “However, there are no good control points in Iowa. It is either an eastern extension of Nebraska or a southwestern extension of Wisconsin. The early Proterozoic history of Iowa remains a great puzzle. The Iowa basement is our biggest gap in knowledge. If I had fifty million bucks for scientific drill holes, I’d spend it there.” An explanation for the gap in knowledge is contained in an expression oft heard in geology: “You could just about drink all the oil that ever came out of Iowa.”
The most comprehensive rock archive of the basement of the Midwest is sort of where it belongs—in the basement of the Geology Department of the University of Kansas. Full of well cores and cuttings from every midwestern state, the archive is about thirty years old. Among the direct results of its existence are maps of the Precambrian midcontinent based less on inference than on fact. Randomly picking up and examining a core there from Buffalo County, Nebraska, Van Schmus said it was a tonalitic gneiss, age 1790. Its biotite and hornblende held its zircons. “The basement of Nebraska is more interesting than the basement of Kansas,” he remarked. “Nebraska has more juvenile, primitive arcs.” For Precambrian purposes, the four best-drilled states in the archive are Oklahoma, Missouri, Kansas, and Nebraska. “In Texas, they get so much oil before they get to the basement they quit,” he said.
The German Republic, in an attempt to be the leader in the field of scientific deep drilling, made a two-hundred-million-dollar hole in Bavaria thirty-two thousand feet deep. On the Kola Peninsula, between the White Sea and the Arctic Ocean, Russia drilled a hole thirty-five thousand feet deep. Beyond the hit-and-miss, oildriven, scrounging mode, scientific drilling in the United States has so far been modest.
Deep in the basement archive seemed as good a place as any to ask Van Schmus to say in his own words what anyone would hope to learn by drilling seven miles through Precambrian worlds.
“First, of course, the Precambrian is nearly ninety per cent of earth history,” he said. “You’ve got to understand it if you’re going to understand the moment when Phanerozoic geology begins. Second, Precambrian shield areas—particularly Archean shield areas—are the hosts of major economic resources: gold, copper, iron, nickel, lead. Diamond pipes are relatively young but they are found only in Precambrian cratons. It has something to do with building up the necessary pressure to form the kimberlites from the mantle”—kimberlite, the species of volcanic neck that is the matrix rock of diamonds. “The Precambrian preserves certain aspects of the earth’s dynamic systems that we can see only in old rocks, because they have been eroded to great depths,” he continued. “We can look at the roots of mountain belts in the Precambrian, whereas in the Phanerozoic all we can see, really, are the middle or upper portions of mountain belts. We don’t know what the roots of the Appalachians look like. We’ve got the folded core of the Appalachians but not the deep collisional roots. We have to look at Precambrian foldbelts to try to understand what goes on in younger mountains at depth. An important thing to recognize now is that with dating techniques we have a way of establishing stratigraphic order in the Precambrian. With mineralogical and geochemical techniques, we have a way of understanding the so-called protolith—or preexisting rock types. We can begin to decipher geologic histories that relate directly to the original formational environments. There are large areas around the world where the Precambrian rocks are preserved in an almost pristine state. They have preserved with them, in local areas, fossil assemblages going back to three and a
half billion years or more. They are the only record we have for tracking the evolution of life.”
In Geology 101, at U-Name-It University, Professor Lucius P. Aenigmatite, scorner of continental drift, used to teach that the Precambrian eons left no fossils. That was virtually the definition of the Precambrian. The arrival of fossils—that is, the abrupt and explosive and unprecedented development of creatures with hard parts—marked the beginning of Phanerozoic time. “There is a large softbodied fauna known as Ediacara that closely precedes the Cambrian,” Van Schmus continued. “What we see before that are just complex bacterial forms, algae, small single-celled organisms. Without that record, and without understanding the environments from which these things evolved, we cannot really understand evolution. There are many so-called chemical fossils in the record, too. Isotopic composition of carbon, for example. It can be monitored through time. It is a reflection of biologic activity on the earth. There’s a number of things in the Precambrian we can look at that sort of get us up to the traditional base-of-the-Cambrian starting point for modern geology.”
Precambrian landscapes had a barrenness beset by weather without vegetal control. The rock summits of high mountains would have looked like the summits of present time, the bare slopes piled in fans of deep scree, like the ranges of Antarctica. Texture rested in topography, color in rock, braided rivers running over the rock. The cycle through which rock is torn apart, ground up, set down, stratified, and made into fresh new rock was unimpeded by so much as a root or a stem, and therefore cycled more rapidly. Only gravity—within its angle of repose—held boulders and gravels to inclined ground. Silts and sands washed down quickly to lakes and seas. Unadorned, unembellished, severely simple, a picture of the Precambrian would present to us the incongruity of desert landscape invaded by white rivers drenched in rain.
