No matter when and where the Morton formed, its birth as an igneous rock raises another question. What was the source of the magma? Magma doesn’t just appear magically, it has to have a parent rock that melts to form a liquid. The process begins in the asthenosphere, the partially molten layer of the mantle made primarily of peridotite, a greenish rock rich in the mineral olivine. The asthenosphere starts about 45 miles below the surface and extends to about 150 miles deep.
When asthenospheric peridotite partially melts, it produces a magma that solidifies to basalt, similar to what erupts in Hawaii. A good analogy is the partial melting of a Popsicle as you eat it outside on a hot day. As you hold your frozen treat, it invariably melts, sending a stream of sugary syrup down your arm. When you take the next bite of your icy confection, you will notice it has a slightly different taste and texture. The Popsicle is still solid but less sweet and more granular with icy particles. When a peridotite partially melts, it loses various elements such as iron, magnesium, and calcium to magma, but still remains a peridotite.
In the geologic process, less than 3 percent of the total peridotite changes to a basaltic magma and the rest remains as peridotite. The most common place to find basalt is at spreading centers where plates begin to pull apart from each other. Most of these spots are what geologists call a midocean ridge, such as occurs in the middle of the Atlantic Ocean or off the coast of Washington State where the Juan de Fuca Plate moves away from the Pacific Plate toward the North American Plate.
When basalt partially melts, the resulting magma can generate a tonalite. This process occurs most often at a subduction zone, where the basalt dives into the planet, warming roughly twenty-four to thirty-two degrees Celsius for every mile it descends. When it reaches temperatures of seven hundred to eight hundred degrees Celsius it partially melts. Because the melt is only partial, on the order of 10 to 15 percent, it is not unusual to find basalt inclusions in a tonalite, sort of as if chunks of Popsicle also dropped onto your arm at the same time the Popsicle melted.
In the Morton Gneiss, basalt appears as solid black rafts floating in the gray and pink swirls. The rafts range in size from a few inches to sixty-five feet across. Geologists refer to these anomalous blocks as enclaves or xenoliths, meaning foreign rock. Mark Gross, who has worked the Morton quarries for Cold Spring Granite for twenty-eight years, called the smaller of these variously shaped blobs “knots” or “cigars” (pronounced see-gars).20 Quarrymen don’t like the knots and cigars because buyers don’t want such imperfections marring their stone.
Unfortunately it is impossible to date the Morton’s basalt rafts because they lack zircons. Bickford believes there is a strong possibility that the basalt originated more that 3,524 million years ago but the rafts could also have formed during a later geologic event, which melted the basalt only enough to generate more basalt that injected itself as dikes into the surrounding rock. Such dikes are not unusual in other rocks that formed from basalt.
After recording the first crystallization of the Morton tonalite, the zircons logged geologic events at 3.42, 3.385, 3.14, and 3.08 billion years ago. Little field evidence for these events remains. The only feature that records one of them may be veins of light-colored rock peppered with black minerals that shoot randomly across the rock. Gross called the veins “big, ugly ropes,” and like knots they lower the value of the stone.
The zircons went haywire 2,680 million years ago. Not only did the Morton rocks get folded like a gymnast, but they also got an infusion of pink magma that finally, after nearly a billion years of boring gray, gave the rocks their distinctive coloring. The fact that several nearby rock units record this same date means that an epic collision altered the Morton. That collision was between the block of rock that was the Morton’s home, known as the Minnesota River Valley terrane, and a much larger mass of land, known as the Superior Province.
Mark Schmitz, a geologist at Boise State University who also studies the Morton, compared that impact to another impact that occurred only 60 million years ago. “Minnesota at 2.6 billion years ago would have looked like the modern Himalayas,” he said. “There would have been a space problem as the two blocks came together. They would start to deform and thicken. As one block slid under the other, pressure and temperature would rise and the rock would start to melt.” Or as another pair of geologists wrote, the collision resulted in “manifestations of constipated subduction.”21
Once again, partial melting would occur, with the tonalite spawning a melt but this time the melt was pink; pink magma that injected itself into the gray tonalite as veins and pools. Heat didn’t just melt rocks, it also weakened the Morton to a taffylike consistency and caused it to swirl, surge, seethe, and eddy. Most of the rock was still solid but the mountainous pile of material above was squeezing and deforming the Morton like toothpaste and metamorphosing it into a gneiss. Similar changes may have occurred at the earlier events recorded by zircons but the continental collision 2,680 million years ago erased previous textures and generated a metamorphic rock consisting of the bands of dark and light minerals that characterize gneiss.
The rock did not respond homogenously. The basalt rafts acted plastically, and either bounced back to their original shape or broke into fragments. The pink granite and tonalite acted like Silly Putty, deforming but not breaking under pressure; but the tonalite was less fluid than the pink granite.
One of the beautiful aspects of the Morton rocks is that you can see this give and take of rock. In one panel pink dominates, in another gray, and in a third, rafts of jet black basalt sit like islands awash in a sea of pink. Some Morton building panels look like still photographs of streams of blood flowing through arteries, a texture that quarry workers call veiny. But the dominant pattern resembles a series of pictures taken while stirring together cans of pink and gray paint. Quarrymen call this texture flurry.22
No matter which texture one sees in the Morton, the rock seems to be constantly in motion. Nothing is static. Although the rock records events that took place between 3.5 and 2.6 billion years ago, it is the most alive rock I have ever seen.
The Morton Gneiss story, however, did not end with collision 2.6 billion years ago. Two billion years ago, plate tectonic action thrust the Morton up and for the first time in its multibillion-year existence, the pink and gray rock was at or very near the surface of the planet. After 2.4 billion years of action, the excitement ended. Its story, though, was not over yet. One more significant geologic event would have to hit the Morton in order for it to be exposed at the surface, but that landscape-altering process would not occur until just thirteen thousand years ago.
Cold Spring Granite still owns and operates the Morton quarry, although they dismantled and scrapped their record-sized boom derrick in 1996. As happens with every product, fashion waxes and wanes and the Morton has been in a long-term wane; Cold Spring quarries stone at Morton for only a few months of the year. When I inquired about visiting the quarry, Dan Rea, vice president for Cold Spring’s Commercial Division, was kind enough to open the site for me.
My guide was Mark Gross. We reached the quarry by driving through a gate with a No Trespassing sign south of town, down a dirt road, and parking next to an abandoned metal shed. Like every other quarry I have seen, the multilevel, football-field-sized stoneyard was strewn with massive blocks of cut stone and rubble piles of cut and broken stone. It also had the requisite scary-looking pit of cloudy water, which usually designates the oldest, now unused, portion of a quarry. Rusty water streaks stained the older cut walls, which towered fifty to sixty feet above the quarry’s main floor and provided good nesting locations for swallows. Other walls were dotted with hundreds of parallel grooves as if troupes of industrious clams had been in a synchronized burrowing competition. These holes had been drilled to break blocks off a quarry wall.
“We now only have a few men work the quarry and they are specialists,” said Gross. Cold Spring removed their big derrick because trucks and front-end loaders are
more efficient. Working a boom required at least four men—one to operate the hoist, one to signal the operator, and two to attach the derrick cable to the block. Operating the newer machines requires only one man and is much less dangerous since multi-ton blocks no longer dangle from steel cables.
Gross called the process of quarrying “building a loaf.” Imagine a squared off quarry wall, flat on the top with two perpendicular, vertical faces, one trending north, the other west, forming a corner. First, a quarryman uses a hydraulic drilling machine, which both pounds and spins a carbide-tipped drill, to cut a horizontal tunnel, up to eighty feet long, into the base of the north face. The three-inch-wide hole runs parallel to and eighteen feet from the base of the west face. In step two, the driller stands on the flat top and pierces the end of the horizontal hole with a vertical shaft as long as eighteen feet. To cut the rock, he threads a diamond-impregnated wire into the vertical shaft and down the horizontal tunnel and makes a loop by reconnecting the wire’s two ends. The loop feeds through a machine that moves the wire like a conveyor belt. As the wire slowly cuts into the rock, the machine tightens the loop and it slices through the rock. This first cut of the loaf takes about fifty-eight hours in the Morton rocks; cutting softer granite takes half the time.
The quarrymen have two options at this point. They can either repeat the drilling and slicing process every six feet and fashion slabs eighteen feet high by six feet wide by eighty feet long or they can make one shorter cut and create a loaf eighteen feet high by eighteen feet wide by eighty feet long. Since both the slabs and loaf are still attached at the base, the driller drills a series of horizontal holes seven inches apart at ground level back to the eighty-foot-long horizontal hole he drilled. He then pushes sticks of Dynashear, roughly equivalent in force to about one-twentieth of a stick of dynamite, back into the holes, and detonates (or shoots) a slab or loaf, which pops free.
A buyer’s need and stone quality dictates whether the quarryman cuts a slab or a loaf. Mausoleums may require wall-sized panels of rock and use of a loaf, which the driller drills into slices, six feet thick by eighteen feet wide by eighteen feet tall. In contrast, cladding or countertops use a long, skinny slab. To work on the slab, drillers drive wedges into the long gap made by the wire saw and force the slab to tip over onto old tires the size of a car or onto a pile of broken-up stone. Smaller blocks are made by drilling, which creates the burrowlike channels I saw on some walls of the quarry.
Unlike sedimentary rock, gneiss does not have to season or cure. It can be worked immediately. No further cutting, however, occurs in Morton. Blocks and slices get moved via a front-end loader onto trucks and transported to the Cold Spring Granite factory, in Cold Spring, Minnesota, about ninety miles north. Again, Dan Rea set up a tour for me.
After putting on a yellow hard hat and clear plastic goggles, I entered the cavernous fabrication, or milling, plant. We started at the gang shot saw, which looked like a bread slicer on steroids. Instead of sharp blades, however, the machine used rows of parallel steel plates that cut through the stone by moving back and forth, like a reciprocating saw. The flat, quarter-inch-thick, two-inch-tall steel plates, each about fifteen feet long, don’t actually cut the stone but grind a slurry of water and steel shot—broken up bits of steel about half the size of a grain of rice—that do the cutting. Operating twenty-four hours a day, the incredibly noisy gang saw, named for its gang of blades, cuts through an eight-foot-thick block in three to four days. It can cut panels as thin as an inch.
Cut panels, which moved through the building via bright yellow overhead cranes, next received a surface finish. The first finishing machine used pie-pan-sized, diamond-encrusted buffers and could produce a finish ranging from glassy smooth to coarse and nonreflective. For a rougher finish, Cold Spring had a machine that resembled a pizza oven and sounded like a jet. Known as a thermal finisher, its eighteen-hundred-degree Celsius torch expanded and exploded surficial feldspar crystals, leaving a textured surface that works well for paving stones. Cold Spring formed rough, natural-looking faces with a stone splitter, which worked like a slow-motion guillotine to crack open a rock. When the stone cracked it sounded like a gunshot.
Cutting took place on the other side of the building. Computer-controlled, diamond-tipped circular saws cut most finished panels. Cold Spring had several with a variety of blades. They use one of the blades mounted on a overhead arm to make round columns by running the blade down the length of a column, rotating the column slightly, and moving the blade slightly out or in and down. Smoothing out the rough edges requires hand grinding and polishing. Humans also perform some of the most high precision work, such as cutting floral patterns into the rock.
For other intricate work, a high-pressure water jet can accurately cut to within one one-hundredth of an inch. The water shoots out at over nine hundred miles per hour at pressures of up to sixty thousand pounds per square inch (psi) and can cut stone up to four inches thick. By comparison, a typical fire hose operates at between one hundred and three hundred psi and a household faucet at between sixty and eighty psi.
Cold Spring primarily sells the Morton Gneiss for buildings, monuments, tombstones, and mausoleums. It is not popular, selling about 8,000 cubic feet annually, compared with Cold Spring’s best sellers—a gray granite from Minnesota and a speckled, red-and-black granite from South Dakota—both of which sell more than 120,000 cubic feet per year. Despite the low sales for the Morton, Cold Spring vice president Dan Rea was optimistic. “Trends change. There is always hope.”23
Twelve years after William Keating and Giacomo Beltrami ascended the Minnesota River,London-born George William Featherstonhaugh (pronounced Fanshaw) traveled up the waterway in a birch-bark canoe that carried eight men and thirty-five hundred pounds of supplies. Known to the native people as the Stone Doctor and to the federal government as Geologist to the United States, he reached the valley of the Minnay So-tor, as he spelled it, in September 1835. Featherstonhaugh noted the unusual gneiss, collected specimens, and made a perceptive observation: “It is evident that in ancient times . . . the volume of the river was many times greater than it is now.”24 He would be astounded by how much water once flowed down the Minnesota River valley.
This giant predecessor to the Minnesota River is called the River Warren.25 When it flooded thirteen thousand years ago, Warren shot down the valley and carried as much as 100,000 times more water than what Featherstonhaugh encountered. Fed by runoff from one of the greatest lakes to cover the planet—Lake Agassiz—Warren spread across the entire valley, purging the land of soil and plants. The turbulent water also stripped away cobbles, rocks, and boulders deposited by previous glaciations, revealing the wonderful pink, gray, and black Morton Gneiss that lay below.
This final chapter of the Morton Gneiss’s 3.5-billion-year-long story began during the last ice age, when a tongue of the Laurentide Ice Sheet retreated north back into Canada. At its maximum the several-thousand-foot-thick glacier had covered most of our northern neighbor and extended as far south as Iowa, but by 13,400 years ago, the climate had begun to warm and the ice was disappearing. Lake Agassiz formed along the southern boundary of the ice, at about modern-day Winnipeg. It spread mostly west to east, but one arm pushed south down into the United States along the modern-day Minnesota/North Dakota border.
Named for the great naturalist Louis Agassiz, Lake Agassiz lasted for over five thousand years and eventually grew to cover 580,000 square miles, about the size of Alaska.26 Four outlets drained the lake, which reached a maximum depth of 2,500 feet. One drainage carried water north to the Arctic Ocean, one east out the St. Lawrence valley to the Atlantic, one northeast through Hudson Bay, and one south to what later became known as the Mississippi River valley—the River Warren. They did not drain at the same time.
For 99 percent of its life, Warren flowed at an average of 1.6 million cubic feet per second, or about three times the flow of the modern-day Mississippi River at its mouth and thirteen hundred times the flow
of the Minnesota River near Morton. But then some environmental change triggered a flood of epic proportions. For days and maybe weeks, the river ripped at up to sixty miles per hour down the valley, a grayish-brown, roaring, churning soup of mud, debris, plants, boulders, and animals. Total estimated volume would have about equaled the combined average annual flow of the ten largest rivers on Earth.
Geologists don’t know how many of these megafloods scoured the Minnesota River valley. The last one hit 12,900 years ago and all of Lake Agassiz drained in a cataclysmic flood 8,400 years ago. Geologists do know, however, that without the River Warren floods and the river’s constant, high-volume flow out of Lake Agassiz, the Morton Gneiss may never have been revealed. Instead, tens to hundreds of feet of glacially deposited sediments would still be covering the rock and one of the planet’s longest geologic stories could not be told.
I find it compelling that the last chapter of the Morton story is also the final global geologic event to hit the planet. From deep time to modern time, the Morton Gneiss has been present. It existed when life evolved on Earth, when modern-style plate tectonics began to operate, and survived the ice age. As one geologist says of the Morton, “It’s got it all.” Plus it is a damned gorgeous rock.
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THE CLAM THAT CHANGED THE
Stories in Stone Page 11
