John Dvorak - How the Mountains Grew: A New Geological History of North America

Geologic Time Scale

I n the far western reaches of South Dakota, among the many towers and great blocks of hard rock that are the Black Hills, the faces of four former presidents stare out from high on a rocky cliff face. These four facesGeorge Washington, Thomas Jefferson, Theodore Roosevelt, and Abraham Lincolncolossal by any standard, were literally blasted out of the hard rock with dynamite. They were then chiseled into their final forms with jackhammers manned by a small army of men who suspended themselves down the steep cliff face at the ends of long steel cables and who sat on broad leather straps that resembled bosuns chairs to do their work.

In all, it took fourteen years to complete the stone carvings at Mount Rushmore. The artist who conceived and directed the work, Gutzon Borglum, would tell politicians and newspapermen who could promote his work that it was his intention to make something more than the biggest in the world. It was his goal to produce a monument that would last through the ages, one that would rival the great stone statuaries of ancient Egypt and of ancient Rome. In that, he succeeded. But there is something more here, something decidedly different if one takes the time to study the four great stone carvings and to explore the surrounding countryside.

Look at George Washington. He and the other three are carved out of a light gray rock. Immediately below Washington is a band of a much darker rock with a distinct pattern of loose lines that sweep up and to the left. The light gray rock is a granite, once a bulbous mass of molten rock that rose up out of the Earths crust where it cooled and solidified. The dark rock was originally composed of ocean sediments that are much older than the granite. Long after those sediments were laid down, they were buried, then heated when the granite was molten and rose up and moved into the crust. The effects of the burial and of the heat changed the mineral content so that a new suite of minerals formed. That transformed the original ocean sediments into a different type of rock, a metamorphic rock that, from the degree of transformation, is known as a schist.

Now hike the trails and drive the roads that run through the Black Hills. One soon discovers that the dark schist forms a ring around the granite. More exploration reveals the ring of schist to be surrounded by three more rings. The first, moving outward, is a yellow limestone, the next a red shale, and finally, a white sandstone. From a high, overhead view, the entire assemblage looks like a giant blister that has risen out of the Earth and had the outer layers of skin scraped away. That, in fact, is not far from the truth.

At the center of the blister is the granite. It and the schist and the other three rings of rock have been pushed up so that a broad dome has formed, the Black Hills. The amount of upward movement has been considerable. The highest point in the Black Hills is Black Elk Peak. It stands several thousand feet above the broad surrounding plain, making it the highest point of land between the nearby Rockies and the distant Pyrenees.

That is the puzzle. Why do the Black Hills, one of the youngest mountain ranges in North America, stand so high, and why do they lie so isolated in the middle of a continent? Most mountain ranges, at least most that are rising today, lie along the edge of a continent where other geologic activity is high, meaning where earthquakes are frequent and where volcanoes are erupting. For North America, that is along the West Coast where the Cascades, the Sierra Nevada, and the Traverse Ranges of Southern California are rising. So why did the Black Hills form where they did? Why so recently? Why is there a mass of rocky cliffs and high peaks standing in the middle of North America?

For a long time, geologists could say little more than the obvious about why mountains grew: There were forces acting inside the Earth that pushed the surface upward, and that formed mountains. Then came the theory of plate tectonics.

This theory, developed in the 1960s, proposed that the surface of the Earth is divided into a dozen or so rigid platestectonic platesthat are in constant motion. As the plates move, the boundaries between the plates push or pull or slide against each other. It is where the pushing or the pulling or the sliding is taking that earthquakes are frequent and where volcanoes are now erupting.

It is where two plates are pushing against each other that mountains form. The Himalayas are rising because the tectonic plate that includes the subcontinent of India is pushing against the massive continent of Eurasia, another tectonic plate. The Andes are a product of a tectonic plate, one that underlies part of the Pacific Ocean, slamming into South America.