How the Earth Moves and Changes
The surface of the earth is in constant, stately motion, though its movements are often too slow for human senses to detect. Continents shift across the globe over millions of years, making once-fixed landmarks merely temporary descriptions. John McPhee explores this shifting reality by traveling with geologists who view the landscape as a flowing narrative of creation and destruction. In New Jersey, the rock walls of the Palisades Sill serve as a starting point for understanding these global forces. This massive layer of dark rock was formed during the Triassic period when the supercontinent began to pull itself apart. As the land strained to break into great blocks, fluid rock from deep within the earth surged upward, cooling slowly beneath the surface to form the crystals visible in the rock today.
To a geologist, the interstate highway system is more than a means of travel. It is a series of portals into the deep history of the planet. Roadcuts, created by the blasting of highways, act like a doctor’s stethoscope or an X-ray, allowing scientists to see freshly exposed rock that has not been obscured by soil or vegetation. These exposures reveal the essential clues of how the land formed, showing ancient lakebeds in Wyoming or shallow oceans that once reached across the eastern United States. While highway departments often try to cover these rocks with grass or vines to make them look more natural, geologists see these exposed rocks as beautiful fragments of a regional story.
Traveling westward across North America reveals a series of distinct geological provinces. After leaving the folded and crumpled Appalachian Mountains of the East, the land settles into the stable interior craton. This is the enduring core of the continent, a vast heartland that has remained relatively undisturbed for hundreds of millions of years. In states like Ohio, Indiana, and Illinois, the ground is a thick sedimentary veneer covering the ancient rock of the continental shield. As one moves further west, the land rises on the debris shed by the Rocky Mountains until the mountains themselves appear. Beyond the Rockies lies the Basin and Range, a unique landscape of north-south mountain ranges separated by wide, flat valleys.
The mechanics of the Basin and Range are defined by the thinning of the earth’s crust. Unlike most mountain ranges, which are created by the mashing together of the crust, these ranges are the result of extension. The crust is being stretched so thin that it has broken into massive blocks. These blocks do not sit flat but instead tilt like seesaws. The high edge of a block becomes a mountain range, while the low side forms a basin that fills with sediment washing down from the heights. This tectonic activity suggests that the continent is splitting open, driven by the hot mantle rising beneath the crust.
Understanding these movements requires a shift in perspective toward deep time. For centuries, the prevailing belief was that the earth was only a few thousand years old, a timeline that left no room for the slow cycles of erosion and uplift. This view was challenged by James Hutton, an eighteenth-century Scottish farmer and scientist. Standing at places like Siccar Point, Hutton observed rock layers that had been turned vertically and then covered by newer, horizontal layers. He realized that for such a formation to exist, an entire mountain range had to be built, worn down to nothing by rain and wind, submerged under a sea to collect new sediment, and then lifted back up.
The geological time scale is built upon this concept of deep time, organized by the history of life preserved in fossils. The Paleozoic era began with a sudden explosion of creatures with hard shells and ended with a massive extinction that wiped out nearly all marine species. The Mesozoic era followed, dominated by dinosaurs, and ended with another great catastrophe that cleared the way for the Cenozoic era and the rise of mammals. These eras are not just abstract concepts but are physically written in the rock. For example, the summit of Mount Everest is made of marine limestone, meaning the highest point on earth was once a quiet seafloor filled with ocean life before the collision of India and Asia drove it five miles into the sky.
The modern understanding of these global changes is defined by plate tectonics, a scientific revolution that occurred in the 1960s. This theory explains that the outer shell of the earth is divided into about twenty rigid plates that move in different directions. New seafloor is created at mid-ocean ridges, where plates pull apart and magma rises to fill the gap. Old seafloor is destroyed at deep ocean trenches, where it dives back into the interior of the earth to melt. When a continent is carried by a plate into a trench, it is too buoyant to sink, so it jams the machinery and buckles upward to form massive mountain chains.
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