Geology: Grand Canyon
Vishnu Basement Rocks Tectonic plates move slowly across earth’s surface. Almost two billion years ago a plate carrying an island chain and the plate that became north America collided. Heat and pressure from this process changed those existing rock layers into dark metamorphic rock, the basement of the canyon. Molten rock squeezed into cracks and hardened as light bands of granite.
Grand Canyon Super Group The red shale, fossil-bearing lime stone, and dark lava of the Grand Canyon Supergroup are revealed in only a few areas. The many strata of the Supergroup accumulated in basins formed as the land mass pulled apart. The expansion caused blocks to tilt, inclining the Supergroup layers. The same process caused Nevada’s alternating basins and mountain ranges.
Layered Paleozoic Rocks Nearly horizon layers of sedimentary rocks comprise the upper two-thirds of the canyon’s walls. These rocks formed near sea level and at the edge of the continent. The remains of marine life accumulated on the ocean floor to form limestone. Rivers deposited sediments in swamps and deltas that then became mudstones. Dunes solidified into sandstone.
The Colorado Plateau Rises About 70 million years ago the Rocky Mountains began to form, pushed up as the north American Plate overrode the Pacific Plate. As result, a large section of what is now eastern Utah, northern Arizona, western Colorado, and a corner of New Mexico rose from sea level to elevations of thousands of feet, forming the Colorado Plateau. This uplift with remarkable little tilting of deformation of sedimentary layers. The stage was set for the carving of the Grand Canyon
Canyon Carving By five or six million years ago, the Colorado River flowed across the Colorado Plateau on its way from the Rocky Mountains to the gulf of California. Each rain washed sparsely vegetated desert soils into the river. A steep gradient and heavy sediment loads created a power tool for erosion. The river’s volume varied seasonally and over time. As the last Ice Age 12,00 years ago, the flow may have been 10 times today’s volume. As the river cuts down, the canyon deepens. Tributaries erode into canyon’s sides, increasing its width. Erosion carves farther into the softer rock layers, undermining harder layers above. With no foundation these layers collapse, forming the cliffs and slopes of of the canyon. Erosion wears away the ridges separating adjacent side canyons, leaving buttes and pinnacles.
Deep time, Changing Landscapes Grand Canyon reveals a beautiful sequence of rock layers that serve as windows into time. The carving of the canyon is only the most recent chapter, a geologic blink of an eye, in a long story. That long story includes rock nearly two billion years old in the bottom of the canyon, land masses colliding and drifting apart, mountains forming and eroding away, sea levels rising and falling, and relentless force of moving water. Many canyons form as rivers cascade among mountain peaks, but Grand Canyon sites incised into an elevated plateau. The desert landscape exposes the geology to view. IT is not hidden under a cloak of vegetation. The strata reveled preserve a lengthy, although incomplete record of Earth’s History
