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Geology of Grand Canyon:
What's the age of Grand Canyon?
The rock layers that make up the walls of Grand Canyon range from the fairly young to the fairly old, geologically speaking. Kaibob Limestone, deposited approximately 270 million years ago, forms the caprock in much of this region. The oldest rocks exposed at the bottom of the canyon, gneiss (NICE) and schist, date as far back as 1.84 billion years. Geologists estimate that East is 4.55 billion years old.
How can Grand Canyon be young if the rocks in it are so old?
While the rocks are old, Grand Canyon itself formed much more recently. Geologists generally agree than canyon carving occurred during the last 5-6 million years -- a geological blink of the eye.
Why here and not somewhere else?
Beginning about 70 million years ago, the pressure of colliding tectonic plates induced mountain building in the American West. Amazingly, the Colorado Plateau was uplifted many thousands of feet, but mostly spared the tilting and deformation that normally accompany the raising of strata.
The section of the canyon viewed from the South Rim cuts through a bulge in the Colorado Plateau called the Kaibab Plateau. "Why does the river cut through the Kaibab Plateau rather than flow around it?" is a question that has plagued geologists since Powell's time. Many hypotheses have been advanced to explain this event. Research conducted today strives to increase our understanding of the origin and evolution of Grand Canyon.
How did it become so deep?
The goal of every raindrop is to get to sea level. As water drains off the western slopes of the southern Rocky Mountains and across the Colorado Plateau, it carries sand, gravel, and rocks that abrade against the rock layers, cutting down through the ancient strata. Without the uplift of the Colorado Plateau, there could not be thousands of feet of rock above sea level to cut through. From Yavapi Point on the South Rim to the Colorado River is an elevation change of 4,600 feet (1,400 m), yet the river is still 2,450 feet (750 m) above sea level.
How did Grand Canyon become so wide?
Simply stated, as the Colorado River cuts down, the walls of the canyon collapse around it. A few key weak layers of rock within the canyon are readily broken down by rain and snowmelt and are washed downhill. Stronger layers sitting atop weaker layers lose their support and collapse. Side creeks erode headward, carving tributary canyons that accelerate the widening of Grand Canyon. Over its 277-river rile (446 km) length, Grand Canyon varies in width as the canyon rims are notably jagged. Along the South Rim, the width varies between 5 and 16 miles (8-26 km), depending on where you choose to measure.
What makes Grand Canyon so grand?
There is only one Grand Canyon. The ensemble of stunning dimensions -- the melding of depth, width, length, color, and form -- sets Grand Canyon apart from all other canyons. Nowhere else if such a dazzling variety of colorful rock layers, impressive buttes, and shadowed side canyons revealed in such a dramatic chasm. Grand Canyon is the canyon against which all other canyons are compared. |