Grand Canyon Reveals the History of the Earth
The great gash of the Grand Canyon also provides perhaps the single most dramatic lesson in geology and the Earth's history on the planet. The Colorado River cut down through the leading edge of the uplifted Colorado Plateau in the past five million years as the same forces that created the Basin and Range Province uplifted the Rocky Mountains. The 1,400-mile-long (2,253 km) Colorado River carries only a fraction of the water that flows down the Mississippi, but since its gradient is 10 times as steep it erodes far more rock and dirt. The Grand Canyon lies on a 277-mile (445.79 km) stretch of the Colorado River that starts in the Rocky Mountains and flows all the way to the Gulf of California. Erosion carved away thousands of feet of sediment as the canyon rose, so that the youngest of the sedimentary layers along the top of the Grand Canyon are about 260 million years old. The river recently has carried away everything laid down earlier than that. The youngest formation at the canyon rim is Kaibab Limestone, made of the compressed skeletons of microscopic creatures living in a shallow sea covering what is now the western United States. The oldest rocks in the Grand Canyon are more than a mile below in the inner gorge, where they are so hard and unyielding that they have produced some of the world's most impressive rapids. Those Precambrian rocks date back nearly 2 billion years and include Elves Chasm gneiss, a metamorphic rock formed under great pressure far beneath the surface and then thrust upward until exposed by the Colorado River.
That means that the Grand Canyon lays out the detailed geological history of the Earth between 2 billion and 250 million years ago, nearly halfway back to its origins. Most of the rocks exposed in the towering walls of the canyon are sedimentary, laid down in the bottom of an endless succession of shallow seas and deep lakes. Some layers mark times when the land rose, the seas fell, and vast fields of sand dunes that dwarf the Sahara left their traces (as can be seen in the color insert on page C-2). Some of the layers mark times when Earth shifted and erupted, leaving great layers of lava. One of the biggest rapids in the bottom of the Grand Canyon marks the place where a flood of lava blocked the Colorado River, creating a deep lake that eventually overtopped the lava dam and roared downstream in a devastating flood.
The geological record in the walls of the Grand Canyon for that 2-billion- year swath of time isn't quite complete. Some periods are completely missing, including the so-called Great Unconformity, which consists of all the sediments laid down between about 500 million and 400 million years ago. This means that during that period the continent was rising, as it is now. All the rocks laid down before were stripped away by erosion, before the uplift stopped and new layers of rock were deposited once again. Aside from these gaps, the Grand Canyon represents one of the most complete records of the geologic history of Earth.