Where did all of the rock come from?

Geologists have this question pretty much wrapped up, aside from some missing layers, or unconformities, that have been completely eroded away. Again there were a number of forces at work and this is where continental drift, vulcanism and climatic change come into play.

The fact that the Earth's continents are not fixed in place but rather float on a sea of molten rock, means that they move around quite a bit, relatively speaking. The surface of the Earth is composed of about twenty of these "plates" which form its crust. Seven of these plates are very large and consist of entire continents or sea floors and the rest are smaller in comparison. The plates are average out to be about 50 miles or 80 kilometers thick and float on top of the Earth's mantle. The plate which contains the Grand Canyon, the North American plate, was at one time considerably further south than its present location and therefore had a much different climate. In time it has gradually moved north and rotated about ninety degrees to its present location and configuration.

The North American Plate is moving west and is colliding the Pacific Plate which is moving towards the northwest. The Pacific Plate is also expanding from its middle and its eastern edge is being subducted beneath the North American Plate as it comes into contact with it. Oceanic plates are typically subducted beneath continental plates because they area heavier. As pressure increases while they are being subducted they tend to get heavier still and to some extent they start to fall and pull more plate along with them. As the Pacific Plate moves beneath the North American Plate the rock of which it is composed is superheated and water is released and begins to rise. This water, which is extremely hot, causes lighter minerals to melt and forms lava which feeds the chain of volcanoes on the eastern edge of the Pacific Rim which runs from Alaska to Chile.

The conflict between the plates is also frequently responsible for mountain building activity. As the plates are forced together they sometimes buckle which causes mountain ranges to be formed along the contact point. This is how the Rocky Mountains, the Sierra Nevada and the costal mountains of California were formed and how the Aleutian Island are being formed today. A much older range of mountains, which geologists suspect were much higher than todays Rocky Mountains and may even have rivaled the Himalayas, now forms the base of the Grand Canyon. The rocks that made up these mountains are about 1.7 billion years old, or about one-third the age of our planet. These mountains have long since eroded away and sedimentary deposits have covered them over.

The sediments that covered the roots of these ancient mountains were deposited by a series of advancing and retreating ocean coast lines. As the climate of our planet warms and cools the median sea level of the planet rises and falls due to the melting and freezing of the polar caps. When the sea level rises, land areas which are close to the coast and relatively low in altitude are sometimes submerged. This was the case with the land area of the Grand Canyon and is why so many different sedimentary rock layers exist. Each of these was formed by a different period in which the ocean moved in and covered the land, stayed for a while, and then retreated again. Limestone deposits are created when the ocean moves in and slates, shales and mudstone deposits are created when the ocean moves out and the area is covered by silts washing into the retreating ocean.

How do we know this?

Well, the fact is that most of the rock in the Grand Canyon is composed of sedimentary rock which can only be formed at the bottom of the ocean or in shallow coastal plains. The Kaibab Limestone which is the current top of the Grand Canyon is composed mostly of a sandy limestone, with some sandstone and shale thrown in for good measure. This means that it was probably formed in a shallow sea near the coast. The fact that it contains fossils of creatures that used to live in the ocean, like brachiopods, coral, mollusks, sea lilies, worms and fish teeth, only tends to reinforce this belief. The intrusion of sandstone and shales into this later means that at times the layer was also above the surface of the water but still very close to the edge. Sandstones are solidified sand which are typically fields of sand dunes or beaches, and shales are solidified mud which are common to river deltas. By dating the fossils found in the rock of the Kaibab Limestone, geologists have determined that it is approximately 250 million years old, and this is the youngest layer.

So where are the younger rocks?

The younger rocks have already been eroded away by the forces of nature, at least in the immediate vicinity of the Grand Canyon. Some of the younger layers, like the Navajo Sandstone of which the Vermilion Cliffs and the rock of Zion National Park are composed, can be found in the region north of the Grand Canyon. Going even further north results in even younger rocks as can be seen in Bryce Canyon. The area from Bryce Canyon down to Grand Canyon is typically referred to as the Grand Staircase.
 

Copyright © 2001, www.kaibab.org, all rights reserved.