Source: NOVA: "Rafting Through the Grand Canyon"
This video segment adapted from NOVA shows a dramatic landscape created by relatively recent rock-forming activity in the Grand Canyon. Volcanic eruptions only a million years ago created the canyon's youngest rocks. In contrast with the much older Vishnu Schist formation, this younger rock has been much more susceptible to physical change. When three-hundred-meter (thousand-foot) lava dams periodically blocked the river, they were quickly eroded away and river flow restored.
Although the rocks exposed in its walls are ancient -- some nearly two billion years old -- the Grand Canyon itself is relatively young. Most scientists believe it started to form only five to six million years ago. In the past 250 million years, the only significant rock-forming activities in the area were the sporadic volcanic eruptions that began about six million years ago and continued until several thousand years ago.
Earth's crust is composed of minerals that form rocks. Igneous rocks -- from the Latin word for "fire" -- solidify from hot, molten material called magma. Certain igneous rocks, such as granite, cool slowly beneath the ground, producing relatively large crystals. These rocks, called intrusive rocks, are exposed at the surface through uplifting processes like mountain building, or as the result of deep erosion. Magma that erupts onto the surface, known as lava, cools very rapidly, so its atoms do not arrange into crystals. Lava forms extrusive igneous rocks such as basalt, the grains of which are so small they may be invisible to the naked eye.
The properties and composition of extrusive rocks vary according to the properties and content of the magma from which they were derived. Silica-poor magmas, such as basalt, have low viscosities and high temperatures. Consequently, they are very fluid on eruption and harden to form dark, fine-grained rock. Silica-rich magmas are more viscous. Many silica-rich magmas reach the surface with high contents of water, carbon dioxide, and other gases, and violently spew out lava as airborne pumice and ash. Others, however, lose their gases during ascent and produce thick, slow-moving lava flows that harden as thick deposits.
Basalt deposits in the western part of the Grand Canyon provide evidence that volcanoes were active there in the past. In fact, scientists know that the Colorado River was dammed at least a dozen times from 1.8 million to 400,000 years ago. Lava cascaded down adjacent side canyons and hardened, and temporary lakes formed behind lava dams that sometimes reached upward of 300 meters (about 1,000 feet) in height -- not unlike how Lake Mead and Lake Powell formed behind the Hoover and Glen Canyon dams. However, each of these natural dams was eventually breached in a testament to the river's erosive power. Relative-age dating and known erosion rates suggest it took about 20,000 years for each dam to form and then be eroded away by the river.
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