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Regional Geology - Arizona


While the Grand Canyon may be the most famous geologic feature in Arizona, it is not the only one. Another important area to learn about is the San Francisco Volcanic Field. We will get back to the Grand Canyon after looking at this even recently active volcanic field.

San Francisco Volcanic Field

There are cones and lava flows in many places on the San Francisco volcanic field. This area covers about 2,000 square miles of the southwestern Colorado Plateau in the Flagstaff vicinity, surrounding the San Francisco Mountain peak (which last erupted 40,000 years ago). These features are the result from several million years of volcanic activity, and have been dramatically active in the last 1,000 years. Powerful underground forces changed the landscape dramatically beginning in the winter of AD 1064-65. Sunset Crater appeared when molten rock sprayed out of a crack in the ground high into the air, solidified, then fell to earth as large bombs or smaller cinders. As periodic eruptions continued over the next 200 years, the heavier debris accumulated around the vent creating a 1,000-foot cone. The lightest, smallest particles blew the farthest, dusting 800 square miles of northern Arizona with ash. Perhaps as spectacular as the original pyrotechnics were two subsequent lava flows: the Kana-A flow in 1064 and the Bonito flow in 1180. They destroyed all living things in their paths. ... In a final burst of activity, around 1250, lava containing iron and sulfur shot out of the vent. The red and yellow oxidized particles fell back onto the rim as a permanent "sunset" so bright that the cone appears still to glow from intense volcanic heat.

Sunset Crater

Sunset Crater is one of the youngest scoria cones in the contiguous United States. The cone is named for the topmost cap of oxidized, red spatter which makes it appear bathed in the light of the sunset. In the 1920's H.S. Colton saved the cone from severe damage by averting the attempt of a Hollywood movie company to blow it up in order to simulate an eruption. This led to the establishment of the National Monument at Sunset Crater.

The Grand Canyon

The rocks at Grand Canyon are not inherently unique; similar rocks are found throughout the world. What is unique about the geologic record at Grand Canyon is the great variety of rocks present, the clarity with which they're exposed, and the complex geologic story they tell. ... These rocks provide a remarkable (but incomplete) record of the Paleozoic Era (550 to 250 million years ago), as well as scattered remnants of Precambrian rocks as old as 2000 million years. The story these rocks tell is far older than the canyon itself. Mesozoic and Cenozoic rocks (250 million years old to the present) are largely missing at Grand Canyon (they've either been worn away or were never deposited). ... Although the rocks exposed in the walls of the canyon are geologically old, the Canyon itself is a fairly young feature. The oldest rocks at the canyon bottom are close to 2 billion years old. The Canyon itself -- an erosional feature that owes its existence to the Colorado River (which is largely responsible for the depth of the canyon) -- has formed only in the past 5 or 6 million years. Geologically speaking, Grand Canyon is very young. Zoroaster Granite

The oldest rocks exposed in the Grand Canyon are the Proterozoic Vishnu schist. Approximately 2 billion years ago, 25,000 feet of sediments and volcanic material were deposited on the sea floor. These sediments were metamorphosed during a mountain building episode 1.7 billion years ago. The rocks were folded, uplifted, and intruded by granites (Zoroaster Granite), resulting in the formation of a mountain range that is believed to have been 5 to 6 miles high. Between 1.7 and 1.2 billion years ago these mountains were eroded till only the roots remained. Uinkaret Volcanic Field and Vulcan's Throne

Late Cenozoic volcanism ( right before the close of the Age of Dinosaurs) extends across a broad region from southwestern Utah to the north rim of the Grand Canyon in western Arizona. The relative ages of lavas show the oldest capping mesas or buttes, and the most recent occupying present drainage valleys. Many of these young flows have no soils developed on them and have well preserved flow features and associated cinder cones. The youngest measured age is 12,500 years ago for a young flow at the Grand Canyon. The Uinkaret volcanic field (1.2 million years ago to 12,500 years ago) at the north rim of the Grand Canyon in the Grand Canyon National Monument is especially noteworthy. Vulcan's Throne is a cinder cone on the rim of the Canyon that has been cut by recent fault movement on the Toroweap fault. Late Cenozoic lava flows have repeatedly flowed down Toroweap Valley and several adjacent valleys into the Grand Canyon, at times forming large lava dams. Flows are exposed on the walls of the Grand Canyon, often interbedded with fluvial sediments, up to 600 meters above present river level. Several of the dams are estimated to have been at least 200 meters high. The most recent flows in the Grand Canyon have cascaded over the rim of the Esplanade to the river 1,000 meters below.