Grand Canyon Geology

GEOLOGICAL FOUNDATIONS

The geological foundation of the Grand Canyon began with a primitive sea floor positioned near a large continent. Over millions of years this region was altered through heat, erosion and uplift and now incorporates the Colorado Plateau Province, an area stretching into the Rocky Mountains of Utah from central Arizona. The Plateau covers approximately 130,000 square miles of the Four Corners states and is the equivalent size of Finland, Italy, Poland or Japan. Rock strata, tributary canyons and geological fault lines reveal a layered history of the Colorado Plateau and the erosive effects of the Colorado River within the Grand Canyon.

It is generally believed that the Grand Canyon is a young formation, but that the rocks contained within the Canyon are very old. Several theories have been advanced about the Grand Canyon’s development. First, that the river was present before the Plateau was uplifted and that the river cut through the Plateau as it rose. This theory was adopted by Major John Wesley Powell, leader of the famous 1869 river expedition through the Inner Canyon.  Second, some believe that the river captured drainages through erosion during and after the Plateau uplift. Finally, it is believed that the river was actively engaged in down cutting, but was interrupted by a period of uplift, which resulted in erosion. The geologic record of the Canyon suffers from large gaps in the rock strata making it difficult to determine the sequence of time between layers. Located west of the Painted Desert, the Grand Canyon is framed by several prominent plateaus, namely Shivwits, Kanab, Kaibab, Paria, Coconino and Marble Platform. It also incorporates portions of the Grand Wash Cliffs and the Echo Cliffs and is closely situated to the Black Mountains and the Hualapai Mountains of Arizona.

THE IMPACT OF THE COLORADO RIVER ON GRAND CANYON GEOLOGY

The Colorado River drops over 2,200 feet in depth within the Inner Canyon, an average of 8’ per river mile. This dramatic elevation change is not only greater than the world’s tallest free standing tower measuring 1,815 feet in height, it is 25 times that of the lower Mississippi River in terms of a total vertical descent.

The force of over 70 major rapids has had a razor-like effect on the Canyon’s various rock stratas including shale-siltstone, sandstone, conglomerate, limestone, dolomite, igneous and metamorphic. A total of 94 rock types have actually been discovered within the Canyon. Over time, the river has etched its way through the region, cutting more than a mile deep in many areas. Though the down-cutting effect of the River has been ten times slower than the lateral erosion of Canyon walls, the strata has been simultaneously impacted by freezing and thawing cycles that have added to the alteration and disintegration of rock types. The impact of the Colorado River has resulted in a Canyon that ranges from 10 miles to 18 miles wide at its furthest point.

DID YOU KNOW? …

Rock strata located within the Grand Canyon covers several geologic periods ranging from 1,700 to 245 million years old, the oldest occurring within the Precambrian period. Vishnu schist forms the oldest and deepest segments of the Canyon with outcroppings of igneous and zoroaster granite.

The Chinle and Moenave Formations are ripe with Mesozoic fossils (245-66 million years ago) including petrified forests, small and large dinosaurs, lungfish and crocodilians.

3 – 10 million years ago, the Gulf of California opened to the Lower Colorado River drainage.

Reptile tracks from the Permian Age (286-245 million years ago) are imbedded in Coconino Sandstone, and fossilized crinoids known as “sea lily” stems have been located in Redwall Limestone.

Nautiloids, which have large chambered shells and are related to the present-day squid and octopus, have been found embedded in the floor of Nautiloid Canyon (River Mile 34.8).

Vulcan’s Anvil (River Mile 178) is the core of a volcano that sits within the Colorado River. It is close to Vulcan’s Throne that is positioned on the north side of the river overlooking Lava Falls. This cinder cone is the largest of many that are found within the Canyon having poured lava into the Canyon less than a million years ago.

Impressive polished walls of Limestone and Hermit shale give Marble Canyon its name rather than marble content. Fossils in the limestone date to the Mississippian period (360-330 m.y.a.) and indicate the presence of a shallow sea floor containing corals, sponges, crinoids and brachiopods.