There can be no better place to commence the story of the fossils of Arizona than the Grand Canyon of the Colorado River. Indeed, in no other place on the planet earth does nature reveal its history so grandly, and with such unvarying simplicity that viewers with no or little geological education can understand and appreciate the vastness of the geologic time, as well as the scale of events which led to the formation of the rocks seen from the rim of the canyon. The Grand Canyon refers to a steep-sided canyon shaped by the Colorado River, in Arizona State, in the United States (Augustin & Kubena, 2009). The canyon is situated inside the Grand Canyon National Park and is managed by the same park in collaboration with the Havasupai Tribe and the Hualapai Tribal Nation. President Theodore Roosevelt was a key advocate of the Grand Canyon preservation, and he visited the place on several occasions to enjoy its breathtaking scenery. The canyon is regarded one of the Seven Natural Wonders of the World (Augustin & Kubena, 2009).
The Grand Canyon is estimated to be 277 miles long, 11 miles wide (with a 15-mile width at its widest point) and 4, 000 feet deep, with an astounding 6,000 feet at its deepest spot (Kiver & Harris, 1999). Almost two billion years of the geological history of the earth have been uncovered because the Colorado River along with its tributaries cut their canals through numerous rock layers, as the Colorado Plateau was being uplifted. Even though the specific timing and geologic processes that led to the formation of the Canyon have been subjects of debate by various geologists, current evidence suggests that Colorado River began its course via the canyon not less than seventeen million years ago (Ranney, 2005). Since then, the Colorado River has continued to erode to form the canyon into its current configuration. For hundreds of years, the Grand Canyon region has been constantly occupied by Native Americans who built their settlements inside the canyon and its several caves. García López de Cárdenas was the earliest European to view the Grand Canyon. According to Mitchell, Lippert and Brunson-Hadley (2004), the canyon was also considered a holy site by the Pueblo people who even made pilgrimages to it. This paper presents a discussion of the geology of the Grand Canyon.
The Grand Canyon
The Grand Canyon has attracted the attention of numerous people around the world for many reasons, but, perhaps, the greatest significance of the canyon lies within its geological record exposed and preserved. The rocks found at the Grand Canyon are not naturally unique - comparable rocks are found all over the world. The uniqueness of the geological record of the canyon lies in the variation of its rocks, the clarity of their exposure, and the multifaceted geological story they tell (Augustin & Kubena, 2009).
There are two different stories behind the Grand Canyon geology. The initial story is shown in the thick rock sequence exposed in the canyon’s walls. The rocks offer an incredible record of the Paleozoic Era i.e. 550-250 million years ago. Found at the base of the canyon, are remains of Precambrian rocks, which are approximated to be 2 billion years old. The story told by the rocks is far much older than the Grand Canyon itself. According to this first explanation, Mesozoic and Cenozoic rocks (250 million years- present) are chiefly missing at the canyon. Either they were never deposited or have been eroded (Augustin & Kubena, 2009).
The second story with regard to the geology of the Grand Canyon concerns its origin i.e. how and when it came to be formed. The existence of the Grand Canyon is owed to the erosional forces of the Colorado River. Other forces of erosion which have significantly shaped and continue to shape the canyon in the present day are running water from rain, tributary streams that enter the canyon via its length, as well as snowmelt (Augustin & Kubena, 2009).
Another vital environmental aspect responsible for the existence of the canyon is semi-arid climate within the surrounding region. The South Rim receives just 38cm of precipitation every year, while the base of the canyon receives below 20cm of precipitation yearly (Augustin & Kubena, 2009). Particularly, during late summers rains usually come abruptly accompanied by violent storms. Consequently, erosion power is more apparent here than other regions that receive more rain. It is crucial to point out that climate plays a fundamental role in the canyon’s appearance. If the Grand Canyon had received more rain, the trees and plants growing within it would be tremendously different. Perhaps, shrubs and cacti growing in the canyon would have been replaced by lush vegetation (Augustin & Kubena, 2009).
The Grand Canyon owes its unique shape to the various layers of rock within the canyon walls. Every rock reacts to erosion in a different manner: some erode faster than others, and others form cliffs or slopes as a result of erosion. The bright colors of numerous rock layers are majorly due to trace amounts of different minerals. Most of them contain iron that imparts yellow, red and green hues to the walls of the canyon (Augustin & Kubena, 2009).
The Geologic History of the Grand Canyon
Even though the breathtaking beauty and grandeur are Grand Canyon’s main attractions, its most valuable and vital aspect lies within the time scale of earth history revealed within the reachable rocks of the Grand Canyon’s walls (Polyak, Hill & Asmerom, 2008). No place in the world can be compared to the Grand canyon in relation to its profound and extensive record of geologic events. However, it is crucial to point out that the canyon’s geologic record is a long way from complete and continuous. It has immense time gaps i.e. numerous millions of years, which are unaccounted for. This is as a result of the gaps in the rock layers that came about due to removal of immense quantities of material via erosion, or due to little or no material deposition (Polyak, Hill & Asmerom, 2008). Accordingly, rock structures of significantly diverse ages are separated by just a distinct, but thin surface, which reveals the huge discrepancy in time. A summary of the geologic history of the Grand Canyon’s strata is discussed below.
The crystallized and twisted unstratified rocks found on the inner ravine at the base of the canyon are schist and granites, which are over 2.5 million years old. On top of these ancient rocks are a layer of Proterozoic sandstones, limestones and shales, aged more than 540 million years (Polyak, Hill & Asmerom, 2008). Overlying the Proterozoic rock strata, is a layer of Paleozoic rock strata made of more limestones, cemented sandstones and freshwater shales that form the greater part of the canyon’s walls and signify a depositional period spanning more than 300 million years. In the ordinary geologic record, a succession of Mesozoic rocks (approximately 250-65 million years old) should be overlying the Paleozoic rock strata. However, rocks that date from the Mesozoic Era have been totally eroded away in the Grand Canyon (Polyak, Hill & Asmerom, 2008). Instead, Mesozoic rocks are available within the nearby southern Utah; there, they have formed precipitous remnants and white, pink and vermilion cliffs. Sheets of volcanic cones and black lava of relatively recent origin (some are approximated to have been active in the past 1,000 years), are situated a few miles southeast of the Grand Canyon, as well as in the western part of the Grand Canyon (Polyak, Hill & Asmerom, 2008).
The Colorado River cuts the Grand Canyon into separate parts. This remains an event of comparatively recent geographic history that started not earlier than 6 million years ago. At that time the river commenced following its current course. The rapid velocity, high water volume and the large amounts of sand, mud, gravel that the Colorado River carry downstream, explain the unbelievable cutting power of the river (Polyak, Hill & Asmerom, 2008). The Grand Canyon’s depth is the outcome of the cutting action of Colorado River. However, the canyon’s vast width can be explained by the chemical erosion, wind, rain, and temperature, with the help of the speedy deterioration of soft rocks, which gradually broadened the canyon.
The Grand Canyon Geology
The Grand Canyon was formed more than 17 million years ago because of the tectonic movement which compelled the Colorado River to create a new route before emptying (Beus & Morales, 2003). Weathering, erosion and climate, played key roles in shaping the canyon that is seen today. The processes allowed the rock to form layers/ strata, which provide a window into the earlier periods. The majority of these layers are composed of sedimentary rocks since the Colorado River and its branches deposited some of the contents on the way to the sea. In due course, the contents have hardened and become the limestone and soft shale we see today (Beus & Morales, 2003).
The specific timing and geologic processes that led to the formation of the canyon have been a subject of the heated debate by various geologists. Past studies have approximated the age of the canyon to 5-6 million years. A recent study by Wilford (2008) on the origins of the canyons, however, found out that the canyon started some 17 million years ago. Another study utilized uranium-lead dating in the analysis of calcite on the walls of the 9 caves throughout the canyon (Beus & Morales, 2003). It was published in Science Journal in 2008, and it suggested that there was a significant departure from the previous widely supported scientific accord.
The main geologic exposures within the Grand Canyon vary from Vishnu Schist found on the base of the Inner Gorge, which is about 2 billion years old, to the Kaibab Limestone found on the Rim of the canyon, which is 230 million years old. There is a gap of approximately a billion years between the stratum and the lower level, which are 500 million and 1.5 billion years old respectively (Beus & Morales, 2003). This vast unconformity shows that there was a period of erosion between the deposition periods.
The majority of the formations were deposited within warm, shallow seas, swamps, and near-shore surroundings like beaches, as the seashore continually advanced and withdrew above the edge of North America. The huge depth of the Grand Canyon, particularly the height of its strata, is attributed to 5,000-10,000 feet of uplift of the Colorado Plateau, beginning approximately 65 years ago (Beus & Morales, 2003). As a result of the uplift, the stream slope of the Colorado River along with its tributaries has steepened, in turn, enhancing their speed and capability to cut through the rock. The course and base level of the Colorado River changed about 5.3 million years ago following the opening of the Gulf of California and the lowering of the river’s base level (Beus & Morales, 2003). Consequently, it increased the erosion rate and cut almost all of the current depth of the Grand Canyon by 1.2 million years ago. The canyon’s terraced walls were created through differential erosion. Volcanic activity, which occurred between 3 million to 100,000 years ago, deposited lava and ash on top of the region, which sometimes totally blocked the river (Beus & Morales, 2003). The volcanic rocks are the youngest rocks in the canyon.
The Grand Canyon’s geology offers a magnificent window for viewing 3 of the 4 main eras of geological time via its different strata, as well as its enormous fossil record. The Grand Canyon is estimated to be 277 miles long, 11 miles wide, and 4, 000 feet deep, with an astonishing 6,000 feet at its deepest spot (Kiver & Harris, 1999). Some of the fossils discovered in the canyon include bacteria spores more than 1 billion years old, dung and mummified hair from more than 11,000 years ago, and body fossils dating back to the Paleozoic era. In addition, the canyon is home to about 1,000 caves, most of which are not catalogued, alongside numerous faults. The uniqueness of the Grand Canyon also lies in its vertical and horizontal faults, which gives seismologists, as well as, other scientists a chance to study them as they move down, thousands of feet into the top layer of the earth: the crust.
Because the Grand Canyon acts as a doorway to the history of the earth, there have been numerous geologic studies being done on it since 1858 (Beus & Morales, 2003). The canyon and its surrounding area are highly instrumental in the study of volcanism and stream erosion. About 2 billion years worth of history are on exhibit through the rock’s cross section present in the canyon walls.
The arid climate surrounding the canyon prevents the quickening of Grand Canyon’s widening. In addition, the Colorado River remains the most significant source of the canyon’s formation. Its flow that starts from the Rocky Mountains, aided by tributaries, offers the most vital water source in the region (Beus & Morales, 2003). Seeps and springs also contribute to the flow of water of the Colorado River, even if it is only slight. Nonetheless, the springs are significant because they are home to different atmospheres where scientists have discovered wildlife species found nowhere else globally.