One Best Hike: Grand Canyon. Elizabeth Wenk
Читать онлайн книгу.
Metamorphic Suite, the Grand Canyon Supergroup, and Paleozoic sedimentary layers. (See page for descriptions of features that identify each rock layer.)
This brief description of the Grand Canyon’s geology is obviously incomplete. If learning a few tidbits piques your curiosity to learn more about the past and present processes that have created the landscape, check out the numerous books written on Grand Canyon geology (see page). Carving Grand Canyon by Wayne Ranney is especially recommended both to learn about what forces combined to create the Grand Canyon and to understand how geologists use field evidence to discern geologic processes. Ancient Landscapes of the Colorado Plateau’s scope (by Ron Blakey and Wayne Ranney) is broader than the Grand Canyon, but it does a superb job of describing historical environments in the Grand Canyon region, both through narrative and maps. Hiking the Grand Canyon’s Geology by Lon Abbott and Terri Cook provides a good introduction to the region’s geology, detailed information on the formation of the many rock layers, and a geologic guide to take with you as you hike along either of the trails described in this book. The U.S. Geological Survey provides an online geologic map and annotated photos from the South Kaibab and Bright Angel trails at: http://3dparks.wr.usgs.gov/grca/index.html.
CATEGORIZING ROCKS BASED ON ORIGIN
Geologists divide rocks into three categories. A sedimentary rock is formed either when mineral grains are transported to a site of deposition and subsequently cemented together or by chemical precipitation at the depositional site. An igneous rock is formed by the solidification of molten rock, or magma. Igneous rock that has solidified above the Earth’s surface is termed volcanic and that below the Earth’s surface is termed intrusive. A metamorphic rock forms when an existing rock is deformed because of high temperature or pressure, causing its mineral composition and/or texture to change. The Grand Canyon contains igneous and metamorphosed sedimentary and igneous rocks (in the Inner Gorge) and sedimentary rocks (the near-horizontal layers above the Inner Gorge).
The tectonic regimes and resultant rocks
The nearly two-billion-year-old rock record at the Grand Canyon shows that a succession of different tectonic regimes occurred over time, which led to the formation of the three different rock groups that outcrop along the Bright Angel or South Kaibab trails. There were also periods of time when little occurred, rocks were being eroded away, or canyons carved.
Collisions, 1.8 billion to 1.4 billion years ago: About 1.8 billion years ago the location that would become the Grand Canyon was an oceanic basin that lay between the incipient North American Plate (to the northwest) and a volcanic island chain (the Yavapai Arc to the southeast). By 1.7 billion years ago the oceanic crust that carried the Yavapai Arc was being pushed over the edge of the North American Plate. In the process, the sediment in the intervening ocean basin was buried, twisted, heated, and hence metamorphosed to form the Vishnu, Brahma, and Rama schists, collectively known as the Grand Canyon Metamorphic Suite, or colloquially as the Vishnu Schist or basement rocks. Meanwhile, deeper sediments were completely melted. The resultant magma rose through cracks in the schist and cooled to form the intermingled Zoroaster Granite (and related rocks), a light-colored, often pinkish rock. Later, there was a collision with a second volcanic island chain, the Mazatzal Arc. These collisions added much material to the edge of North America, moving its boundary well south of the Grand Canyon region.
PLATE TECTONICS
The surface of the Earth is composed of thin, rigid pieces termed plates. The 14 larger plates and many smaller microplates float and rotate slowly atop the more liquid inner layers of the Earth. Each of these plates is constantly moving—and in different directions from one another, so the plates collide, slide past one another, and pull away from one another, changing their position on the Earth’s surface in the process. Colliding plates have created—and continue to create—the world’s mountain ranges. In some cases, two plates move toward one another, the type of collision that created the European Alps. In other cases, one plate collides into and is shoved beneath a second plate, a process called subduction. Plates sliding past one another create large strike-slip faults like the San Andreas Fault in western California. Plates pulling apart create new and ever larger ocean basins, a process currently occurring in the Red Sea.
Little tectonic activity, 1.4 billion to 1.2 billion years ago: Having the plate boundary south of the Grand Canyon set the stage for a long period of tectonic calm in the region. The mountain range that had formed from the collisions was slowly eroded, eventually allowing the deeply buried Vishnu Schist and Zoroaster Granite to rise to the surface. Some of their mass was eroded, flattening them by 1.2 billion years ago. The eroded surface is termed the Greatest Unconformity.
Formation and existence of supercontinent Rodinia, 1.2 billion to 750 million years ago: While the Grand Canyon was experiencing a period of tectonic calm and associated erosion, the global stage was being set for a set of massive collisions that formed the supercontinent Rodinia. Rodinia incorporated most of the Earth’s land masses. Australia and Antarctica were welded onto the western edge of the North American continent, west of the Grand Canyon. Along the eastern edge of North America, the Grenville Orogeny, beginning 1.1 billion years ago, created the mountains of the eastern seaboard and apparently caused the western edge of North America to tip downward, creating a narrow sea at the border of North America and the Australian/Antarctic landmasses.
When sedimentary rocks are horizontally layered, the layers indicate the order in which the sediment was deposited—the oldest sedimentary layer is at the bottom and the youngest on top.
The Grand Canyon region was now a costal environment and the sediment that comprises the Grand Canyon Supergroup began to be deposited. Initially the shallow Bass Sea covered the area, depositing the calcareous sediment that constitutes the Bass Limestone. Subsequently, a decrease in water level led to the deposition of mud atop the limestone, creating the Hakatai Shale. Additional decrease in water level caused beach sands to be deposited, coalescing into the very erosion-resistant Shinumo Quartzite. What followed were small-scale encroachments and retreats of the sea, creating bedding shales and sandstones, the Dox Formation. The final formation in the first group of Supergroup rocks is the Cardenas Lava, dating to 1.1 billion years ago, coinciding with the Grenville Orogeny and small-scale rifting in the Grand Canyon region. These first five formations are collectively known as the Unkar Group. The remaining Supergroup formations were deposited in the sea deep within Rodinia. Since they outcrop on neither the South Kaibab nor the Bright Angel trails, they are not described here.
WHY ARE THERE SO MANY DIFFERENT TYPES OF SEDIMENTARY ROCK?
The type of sediment that is deposited is determined by a location’s position on the landscape. Consider a shoreline environment: dunes along the coast (or farther inland) and beach sands become sandstones or quartzites (metamorphosed sandstones); mud and silt are deposited farther out to sea and become mudstones, siltstones, and shales; calcite accumulates in shallow tropical waters, both precipitating from the water and from the deposition of sea creatures. Fewer sediments are deposited and preserved in the interior of continents, which is why most sedimentary rocks are from shores, deltas, or shallow marine environments.
For different types of sediment to overlie each other, the shoreline’s location must keep shifting. Many factors lead to never-ending movement in the position of the shoreline, including continuous variation in the strength of the sun’s radiation and consequent changes in the amount of the Earth’s water stored as ice. Over hundreds of thousands to millions of years, a single location will experience different sedimentary environments—a history preserved as consecutive layers of sedimentary rock.
Breakup of supercontinent Rodinia, 750 million to 525 million years ago: By 750 million years ago Rodinia was beginning to be pulled apart, as Antarctica and Australia headed westward. As the continents were separated, a series of large faults formed in the Grand Canyon region, including the Bright Angel Fault. These were normal faults, which form as a region is stretched and expanded, and result in some blocks of rock being dropped downward. The formation of these faults caused the Grand Canyon Supergroup strata to be tilted and blocks of Grand Canyon Supergroup rocks to be “dropped into” the basement rocks.
The period of breakup and faulting was also one of