One Best Hike: Grand Canyon. Elizabeth Wenk

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Natural History

      In many locations, people interested in natural history, which includes all information about the natural world, focus their attention on the plants and animals, and think of the geology as a backdrop. In the Grand Canyon the geologic features strike even the most ardent biologists. Not only is the geology visually grabbing, but also it is quickly apparent what a strong influence the geology, from rock type to topographic features, has on where plants and animals live. Consider for instance that different rock layers decompose to form soils with different nutrients and textures, affecting water-holding capacity and water availability, and therefore plant cover.

      Even more apparent to a casual hiker, consider how variable the topography can be. For instance, some rock layers erode easily forming slopes, while others are cliffs, each creating unique habitats required and tolerated by different species. Where a north-facing cliff and gentler topography meet is a small patch of real estate that remains shadier and cooler, allowing a unique collection of species to establish. Rock formations with alternating layers of sandstone and shale, may create small platforms of soil underlain by fractured rock—if a tree takes root, it might find moister soil deep down. A wash that carries water occasionally will host different species than the dry terraces to either side. This variation in physical features creates patchiness in resources and in turn leads to a surprising diversity in plants and animals. As you explore the inner canyon, consider the interactions between the physical and biological worlds.

GEOLOGY

      Quite simply, the Grand Canyon is unbelievable because of its geologic features. As you stand at a vista point and peer into and across the canyon, you are marveling at the geology: staring at the exceptionally wide and deep canyon and the nearly horizontal layers of colorful rock. Generations of talented geologists have sought to understand what combination of geologic events created this landscape. Individuals with different geologic specialties have contemplated different aspects of the picture, weaving together their conclusions with the data collected by others to present a coherent story of the Grand Canyon’s geologic history. However, the sleuthing continues—evidence to decipher some pieces of the story is simply missing. The story of the Grand Canyon presented today, in this book and at vistas throughout the park, may stand the test of time or may be quite different if you revisit the park in a generation.

      Questions will probably leap to mind as you gaze at the canyon. You may wonder why such a deep canyon exists and how it formed. Or you may contemplate why there are so many different layers or rock, how they got there, and why some are flat, but others steep?

      To answer these clusters of questions, and others, I describe the tectonic regimes to which the Grand Canyon area was subjected from 1.8 billion years ago until today, as the tectonic surroundings dictate many of the geologic processes that are recorded. These descriptions include information about the environments that led to the creation of the three main rock groups in the Grand Canyon: the Grand Canyon 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 СКАЧАТЬ