Читать книгу One Best Hike: Grand Canyon. Everything You Need to Know to Successfully Hike from the Rim to the River—and Back онлайн

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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.