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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.