Geology
The Grand Canyon is part of the Colorado River basin which has developed over the past 40 million years. A recent study places the origins of the canyon beginning about 17M years ago.
The major geologic exposures in the Grand Canyon range in age from the 2-billion-year-old Vishnu Schist at the bottom of the Inner Gorge to the 230M-year-old Kaibab Limestone on the Rim. There is a gap of about a billion years between the 500M-year-old stratum and the level below it, which dates to about 1.5 billion years ago. This large unconformity indicates a period of erosion between two periods of deposition.
Many of the formations were deposited in warm shallow seas, near-shore environments (such as beaches), and swamps as the seashore repeatedly advanced and retreated over the edge of a proto-North America. Major exceptions include the Permian Coconino Sandstone, which contains abundant geological evidence of aeolian sand dune deposition. Several parts of the Supai Group also were deposited in non–marine environments.
Weather
Weather in the Grand Canyon varies according to elevation. Conditions in the Grand Canyon region are generally dry, but substantial precipitation occurs twice annually, during seasonal pattern shifts in winter and in late summer. Average annual precipitation on the South Rim is less than 16 inches , with 60 inches of snow; the higher North Rim usually receives 27 inches of moisture, with a typical snowfall of 144 inches and Phantom Ranch, far below the Canyon's rims along the Colorado River at 2,500 feet gets just 8 inches of rain, and snow is a rarity.
Temperatures vary wildly throughout the year, with summer highs within the Inner Gorge commonly exceeding 100 °F and winter minimum temperatures sometimes falling below zero degrees Fahrenheit along the canyon's rims. Visitors are often surprised by these potentially extreme conditions, and this, along with the high altitude of the canyon's rims, can lead to unpleasant side effects such as dehydration, sunburn, and hypothermia.