Arizona’s geography has two main parts, split by a cliff. The north is the Colorado Plateau, a high, layered tableland cut by the Grand Canyon, which is 446 km (277 mi) long and up to 1,857 m (6,093 ft) deep. The south is the Basin and Range, where short, steep mountain ranges rise between wide desert basins of the Sonoran Desert. Between them runs the Mogollon Rim, an escarpment about 320 km (200 mi) long that marks the plateau’s southern edge. The highest point is Humphreys Peak in the San Francisco Peaks near Flagstaff, 3,852 m (12,637 ft), a dormant volcano; the lowest is the Colorado River where it crosses into Mexico near Yuma, about 21 m (70 ft). The Colorado is the state’s great river, and Arizona covers 295,253 km² (113,998 sq mi), the sixth-largest state, at an average elevation of about 1,200 m (4,000 ft).
Nearly every one of those features can be tied to a datable event, so this guide takes them from oldest to youngest.
Vishnu Schist and the Great Unconformity: 1.75 Billion Years Ago
The oldest rocks in Arizona are exposed at the very bottom of the Grand Canyon, in the dark walls of the Inner Gorge. Between about 1.8 and 1.6 billion years ago, at least two chains of volcanic islands collided with the edge of the young North American continent. The collisions crushed the sand, mud and volcanic ash of an offshore basin, buried it some 19 km (12 mi) deep and cooked it into metamorphic rock: the Vishnu Schist and its companions, the Brahma and Rama schists, formed between 1.75 and 1.73 billion years ago. Blobs of magma rose into them and cooled as the pale Zoroaster Granite. The same collisions thickened the crust enough to raise the Mazatzal Mountains, which may have stood 8 to 10 km high.
Those mountains did not last. About 300 million years of erosion wore them down to low hills, and later rifting opened a shallow seaway where the Grand Canyon Supergroup was laid down between 1.2 billion and 740 million years ago. Its layers include the bright orange Hakatai Shale, which gives Red Canyon its name, and the Cardenas Basalt, lava flows roughly 1.07 billion years old. About 800 million years ago the Grand Canyon Orogeny tilted these beds by about 15 degrees and broke them into fault blocks, and erosion removed most of them again. Where younger rock now rests directly on the ancient schist, the gap in time is the Great Unconformity, the name given by John Wesley Powell.
Paleozoic Seas: From the Tapeats Sandstone to the Kaibab Limestone
The banded walls that most visitors photograph were built during the Paleozoic era, when this part of North America lay near the equator on the quiet edge of a continent. A sea advanced from the west about 550 million years ago. Its beaches left the Tapeats Sandstone, about 525 million years old and full of trilobite tracks, and its deeper water left the Bright Angel Shale and Muav Limestone. Together these form the Tonto Platform, the broad bench that runs above the Colorado River.
Higher up comes the Redwall Limestone, a cliff 120 to 240 m (400 to 800 ft) tall laid down in a shallow tropical sea during the Mississippian period, rich in fossil crinoids, corals and brachiopods. Above it the red slopes of the Supai Group and Hermit Formation record coastal plains and rivers draining the Ancestral Rocky Mountains, between about 320 and 280 million years ago.
Then the climate dried out. Around 275 million years ago a sand sea spread across the region, and its dunes hardened into the cream-coloured Coconino Sandstone, up to 183 m (600 ft) thick. The sea returned one last time to deposit the Toroweap Formation and finally the Kaibab Limestone, about 270 million years old. The Kaibab is the pale rock that visitors stand on at both rims, and it also forms the surface of the Kaibab Plateau north of the canyon and the Coconino Plateau to the south.
Triassic Rivers of the Painted Desert and the Petrified Forest
By the Late Triassic, about 225 to 207 million years ago, northeastern Arizona was a humid lowland near the equator on the southwest edge of the supercontinent Pangaea. Streams from nearby highlands carried mud, sand and volcanic ash across the plain, and with them whole trees, which were buried and slowly replaced by silica. These deposits make up the Chinle Formation, the banded red, purple and grey clays of the Painted Desert, which reach a maximum thickness of a little over 520 m in the south.
Petrified Forest National Park, about 900 km² (346 sq mi) east of Holbrook, protects the richest beds of those fossil logs. It became a national park in 1962. Erosion has stripped away almost everything that once lay above the Chinle here; in places, rock only 4 to 8 million years old sits directly on top of it, a gap of about 200 million years.
In the Jurassic that followed, another desert of wind-blown dunes covered the region and left the Navajo Sandstone. Near Page, on Navajo land, flash floods pouring off the plateau have carved this sandstone into the narrow, curving slots of Antelope Canyon. On the Utah line, the buttes of Monument Valley, the largest standing about 300 m (1,000 ft) above the valley floor, preserve the Organ Rock Shale, the de Chelly Sandstone and a cap of Moenkopi Formation and Shinarump Conglomerate.
The Laramide Uplift and the Mogollon Rim, From 75 Million Years Ago
For most of this time the layers lay close to sea level. That changed about 75 million years ago with the Laramide orogeny, the mountain-building episode that also raised the Rocky Mountains. Over tens of millions of years the Colorado Plateau was lifted an estimated 3.2 km (2 mi) as a single, largely unbroken block. Because the layers stayed nearly flat, northern Arizona became a land of mesas, cliffs and plateaus rather than folded ridges.
The plateau’s southern edge is the Mogollon Rim, named after Juan Ignacio Flores Mogollón, Spanish governor of New Mexico from 1712 to 1715. Its cliffs are made of the same Kaibab Limestone and Coconino Sandstone seen at the Grand Canyon. Land below the rim lies at 1,200 to 1,500 m (4,000 to 5,000 ft), and the escarpment rises to about 2,400 m (8,000 ft). Ponderosa pine forests cover the rim and the plateau behind it; Arizona has the largest stand of ponderosa pine in the world, and about 27 percent of the state is forest.
Basin and Range: Faulted Mountains and Sky Islands
About 18 million years ago the crust southwest of the plateau began to stretch. As it pulled apart, it broke into long blocks: some dropped to form basins, others tilted up to form ranges, and the basins filled with gravel and sand eroded from the mountains. This is the Basin and Range Province, which covers southern and western Arizona and gives the Phoenix and Tucson regions their pattern of flat valleys hemmed in by abrupt ranges.
Some ranges also hold the remains of volcanoes: in the Superstition Mountains east of Phoenix, the spire of Weavers Needle is an eroded volcanic remnant. In the southeast the ranges are tall enough to hold forests: the Santa Catalina Mountains rise to Mount Lemmon, 2,795 m (9,171 ft), above Tucson, and Mount Graham in the Pinaleño Mountains reaches 3,267 m (10,720 ft). These Madrean Sky Islands, cool woodlands surrounded by Sonoran and Chihuahuan desert, are home to more than 7,000 species of plants and animals. The basins between them hold the state’s low desert; see our guide to the deserts in Arizona for those.
The Colorado River Cuts the Grand Canyon, 6 Million Years Ago to Today
The Grand Canyon is young compared with the rocks it exposes. The Gulf of California opened about 6 million years ago, and a river working its way northeast from the gulf captured older drainage on the plateau to form the ancestral Colorado River. Several recent studies support the idea that the river took its present course through the canyon area about 5 to 6 million years ago. Wetter ice-age climates from about 2 million years ago sped up the cutting, and the canyon was nearly as deep as it is now by 1.2 million years ago. Between about 1.8 million and 500,000 years ago, lava flows built at least 13 dams across the river, holding back lakes up to 610 m (2,000 ft) deep before the river cut through them. Our article on how old the Grand Canyon is sets out the competing ideas.
Today the Colorado runs 2,330 km (1,450 mi) from the Rockies to Mexico. In Arizona it crosses the plateau, turns south at Lake Mead on the Nevada border, and forms the state’s western boundary down to its lowest point near Yuma.
San Francisco Peaks, Meteor Crater and Sunset Crater: the Youngest Landforms
While the river was cutting down, the plateau around Flagstaff was building up. The San Francisco volcanic field covers about 4,700 km² (1,800 sq mi) and contains more than 600 volcanoes, from nearly 6 million years old near Williams to less than 1,000 years old farther east; the belt of eruptions has moved eastward over time. Its centrepiece is the San Francisco Peaks, a stratovolcano whose eastern side was blown or slid away in a collapse like the 1980 eruption of Mount St. Helens. Before that collapse it may have stood 4,600 to 4,900 m (15,000 to 16,000 ft) high; its remaining summit, Humphreys Peak, is still the highest point in Arizona, and small glaciers carved its slopes during the ice ages.
About 50,000 years ago, when the plateau was cooler and grassy, an iron meteorite struck the plain about 29 km (18 mi) west of Winslow. Meteor Crater, also called Barringer Crater, is about 1,200 m (3,900 ft) across and 170 m (560 ft) deep, with a rim rising 45 m (148 ft) above the plain, and is often called the best-preserved meteorite crater on Earth.
The youngest volcano in the field is Sunset Crater, a cinder cone 340 m (1,120 ft) high just north of Flagstaff. Its eruption, now dated to around AD 1085, sent lava flows several kilometres across the plateau and spread ash over more than 2,100 km² (810 sq mi), forcing local Sinagua settlements to be abandoned for a time.
Arizona’s surface is therefore a stack of very different ages, from schist in the canyon floor to faulted ranges barely 18 million years old and a volcano that erupted in the 11th century.