Grand Canyon

The Grand Canyon lies in northern Arizona, where the Colorado River crosses the elevated Colorado Plateau. The gorge is about 445 kilometres (277 miles) long and reaches roughly 1.5 kilometres (nearly a mile) in depth. Its width varies considerably, from narrow stretches to about 30 kilometres (19 miles) across. Much of the canyon is protected within Grand Canyon National Park, although the park also encompasses a broader landscape of plateaus, forests, desert country, streams and volcanic features.

The canyon's walls expose a sequence of rock layers that geologists study as a record of changing environments. The oldest rocks, deep in the inner gorge, are ancient igneous and metamorphic basement rocks; above them lie sedimentary formations deposited at different times. Some of those layers formed in marine settings, and fossils preserved in the strata help reconstruct past life and environments. Erosion also removed material and left unconformities—gaps in the local rock record—so the exposed sequence is not a complete account of every interval in Earth's history. The rocks are far older than the canyon carved through them.

Tectonic forces raised the Colorado Plateau, allowing rivers to cut downward through the rock. The Colorado River's flow, carrying sediment, eroded a channel into the plateau; tributary streams and weathering widened the gorge and shaped side canyons, cliffs and mesas. The National Park Service gives about five to six million years as an estimate for the river's incision of the modern canyon, while the exposed rocks record much older events. The history is more complicated than one date: the upper and lower portions of the Colorado River may have developed at different times, and geologists continue to investigate how the present through-flowing river formed and how earlier drainage patterns contributed to the canyon.

The canyon's elevation changes create distinct local climates and habitats within a relatively small area. The National Park Service and UNESCO describe five of North America's seven life zones within the canyon, supporting diverse plant and animal communities. The area is also an enduring cultural landscape: Indigenous peoples have ancestral and contemporary connections to the canyon and surrounding lands, and UNESCO records traditional associations with multiple federally recognized American Indian tribes. River regulation upstream has altered water flows and ecological conditions along parts of the Colorado River within the park, making conservation and management an ongoing concern.

Grand Canyon National Park protects much of the canyon and its surrounding landscape. The park is recognized by UNESCO as a World Heritage property for its exceptional geological record, landforms, natural beauty and varied ecosystems. Its significance therefore extends beyond the dramatic scenery: the exposed rocks, river processes, habitats and cultural relationships all contribute to the area's importance.