Canyon
Very narrow, deep valley cut by a river through resistant rock and having steep, almost vertical sides.
Canyons occur most often in arid or semiarid regions. Some canyons (e.g., the Grand Canyon) are spectacular natural features. See also submarine canyon.
Submarine canyon
Narrow, steep-sided underwater valley cut into a continental slope.
Submarine canyons resemble river canyons on land, usually having steep, rocky walls.
They are found along most continental slopes. Those of the Grand Bahama Canyon,
which are thought to be the deepest, cut nearly 3 mi (5 km) deep into the continental slope.
Most submarine canyons extend only about 30 mi (50 km) or less, but a few are more than 200 mi (300 km) long.
submarine fan
Accumulation of land-derived sediment on the seafloor; a fan is shaped like the section of a cone, with its apex at the mouth of a subbmarine canyon.
The sediments consist largely of sandy material that drops from the canyon current in successively finer layers. Submarine fan valleys, with low relief and natural levees, often occur on submarine fans. Several fans may coalesce laterally.
submarine slump
In a submarine canyon or on a continental slope, a relatively rapid and sporadic downslope moving mass composed of sediment and organic debris that has built up slowly into an unstable or marginally stable mass.
After an individual slump in a canyon, however, the material tends to continue falling in a series of slumps until the sediment mass attains a shallower, more stable slope. A slumping episode may also trigger other slumps farther down the canyon.
Grand canyon series
Major division of rocks in northern Arizona dating from Precambrian time.
The rocks of the series consist of about 10,600 ft (3,400 m) of quartz sandstones, shales, and thick sequences of carbonate rocks. Spectacular exposures of these rocks occur in the Grand Canyon.
Grand canyon
Extensive canyon system cut by the Colorado River, northwestern Arizona, U.S.
Noted for its rock formations and coloration, it is about 0.1–18 mi (0.2–29 km) wide and extends from northern Arizona to Grand Wash Cliffs, near the Nevada border, a distance of about 277 mi (446 km).
The deepest section, 56 mi (90 km) long, is within Grand Canyon National Park, which covers the river's length from Lake Powell to Lake Mead. The surrounding plateau is 6,000–9,000 ft (1,800–2,750 m) above sea level, and the canyon is in places more than 1 mi (1.6 km) deep. The national park, now containing 1,904 sq mi (4,931 sq km), was created in 1919. The former Grand Canyon National Monument, established in 1932, was added, with other lands, in 1975. In 1979 the Grand Canyon was designated a UNESCO World Heritage site.
colorado River
River, North America.
Rising in the Rocky Mountains of Colorado, U.S., it flows west and south 1,450 mi (2,330 km) to empty into the Gulf of California in northwestern Mexico. It drains a vast sector of the North American continent, about 246,000 sq mi (637,000 sq km). No other river in the world has cut so many deep trenches, of which the Grand Canyon is the largest and most spectacular. It is important for hydroelectric power and irrigation; more than 20 dams, including Hoover Dam, have been built on the Colorado River and its tributaries.
Mead, lake
Reservoir of the Hoover Dam, on the Arizona-Nevada border in the U.S.
One of the largest man-made lakes in the world, it was formed by the damming of the Colorado River. Lake Mead is 115 mi (185 km) long and 1–10 mi (1.6–16 km) wide; it has a capacity of over 31 million acre ft (38 billion cubic m), with a surface area of 229 sq mi (593 sq km). It was named after Elwood Mead, commissioner of reclamation. Lake Mead National Recreation Area (established 1936) has an area of 2,338 sq mi (6,055 sq km) and extends 240 mi (386 km) along the river.
Turbidity current
Underwater current of abrasive sediments.
Such currents appear to be relatively short-lived, transient phenomena that occur at great depths. They are thought to be caused by the slumping of sediment that has piled up at the top of the continental slope, particularly at the heads of submarine canyons. Slumping of large masses of sediment creates a dense slurry, which then flows down the canyon to spread out over the ocean floor and deposit a layer of sand in deep water. Repeated deposition forms submarine fans, analogous to the alluvial fans found at the mouths of river canyons.
