DK Earth: Mountains
A mountain is a steep-sided mass of rock, rising at least 2,000 ft (600 m) above sea level. Mountains are found on land and under the sea. Some are isolated peaks, but most are found in a RANGE.
Mountains are formed by movements of the huge tectonic plates that make up Earth?s crust. Fold mountains are formed when plates collide. Block mountains occur when a slab of land is forced upward. Volcanic mountains are built up from layers of cooled and hardened lava and ash.
Some relatively young mountains are still rising, as colliding plates continue to force the land at their edges upward. At the same time, mountains are constantly eroded by ice, rain, and the wind.
Mountaintops are cold because the thin air high up does not hold the Sun?s heat well, and the temperature falls 1.8F (1C) for every 500 ft (150 m) of height. It is therefore cold enough to snow over high peaks (even on the equator), and since the temperature rarely rises above freezing, the snow never melts.
Some of the world?s tallest mountains lie mostly underwater, with only their summits breaking the surface. Mauna Kea in Hawaii rises 33,480 ft (10,205 m) from the ocean bed, which makes it taller than Mount Everest.
Most mountains are found in groups called ranges, such as the Jura Mountains in Europe and the Sierra Nevada Mountains of California. Often, a series of ranges is connected in a larger chain of mountains called a cordillera.
The longest mountain chain on land is the Andes, which runs for 4,470 miles (7,200 km) down the western edge of South America. An undersea mountain chain called the Mid-Atlantic Ridge is even longer. It stretches 7,000 miles (11,300 km) down the center of the Atlantic Ocean.