Some mountains are formed by volcanic action and others by tremendous lateral (sideways) pressure that distorts and displaces formerly flat layers of rock. They are found in regions of crustal instability, along or near the edges of the rigid plates that form the Earth’s crust.
Volcanic mountains occur where plates are moving apart along the ridges that extend throughout the oceans. As the plates move apart, hot magma (molten rock) wells up to fill the gap, and new crustal rock is formed. Sometimes the magma piles up on the sea bed to form undersea volcanoes that may emerge above the waves as islands.
When two oceanic plates collide, one plate is pushed down beneath the other along deep oceanic trenches. The descending plate is melted, and some of this magma rises to the surface to form chains of volcanic islands, such as those in Japan, the Philippines and Indonesia.
Rising magma does not always reach the surface through volcanoes. Instead, it some- times arches up the overlying rocks to form a dome mountain, such as the Black Hills of Dakota in the United States. The rising magma finally cools to form granite, which is exposed later by the forces of erosion.
The world’s longest and most majestic mountain ranges on land, including the Himalayas, the Andes, the Rocky Mountains and the Alps. Consist of folded rocks, formed mostly from highly twisted and contorted layers of sedimentary rocks that accumulated on the beds of ancient oceans.
Some sedimentary rocks form in shallow seas from mud, sand and silt that were swept into the sea by rivers. Farther out, limestone rocks form in deeper water from the remains of dead organisms, which survive as fossils.
Over periods of at least 100 million years, these marine sediments form deposits often tens of kilometres thick. The enormous weight buckles the sea floor, causing large areas called geosynclines to subside.
Scientists once thought that, as the sea floor subsided, the sedimentary rocks in the geosynclines were squeezed together into folds. The folds then rose upwards to form mountains. Today, however, most scientists believe that plate movements provide the pressure that produces Fold Mountains.
Plates move only a few centimetres a year. But when two plates bearing land masses collide, the continental rocks along the edges of the plates, together with the sedimentary layers on the sea bed between the continents, are gradually pushed upwards into mountain ranges.
Plate movements generate great heat and pressure which deforms and weakens many rock layers which then yield like plastic to form gigantic folds. Other cold or strong rock layers are fractured (cracked) and often severed from their roots.
During a mountain building period, heat also creates magma near the base of the continental crust. Huge bodies of magma rise and harden to form granite cores beneath the Fold Mountains.
Old Fold Mountains, which have long ceased to rise but have not yet been worn away, are evidence of ancient continental collisions. For example, the mountains of north-eastern North America, eastern Greenland, western Ireland and Scotland, together with the mountains of Norway and Sweden, were originally formed when North America and Europe collided and became a single vast continent which was later fragmented when the Atlantic Ocean opened up around 100 million years ago.
Folds can often be seen in exposed cliffs in mountainous and other regions. The simplest folds are anticlines (up folds) and synclines (down folds). Some folds are overturned and become recumbent folds. Others, called napes, shear away from their roots and the upper parts of the folds are thrust forwards often for several kilometres.
Plate movements cause rocks near plate edges to fracture, forming long faults (cracks) in the rocks. Blocks of land are sometimes pushed upwards along faults to form tilt Block Mountains, such as the Sierra Nevada in California. Other blocks are pushed upwards between two roughly parallel fault lines. The edges of Block Mountains often rise steeply from the surrounding terrain.
Even as mountains rise, so the forces of erosion start to wear them down. Erosion is especially effective in mountain regions because slopes are steep, while the effect of gravity is at its greatest. As a result, frost- shattered rocks tumble downhill and are transported away by glaciers that form from compacted snow in mountain basins.
Other frost-shattered rocks pile up in huge heaps called scree or talus. The loose rocks are eventually carried away by fast-flowing mountain streams. All these natural forces combine with plate tectonics to shape spectacular mountain scenery.