Mountains

A mountain is a large-scale topographic feature that is set apart from the local landscape by being much higher in elevation (topographic means having to do with the shape of the land surface).

Island arcs

When the edge of a plate of Earth's crust runs over another plate, forcing the lower plate deep into Earth's elastic interior, a long, curved mountain chain of volcanos usually forms on the forward-moving edge of the upper plate. When this border between two plates forms in the middle of the ocean, the volcanic mountains form a string of islands, or archipelago, such as the Antilles and the Aleutians. This is called an island arc.

Continental arcs

When the upper plate is carrying a continent on its forward edge, a mountain chain, like the Cascades or the Andes, forms right on the forward edge. This edge, heavily populated with volcanos, is called a continental arc.

Collisional mountain belt

A continent or island arc runs into a continent, shattering and deforming the rocks of the collision area, and stacking up the pieces into a mountain range. This is how the Appalachians, Alps, and Himalayas were formed: the rocks of their continents were folded just as flat-lying Mountain in Banff National Park, Canada. This mountain shows rock folds which are the result of the collision of the earth's plates. JLM Visuals. Reproduced with permission. cloth folds when pushed. Imagine how much taller your school would be if it were squeezed by bulldozers so it remained the same length east to west as it is now, but from north to south measured the width of a school bus. The result would be a tall wall of compressed material, and that is just what a collisional mountain belt is. Collisional mountain belts are one of three types of boundary between plates of the earth's crust, along with mid-ocean ridges and inter-plate strike-slip faults. Mountains rise relatively quickly, over a few million years, such as the Appalachians did more than 200 million years ago. As these mountains begin to erode, the topography continually changes and develops. Hard rock layers influence the development of streams, because they resist erosion and form the ridgetops in the mountain range.

One special type of orogeny that can happen during a continental collision is the rise of ophiolite mountains. On rare occasions the crust beneath the ocean floor fractures along the tectonically active coast of a continent, and oceanic crust is thrust up over the shore and forms mountains. This spectacular form of plate-tectonic back-fire is not supposed to happen, yet it does often enough to have its own name: obduction, meaning "over" (ob-) "leading" (-duction). A piece of oceanic crust, and the mantle rock beneath it, is heaved up onto the land to form mountains. The Taconic Mountains that rose in upstate New York 430 million years ago were an obducted ophiolite, as are the uplands around Troodos in Cyprus.

Fault block mountains: When a continent-sized "layer cake" of rock is pushed, the upper layers can be pushed more readily than the lower layers. The easy-to-push upper layers split from the deeper rocks, and a broad sheet of the upper crust, a few miles thick, begins to move across the continent. This thrust sheet floats on fluid pressure between the upper and lower sections of the crust. The horizontal split in the crust that separates the motionless lower crust from the floating upper layers is called a detachment fault in English, or a decollement in French.

Like a hydroplaning tractor trailer (viewed in very slow motion), the upper fault block glides until it runs into something. When the thrust sheet runs into something that resists its forward motion, the detachment fault turns into a ramp, leading up to the surface. The moving layer of upper crust is pushed up the ramp-like fault, and the front of the fault block rises out of the ground. The mountains thrown up where the thrust fault reaches the surface are one kind of fault block mountains. The mountains of Glacier National Park slid along a thrust fault over younger rocks, and out onto the Great Plains. Chief Mountain, a remarkable square mountain in Montana, moved to where it is now by sliding out onto the prairie on a thrust fault. The broad, flat fault block it belonged to, called a thrust sheet, has long since disappeared, leaving Chief Mountain standing alone.

Another kind of fault block mountain comes from stretching of Earth's crust. As the crust stretches, it pulls apart, making long faults that run perpendicular to the direction of pulling. These faults grow and connect with each other, isolating mountain-sized, wedge-shaped fault blocks. Some of these fault blocks begin slipping downward between more stable blocks that still rest on a firm foundation of deep rock. The stable blocks are called horsts, and the sinking blocks, that form valley floors, are called grabens.

Mid-ocean ridge

The longest mountain chain on Earth, the mid-ocean ridge system is entirely under water. Twisting down the center of the Atlantic Ocean, it continues through the Indian and Pacific oceans. It is one of three types of boundary between plates of the crust, along with inter-plate strike-slip faults and collisional mountain belts. Along this ridge, lava continuously erupts, releasing heat from the planet's interior and extruding new strips of ocean floor.

Stratovolcanos

Popocatepetl, Mt. Fuji, Vesuvius, and Mt. Ararat are all stratovolcanos. The prefix strato-refers to these moun tains' characteristic layers, the result of alternately erupting ash and lava. Spectacularly tall and pointed, stratovolcanos may grow to an elevation of 2-3 mi (3.2-4.8 km) before collapsing. It is not certain that every stratovolcano collapses into a crater of superheated steam and molten rock. But the continents are dotted with the remains of these mountains' self-annihilations, some of whose like has not been witnessed in human history.

Cinder cone

These volcanos build a pile of pyroclastic gravel and boulders (pyroclastic is derived from "fire" and "broken pieces") that forms a pointed or rounded cone. Because they are made of loose material, they quickly erode away unless further eruptions continue to build them.

Shield volcanos

Often solitary volcanic mountains form as a volcano piles up rock above the ocean floor over millions of years. Hawaii, Bermuda, and the Canary Islands are shield volcanos. These islands, and others like them, are the work of hot spots (hot spot is a volcanically active site heated from below by a concentrated flow of heat out of the earth's mantle). Iceland is a hot spot that sits astride the mid-ocean ridge system. Shield volcanos also occur on continents, particularly in rift valleys where a continent is being ripped in two. Kilimanjaro is the classic example of a continental shield volcano. Olympus Mons on Mars is another classic shield volcano, and is the largest known mountain in the solar system.

Volcanic necks

In a cinder cone, lava rises through a vertical pipe before it erupts. The mountain resembles a huge pile of gravel. After an old cinder cone becomes extinct, the underground pipes that brought it lava from below solidify, and the pile of erupted material begins to wear away. Solid lava, usually a very hard rock, often fills the extinct volcano's vent. In a cinder cone, the solidified lava will resist the forces of erosion far longer than the ash, cinders, and other loose material of which the volcanic pile is made. Thus, as rain, wind, and frost scrub the soft exterior of the volcano away from the hard interior, a columnar mountain emerges. Shiprock, in New Mexico, and Devil's Tower, in Wyoming, are classic examples of these mountains, called volcanic necks.

Exposed plutons

Plutons are masses of hard, visibly crystalline igneous rock that form deep in Earth's crust. Plutons rise through the earth's crust when they are molten, and freeze into solid rock far below the surface. Plutons can be as small as a highway roadcut, or as large as an entire mountain range. Mountains emerge from a landscape as erosive forces strip away the rocks that cover a pluton. A small pluton called a stock forms the granite core of Mount Ellsworth in southern Utah. The Sierra Nevada mountains are entirely made up of massed plutons, collectively called the Sierra Nevada batholith. The Yosemite Valley cuts into the solid granite interior of these mountains.