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How Do You Make and Break a Mountain?

How Physical Processes Form Physical Features

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Mount McKinley

Mount McKinley in Spring Snow, Alaska Range.

Eastcott Momatiuk/Getty Images
“Water carries the mountains down to the sea a teaspoon at a time. A day becomes a million days, and a mountain of rock changes shape.” (from the film "Planet of Man: The Uneventful Day")

Geographers believe that the Earth’s physical features are created by physical processes--constant, ongoing actions of nature that change the physical environment. In physical geography, we study the physical features and the physical processes that create, shape, move, destroy, or recreate them.

One of the best ways to examine these processes is to look at the life cycle of a mountain.

Building a Mountain

A mountain is an elevated landform with a summit and steep sides. According to scientific theory, mountains are created by a physical process called plate tectonics. The theory of plate tectonics says that the earth’s solid surface (crust) is broken up into massive pieces, called plates, and each plate is squeezed against other plates. Plates move slowly but constantly, the result of convection currents or slab pull, and not all at the same speed or direction. As plates move, so much pressure and stress build at the places where the plates meet (plate boundaries) that the crust (rock) there starts to bend, fold, or get crumpled. After millions of years, when the force is great enough, the pressure is released in sudden, brief, violent events as plates slide under, into, by, and away from each other, breaking rocks or pulling them apart. A mountain starts to build when colliding plates push up the rock between them. At the rate of just a few millimeters a year, building an entire mountain will take millions and millions of years. The mountain stops growing when tectonic forces no longer act on it and crust is no longer being uplifted.

Mountain Breaking

The first step in the process is weathering. Weathering breaks down the mountain’s surface into tiny pieces called sediment. Over time, the forces of weathering (wind, water, rain, ice, waves, chemicals, gravity, and organisms) wear down and eventually level the mountain by breaking up or dissolving its rock into smaller and smaller pieces.

The next step in the process is erosion. Erosion is the carrying away, movement, or removal of weathered rock, dirt, and other bits of earth from one place to another by wind and water in various forms. One of the more forceful agents of erosion is running water, which picks up and transports weathered material. This is how sediment finds its way to a river that moves these weathered materials downstream to new locations.

The next step in the process is deposition. Deposition occurs when sediment carried and transported by a flowing river gets deposited at other places on the Earth's surface. This typically happens where the current slows so much that it can no longer move or carry the sediment. As the river approaches an ocean, for example, it tries to flow downstream, but the ocean pushes it back. At these locations, such as in the mouth of a river, millions of tons of the weathered mountain drop out and are left behind. Over time more and more sediment drops out of the river and is deposited at the same place, building up and forming a solid land mass. This new land mass takes on a triangular, fan shape because the river slows down and veers off course as it approaches the ocean, splitting into different channels that slice the new landform into sections. The result is a delta, a triangular landform formed from the sediment that flowed downstream and was deposited at the mouth of a river or stream where it enters a larger, quieter body of water, like an ocean or lake.

Physical Processes and Mountain Building

Tectonic processes build landforms such as plateaus, volcanoes, valleys, rift valleys, and certain kinds of islands, as well as mountains. Weathering breaks down landforms, while erosion carries away landforms, and together they reshape the earth’s surface by creating landforms such as canyons, buttes, mesas, inselbergs, fjords, hills, lakes, valleys, and sand dunes. Thanks to deposition, what gets worn down gets new life somewhere else as an alluvial plain, island, beach, or a delta. Tectonic activity, weathering, erosion, and deposition are actually not steps, but rather ongoing simultaneous forces at work on the Earth’s surface. Even as a mountain is growing, the physical processes of weathering, erosion and deposition are slowly but relentlessly breaking and taking its surface and depositing it somewhere else.

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