How mountains form

Mountains look like they never change.

But they grow very, very slowly. Giant pieces of the Earth push and squeeze. The rock is lifted up high into the sky.

Mountains look as if they have been there forever, but every range had a beginning. The U.S. National Park Service (NPS) explains that the motions of Earth's tectonic plates fold, break and stretch rock, and on a massive scale build mountains.

There is more than one way to make a mountain. Rock can be squeezed and folded as continents crash together. It can crack along faults, with one block pushed up and the next dropped down. Or melted rock can erupt and pile up into a volcano. Each kind of mountain looks different.

Mountain ranges mark where plate motion has deformed the crust. The NPS lists the main processes as faulting, folding, extension and, on a massive scale, mountain building, and groups mountains into fold mountains at collisional boundaries, fault-block mountains where the crust is pulled apart, and volcanic mountains above subduction zones and hotspots.

The NPS points out that knowing a place's plate tectonic history and setting helps make sense of its landforms and scenery. The Himalayas and Alps are fold mountains from collisions, the Tetons are a fault-block range, and Mount Fuji and the Hawaiian volcanoes are volcanic mountains. Whatever builds them, rain, rivers and ice start wearing them down, and the last section of this page shows how a range's age shows in its shape.

Sometimes two big pieces of land push into each other.

The rock bends and folds, like a rug pushed against a wall. The folds pile up into tall mountains, like the Himalayas.

When two continents push together, neither one sinks, because continental rock is too light. Instead, the rock between them is squeezed, crumpled and folded upward. The NPS compares it to a swimmer lying on a beach ball: the swimmer rises up out of the water. The crust gets thicker and rises higher.

The Himalayas are the highest mountains on Earth today. They are so high because India is pushing the whole thickness of its crust beneath Asia. The Alps in Europe come from another collision, between Africa and Europe. Inside fold mountains, layers of rock are bent into long upward arches and downward troughs.

Collisional, or fold, mountain ranges form when an ocean closes completely and blocks of thick continental crust collide; the crust is compressed, crumpled and thickened, and like a swimmer resting on a beach ball, the thickened crust floats higher (NPS). The Himalayas owe their height to the full thickness of the Indian subcontinent shoving beneath Asia; the USGS dates the start of the India-Asia collision to about 50 million years ago, with most of the uplift in the past 10 million years. The British Geological Survey (BGS) cites the Eurasian-African collision for the Alps.

Structurally, the NPS describes fold mountains as alternating anticlines (upward arches) and synclines (downward troughs) running nearly parallel; the folds may be overturned or thrust over one another along reverse (thrust) faults, which BGS associates with ranges such as the Himalayas and the Alps.

Sometimes the ground stretches and cracks.

One side of the crack goes up. The other side goes down. The side that goes up becomes a mountain.

In some places the crust is being stretched instead of squeezed. As it stretches, it cracks along faults. One block of rock is lifted up and the block next to it drops down, making a fault-block mountain beside a flat valley.

The Teton Range in Wyoming is a famous example. The NPS explains that it rises one earthquake at a time along the Teton fault, about 40 miles (64 kilometers) long. In an average earthquake the fault moves about 10 feet (3 meters): the mountains go up 2 to 3 feet and the valley drops 4 to 6 feet. Thousands of steps like that, over less than 10 million years, built the Tetons.

Where the crust is extended, as in the Basin and Range Province of the western United States, it breaks along normal faults: valley floors drop down and adjacent fault-block mountains rise, producing steep range fronts (NPS).

The Teton Range shows the process clearly. It uplifts one earthquake at a time along the 40-mile Teton fault as the region stretches east to west. On average each earthquake offsets the fault by about 10 feet, two to three feet up and four to six feet down. In the less than 10 million years since the fault began moving, the total offset is probably over 25,000 feet: the Flathead Sandstone on top of Mount Moran sits about 6,000 feet above the valley, while the same layer lies roughly 20,000 feet below the valley floor on the other side of the fault. The fault is still active; the NPS says it could produce a magnitude 7.0 to 7.5 earthquake.

Some mountains are volcanoes.

Hot, melted rock comes out of the ground. It cools and piles up, layer by layer. Over a long time, it makes a mountain.

Volcanoes build mountains a different way: by piling up rock on top of the land instead of pushing it up from below. Each eruption adds lava or ash.

Where magma is sticky, it builds tall, steep cones called stratovolcanoes, like Mount Fuji in Japan. Where lava is runny, it flows far and builds wide, gently sloping shield volcanoes. Mauna Kea and Mauna Loa in Hawaii are shield volcanoes that rise more than 9 kilometers above the sea floor around the island. Chains of volcanic mountains form above subduction zones, like the Cascades and the Andes, and over hotspots, like Hawaii.

Volcanic mountains are constructional: they grow by accumulation rather than by uplift. The NPS notes that a volcanic arc can form where a subducting plate releases hot water that melts rock above it, as in the Cascades, and that hotspots, areas of concentrated mantle heat that can persist for millions of years, build features like the Hawaiian Islands.

The form depends on the magma. BGS explains that viscous magma builds steep stratovolcanoes with slopes of about 30 to 35 degrees, such as Fuji, while fluid basalt builds shield volcanoes with slopes of less than 10 degrees; Mauna Kea and Mauna Loa rise over 9 km above the surrounding sea floor. Volcanoes can also destroy their own summits: Crater Lake in Oregon partly fills a depression formed when a composite volcano erupted and collapsed 7,700 years ago (NPS).

Wind, rain and ice wear mountains down.

Young mountains are tall and pointy. Very old mountains are lower and rounder. It takes millions of years.

As soon as a mountain rises, rain, rivers, ice and wind start wearing it away. That is why the age of a range shows in its shape.

The NPS calls the Tetons "adolescent" mountains: they have been rising for less than 10 million years, so their peaks are still jagged. The Rocky Mountains are "middle-aged" at 50 to 80 million years. The Appalachians in the eastern United States are "elderly," more than 300 million years old. They may once have had peaks as high as today's Himalayas and Alps, but they have been worn down to much lower, rounded mountains.

Erosion begins with uplift. The NPS contrasts the Tetons, uplifted over less than 10 million years and still jagged because erosion has had little time, with the Rocky Mountains (50 to 80 million years old) and the Appalachians (more than 300 million years old). The Appalachians formed in a continental collision 500 to 300 million years ago and in their prime probably had peaks comparable to those in today's collision zone from the Himalayas to the Alps; 300 million years of erosion have reduced them to modest heights.

Ice is a powerful sculptor. In the Tetons, Ice Age glaciers widened V-shaped river canyons into U-shaped glacial canyons, gouged out lake basins and sharpened the high peaks; the most recent advance lasted from roughly 50,000 to 12,000 years ago (NPS). The mountains we see have all been both pushed up from below and worn down from above.