When two tectonic plates come together at a , mountains are formed.

Mountains are landforms that rise above the surrounding terrain, typically thousands of feet high. Some mountains stand on their own; others are part of long chains called mountain ranges. Mountains form in one of three ways:

• Volcanic explosions
• Tectonic faults that occur when tectonic plates slide past each other
• Tectonic collisions

The height of a mountain depends, in part, on where it originates. Mountains that start under the sea are taller, from top to bottom, than those that originate on land. Another important factor is the age of the mountain. Older mountains have had more time to erode, making them smaller (in general) than newer mountains.

There are between 15 to 20 tectonic plates on Earth, either beneath the sea or on land, that fit together like puzzle pieces. Beneath the tectonic plates, which make up the Earth's lithosphere (outer two layers), lies a molten sea of rock. The tectonic plates float on the molten rock and, due to the heat from radioactive processes, shift toward and away from one another. While the plates move incredibly slow, this movement has led to vast changes on the Earth's surface. The continent, oceans, seas, and mountains we know today all exist because of the movement of tectonic plates.

All mountains are formed by the movement of tectonic plates, which lie under the Earth's crust and upper mantle (the layer just below the crust). When tectonic plates move apart or come together, the impact can be explosive. Below are three tectonic-plate movements that create geological change.

When the boundaries between two tectonic plates move further apart, the result is described as a divergent boundary. Molten rock (magma) rises from between the plates. As the magma cools, it creates new oceanic crust. In the process, however, the magma may explode upward in the form of a volcano. In fact, the most volcanic parts of the planet — the Mid-Atlantic Ridge and the Pacific Ring of Fire — are the result of diverging tectonic plates.

When two plates collide, the outcome is called a convergent boundary. The incredible force of the collision may cause parts of the tectonic plates to move upward to form mountain ranges. Earthquakes are often the result of two tectonic plates colliding. Alternatively, a plate may move down to form an ocean trench. When that happens, magma rises up through the ocean floor and solidifies, forming granite.

When two tectonic plates slide past one another, earthquakes occur. The San Andreas Fault is a major example of a point at which this is taking place. Earthquakes occur at these locations, but because the magma under the Earth's surface is not disturbed, no new crust is created or destroyed. This is called a transform plate boundary.

Volcanic, fault-block, and fold mountains all occur as the result of the movement of tectonic plates. The process can be quick, as in the case of an exploding volcano, or it can take millions of years. Erosional mountains are actually fold mountains that are so old, they have been eroded from huge peaks to become much smaller, gentler mountains, such as those found in the Catskills of New York.

Volcanoes form when molten rock builds up in an underground chamber. As the pressure builds, the magma is forced upward. It can escape as a slow flow of lava or as an explosive event. In either case, the magma hardens into volcanic rock, creating new land.

Volcanic events occur at the bottom of the sea and on land. When they occur in the sea, the volcano can grow into a mountain which, in the long run, appears on the surface as an island. In some cases, islands form almost instantly as a result of an erupting undersea volcano.

Mauna Loa is an active volcano on the island of Hawaii which rises 13,100 feet above sea level. For context, Mount Everest rises 29,032 feet. Yet Mauna Loa is actually a taller mountain than Everest because its base is far beneath the sea where volcanic activity is still taking place. Mauna Loa is also still an active volcano — the largest in the world — and it is still growing. From base to summit, Mauna Loa rises 55,700 feet, while its nearby sister, Mauna Kea, rises even taller.

Faults are locations where two tectonic plates slide above and beneath one another. Earthquakes occur, and new landforms, called fault-block mountains, emerge.

The Sierra Nevada mountains, along with the Grand Tetons, are examples of fault-block mountains. Fault-block mountains are formed when tectonic plates slide above and beneath one another. Blocks of rock are raised and tilted during fault events, while other areas are tilted downwards. The raised blocks become mountains; erosion from the mountains fills the depressions below.

Two vast tectonic plates collide, very slowly. As they press together, their boundaries move upwards and start to fold. This process continues for millennia until the folds become vast mountain ranges like the Himalayas, the Andes, and the Alps. While some fold mountain ranges are huge, others, like the Appalachians, are so old that they have eroded to more gentle hills. At one point in the planet's history, however, the Appalachians were even taller than the Himalayas.

There are more fold mountains than any other type of mountain, and there are many different types of folds. Synclines and anticlines are the up and down folds that result from compression. Domes are folds that are shaped like hemispheres, while basins are dips in the surface of the Earth. Most mountains include multiple types of folds.

Updated April 20, 2018

By Laurie Brenner

About 45 million years ago, when the Eurasian continent collided with the Indian subcontinent, the Himalayan mountains formed. In plate tectonics, the scientific theory that explains the structure of the Earth’s crust and how it moves, the planet has roughly nine major plates and many smaller ones, arranged in puzzle pieces around the globe. These plates skate over the mantel of the Earth, an inner layer composed of rocks that surround the Earth’s core. As a unifying theory in geology, most geologists subscribe to plate tectonics as it helps them describe these changes that occur to the Earth’s crust.

When continental plates collide, mountains form. The least understood of all tectonic boundaries, continental plates have greater density, sometimes reaching lower than the mantel. When these plates collide, it’s reminiscent of the force of two bulls butting heads. While some subduction can occur, the effects at these boundaries often include a wide-ranging and creased mountain range, intense crumpling, faulting and a condensed, thickened area inside the collision zone.

Where plates meet in plate tectonics, three types of boundaries form: convergent, divergent and transform. Convergent boundaries include when two continental plates collide, two oceanic plates converge or when an oceanic plate meets a continental plate. Several events can occur. Generally, when the oceanic plate hits a continental one, the continental plate uplifts, and the oceanic plate goes beneath it or subducts. When two oceanic plates collide, the older, heavier plate usually subducts beneath the other.

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Continental plates typically do not subduct beneath oceanic plates because of how thick and buoyant they are. Instead, continental plates typically bend, break and crumple, creating folds, thick creases and mountain ranges like the Andes, Swiss Alps and the Himalayas. Rocks trapped within the collision zone undergo changes because of the extreme heat and squeezing. Called metamorphic rocks, you can find slate, gneiss and schist in these mountain ranges. This includes the eroding Appalachians, which at one time stood as high or higher than the Himalayas, and formed when the North American plate collided with Gondwana, a super continental plate that included South America and Africa at one time.

In areas where oceanic plates collide with continental plates, volcanoes often form, like the volcanoes that circle the Pacific Ocean called the Ring of Fire. Along the Pacific Plate in the Northwestern United States, the Cascade Mountain range consists of several volcanoes formed by the oceanic plate subducting beneath the continental one. Transform boundaries also form, like the San Andreas fault zone, where the two sides of the fault move in opposite directions sliding past each other. The Pacific Plate on the west grinds horizontally to the southeast, while the North American plate moves northwest.