What Is a Mountain?
A mountain is a landform that rises steeply above the surrounding land, usually to a height of over 600 metres (though there is no universal agreed definition). Mountains often have:
- Steep sides (slopes)
- A summit (the highest point)
- A base that merges with the surrounding land
Mountain ranges are long chains of mountains that were formed by the same processes. Examples include the Himalayas, the Andes, the Alps, and the Rockies.
The Theory of Plate Tectonics
Earth's surface is made up of large, moving pieces called tectonic plates. There are about 15 major plates. They float on the semi-molten rock of the mantle beneath the crust and move a few centimetres per year — about the speed your fingernails grow.
When and where plates move, mountains can form.
Evidence for tectonic plates includes matching coastlines (the east coast of South America fits the west coast of Africa like puzzle pieces), identical fossils found on continents now separated by oceans, and continuing earthquake and volcanic activity at plate boundaries.
Types of Mountain Formation
1. Fold Mountains — formed at convergent (colliding) plate boundaries: When two continental plates collide, neither can sink below the other (both are thick and less dense). Instead, the rock layers are squeezed and buckled upward into folds, forming mountain ranges. The Himalayas formed (and are still forming) where the Indian plate crashes into the Eurasian plate.
2. Block (Fault-Block) Mountains — formed by faulting: When the crust is stretched or compressed, cracks called faults form. The land between two faults can be pushed upward (a horst) or the surrounding land can drop down (a rift valley). The Sierra Nevada in California and the Harz mountains in Germany are examples.
3. Dome Mountains — formed by uplift: Hot magma pushes up beneath the crust, doming the surface upward without breaking through. The Black Hills of South Dakota, USA, are a dome mountain.
4. Volcanic Mountains — formed by eruptions: Magma erupts and builds up layers of lava and ash. Mount Fuji (Japan) and Mauna Kea (Hawaii) are volcanic mountains.
Mount Everest (8849 m) is the world's highest mountain above sea level, in the Himalayas. However, Mauna Kea in Hawaii is the tallest mountain if measured from its base on the ocean floor — it rises about 10,200 m from the sea bed, even though only 4205 m is above sea level.
Mountains Are Still Moving
Mountain building (orogenesis) is ongoing. The Himalayas are still being pushed upward at about 5 mm per year as India continues to move northward into Eurasia. At the same time, erosion by wind, water, and ice constantly wears mountains down.
Real world: The Alps — where continents collide
The Alps stretch across eight countries including Austria, Switzerland, France, and Italy. They formed when the African Plate collided with the Eurasian Plate millions of years ago — the same fold-mountain process that built the Himalayas. Today, millions of tourists ski and hike in the Alps every year, and the mountains feed major rivers including the Rhine and Danube.
Real world: The Himalayas are still growing
The Himalayas formed where the Indian Plate crashes into the Eurasian Plate — and they are still rising by around 5 mm per year. This ongoing collision makes Nepal and surrounding countries earthquake-prone. The 2015 Nepal earthquake (magnitude 7.8) killed nearly 9,000 people, a direct result of the forces that also built Earth's tallest mountains.
Real world: Hawaii's volcanic mountains
The Hawaiian Islands are volcanic mountains built over a hotspot in the Pacific Ocean. As the Pacific Plate slowly moves, new volcanic islands form in a chain. Mauna Loa — one of Hawaii's active volcanoes — last erupted in 2022, and Mauna Kea is so high that world-class telescopes are built on its summit above most of the atmosphere.
Where do you see this in real life? Look up the Alps, Himalayas, or Andes on Google Maps — notice how mountain ranges form long chains tracing plate boundaries across whole continents.
Practice Problems
Why do fold mountains form at convergent plate boundaries between two continental plates rather than one sinking beneath the other?
Show answer
The Himalayas are the highest mountain range on Earth. What two tectonic plates are colliding to form them, and are they still growing?