28.0°N 86.9°EMount Everest
Plates set the stage
Earth's outer shell is broken into large plates that move a few centimeters a year, about as fast as fingernails grow. Where they collide, rock crumples and thickens. Where one plate sinks beneath another, melting rock feeds volcanoes. Where the crust stretches, blocks break and tilt. Each process makes a different kind of mountain.
Four kinds of mountains
| Type | How it forms | Examples |
|---|---|---|
| Fold mountains | Two plates collide and the rock between them folds and stacks up | Himalaya, Alps |
| Fault-block mountains | Crust stretches and cracks, and blocks rise or tilt along faults | Sierra Nevada, Teton Range |
| Volcanic mountains | Lava and ash pile up around a vent | Cascade volcanoes, Mount Fuji, Kilimanjaro |
| Dome mountains | Rising molten rock pushes the crust up into a bulge, then erosion exposes it | Black Hills |
Many real ranges mix types. The Andes have folded and faulted rock as well as a long chain of volcanoes, because an ocean plate is sinking under South America along their whole length.
The Himalaya: a collision still going on
The Himalaya began rising when the plate carrying India collided with Asia, and the two are still pushing together. That is why the region has frequent earthquakes. Mount Everest, the highest point above sea level, stands about 8,850 m (29,030 ft) according to recent surveys. The exact figure changes a little with each survey method.
Old mountains wear down
As soon as mountains rise, erosion starts on them. The Appalachians in eastern North America were once a far higher range; hundreds of millions of years of weathering have worn them into rounded ridges. Sharp, jagged peaks usually mean a young range or recent glacial carving. Smooth, rounded summits usually mean an old one.
Some mountain records
- The Andes are the longest mountain range on land, at about 7,000 km (4,300 miles).
- The mid-ocean ridge, mostly under the sea, is far longer, winding more than 60,000 km around the globe.
- Mauna Kea in Hawaii rises more than 10,000 m from its base on the sea floor, though only about 4,200 m of that is above sea level.
How mountains shape weather
Mountains force moving air upward. As it rises it cools, and its moisture condenses into cloud and rain on the windward side. By the time the air crosses the crest and sinks, it is drier and warmer, leaving a rain shadow on the far side. The dry lands east of the Sierra Nevada and the Cascades are rain shadows. Height matters too: air cools by about 6.5°C per 1,000 m on average, so mountains have their own climate bands, from forest at the foot to bare rock and snow at the top. The climate zones post covers this in more detail.
Mountains on maps
On a topographic map, summits show as tight closed loops with spot heights, passes show as saddles, and steep faces show as crowded lines. Ranges often line up with plate boundaries, which you can see by comparing a relief map with a map of earthquakes. The landforms post explains the valleys that ice and rivers cut into them.
Mountain range FAQ
Are mountains still growing?
Some are. Ranges at active plate boundaries, such as the Himalaya, are still being pushed up, while erosion wears them down at the same time. Whether a peak gains or loses height depends on which process is faster.
What is the tallest mountain in the world?
Everest is the highest above sea level. Measured from base to summit, Mauna Kea is taller, because most of it is under the ocean.
Why is it colder on mountains if they are closer to the sun?
The extra distance is tiny compared with the distance to the sun. Air is heated mostly from the ground, and higher air is thinner and expands as it rises, which cools it.