What is a mountain and how do mountains form?

Earth's outer layer is broken into giant slabs called tectonic plates. They move very slowly, a few centimetres a year, about as fast as your fingernails grow. Over millions of years, that movement builds mountains.

Fold mountains form when two plates push together and the rock crumples upwards, like a rug pushed against a wall. The Himalayas, the Alps and the Andes formed this way. The Himalayas are still rising, because India keeps pushing into the rest of Asia. Volcanic mountains grow as lava and ash pile up during eruptions; Kilimanjaro and Mount Fuji are examples (see volcanoes). Block mountains form when cracks in the crust let huge blocks of land rise or sink.

There is no single world rule for when a hill becomes a mountain. Generally, mountains are higher and steeper and have a clear top, or peak. A line of mountains is a range.

Mountains have their own weather. Air gets colder as you go higher, so peaks can be covered in snow even in hot countries. Many rivers begin as rain or melting snow on mountains.

Highest or tallest? Mount Everest, on the border of Nepal and China, is the highest above sea level at 8,848.86 m. But Mauna Kea in Hawaii starts deep under the ocean. Measured from its base on the sea floor, it is over 10,000 m, taller than Everest.