Fold mountains are the most common type of mountains on Earth.
Forming many spectacular ranges.
They're old in human terms, but as geological features many fold mountains are still growing up.
Fold mountains are formed through the movement of the Earth's crust.
Crust - Earth's rocky outermost layer
The crust is split into huge segments called tectonic plates.
The plates are moving incredibly slowly - by only centimeters each year.
Sediment on the sea floor is gradually compressed into sedimentary rock.
Formed by compression of sediment
If the two plates move towards each other, they push together along their margins in a very slow but powerful collision.
One plate forces the other underneath it, a process called subduction.
One tectonic plate descending below another
The extreme pressure created causes the sedimentary rock to fold, and forces it upwards.
Over time, this rock is pushed above sea level.
To form a range of fold mountains.
Occurring over many millions of years, the whole process of fold mountain formation is called orogeny.
Orogeny has formed many of the world's most famous mountain ranges.
The Alps, South-central Europe
In Europe, the Alps were formed by the meeting of the African and Eurasian tectonic plates.
Their collision began around 55 million years ago, which makes them young for mountains.
The Rocky Mountains,
Western North America
The Rocky Mountains of North America.
Western South America
And the Andes of South America stretch down the western side of each continent.
Both were formed by the subduction of oceanic plates beneath continental plates.
The highest fold mountains are Asia's Himalayas.
Including Earth's highest mountain.
Mount Everest 8850m high
The discovery of ancient seashells near the peaks of these great fold mountains confirmed that they were once underwater and in fact fold mountains are still gradually growing.