Fold mountain

Also called: Young fold mountains · Topic: Continental Drift, Sea-floor Spreading and Plate Tectonics · NCERT: Class 6, Ch 3 "Landforms and Life"; Class 7, Ch 1 "Geographical Diversity of India"; Class 9, Ch 2 "Shaping of the Earth's Surface"

Meaning

A fold mountain is a mountain made when layers of rock are squeezed from the sides and bent into up-folds and down-folds where two plates push into each other. Fold mountains are the highest and longest mountain chains on Earth, such as the Himalayas, Alps, Andes and Rockies. For India they matter a great deal: they shape its climate, feed its rivers and bring the risk of earthquakes.

Explanation

How fold mountains form (cause → process → result)

• Cause: two plates move towards each other.
• This kind of meeting place is called a convergent boundary (a boundary where plates move towards each other and crust is destroyed).

• It is the opposite of a divergent boundary, where plates move apart and new crust forms at mid-ocean ridges.

• Process: the rock layers get squeezed and bent.

• Rock layers that lie between the plates, often old sea-floor sediments, are pushed from both sides.
• They bend into up-folds called anticlines and down-folds called synclines.

• When the push is very strong, the rock also breaks along faults (cracks where rock blocks move) and the broken slices pile up on top of each other.

• Result: the crust becomes very thick and rises into long mountain chains.

• The chains run along the edge where the plates meet.

Two ways plates build fold mountains

• Continent-continent convergence (great fold mountains)
• Both plates are made of light continental crust, so neither plate sinks easily.
• First, the ocean between them closes as its floor sinks into the mantle.
• Then the two continents crash into each other. Rock crumples, folds, breaks and stacks up, and the crust becomes very thick.
• Examples are the Himalayas (India meeting Eurasia) and the Alps (Africa meeting Europe).

• There is no real subduction here, so this type has no major volcanoes.

• Ocean-continent convergence (volcanic fold mountains)

• Heavy oceanic crust (basalt) sinks under lighter continental crust (granite-type rock). This sinking is called subduction.
• A deep trench (a long, narrow, very deep valley in the ocean floor) forms just offshore.
• The edge of the continent is squeezed and lifted. Magma (molten rock) rises up and builds volcanoes on it.
• The result is volcanic fold mountains, such as the Andes.

The Himalayas: how a young fold mountain was made

• The Tethys Sea came first.
• About 225 million years ago, India lay off the coast of Australia, and the Tethys Sea separated it from Asia [3].

• About 80 million years ago, India was about 6,400 km south of Asia and was moving north at about 9 m per century [3].

• Then came the collision.

• India hit Asia about 40-50 million years ago [3].
• After the crash, India's speed towards the north fell by about half [3].
• The Indian and Eurasian plates crumpled up into the Himalayas [2].

• Class 7 NCERT puts it this way: sediments of the Tethys Sea were squeezed and lifted up.

• Fossils show the rocks came from the sea.

• Sandstone layers in the Himalayas hold plant and animal fossils. These layers were laid down when the Tethys Sea flooded the area [3].

• The mountains are still rising.

• The Himalayas rise more than 1 cm a year, which adds up to 10 km in a million years [3].
• The nearby Tibetan Plateau has an average height of about 4,600 m [2].

Young and old fold mountains

Feature	Young fold mountains	Old fold mountains
Examples	Himalayas, Alps, Andes, Rockies	Aravallis (India)
Age	Tens of millions of years	About 2.5 billion years (NCERT Class 7)
Height and shape	High, with sharp peaks	Worn down, low and rounded
Activity	Still rising; frequent earthquakes	Stable; worn down by erosion (slow wearing away by wind, water and weather) over a very long time

Where it occurs

• Himalayas (northern India and its neighbours): young fold mountains formed where the Indian plate pushed into the Eurasian plate. They run across Jammu and Kashmir, Himachal Pradesh, Uttarakhand and the north-east.
• Aravallis (India):
• They run from north-east to south-west, mainly through Rajasthan, from near Delhi to Gujarat.

• They are among the oldest fold mountains in the world.

• Alps (Europe): formed as the African plate pushes north into Europe and folds the rock between them.

• Andes (western South America):
• Formed where the Nazca plate sinks under the South American plate, which also creates the Peru-Chile trench [2].

• The region has strong, destructive earthquakes, and the mountains rise quickly [2].

• Rockies (western North America): young fold mountains.

• Cascade Range (Pacific north-west of the USA): active volcanoes formed by ocean-continent convergence, like the Andes [2].

Don't confuse with

• Island arc: a curved chain of volcanic islands made by ocean-ocean convergence, for example Japan, the Marianas or the Andaman-Nicobar arc. Fold mountains like the Himalayas and Alps come from continent-continent collision, so they are not island arcs.
• Volcanic mountain: made by lava and ash piling up around a vent. A fold mountain is made by rock layers being squeezed and bent. The Himalayas are fold mountains and have no active volcanoes.
• Old vs young fold mountains: both form by folding. The difference is age and shape. The Aravallis (about 2.5 billion years old) are low and rounded. The Himalayas are young, high, sharp-peaked and still rising.
• Trench: also forms at convergent boundaries, but it is a deep valley in the ocean floor where a plate bends downwards. A fold mountain is land pushed upwards.

Prelims Hooks

• Young fold mountains: Himalayas, Alps, Andes and Rockies. Old fold mountains: Aravallis (about 2.5 billion years old, NCERT Class 7).
• Process–landform pairs: continent-continent collision → Himalayas (India-Eurasia) and Alps (Africa-Europe). Ocean-continent convergence → Andes (Nazca plate under the South American plate) [2].
• Trap: the Himalayas have no active volcanoes, because continent-continent convergence has no true subduction. The Andes do have volcanoes.
• Order of events: Tethys separates India from Asia (about 225 million years ago) → India about 6,400 km south of Asia (about 80 million years ago) → collision (about 40-50 million years ago) [3].
• Still rising: the Himalayas gain more than 1 cm a year [3]. Marine fossils in Himalayan sandstone show the rocks were once under the Tethys Sea [3].
• Aravallis: run north-east to south-west, from near Delhi through Rajasthan to Gujarat.

Mains Points

• Young means unstable (GS-I and GS-III, disasters):
• India is still pushing into Eurasia, and the Himalayas are still rising more than 1 cm a year [3].
• So the Himalayan belt is young, unstable and prone to earthquakes and landslides.

• This puts dams, roads, tunnels and towns in Uttarakhand, Himachal Pradesh, Jammu and Kashmir and the north-east at risk. Building and planning there must take this into account.

• Young fold mountains as climate barriers and water towers (GS-I):

• The Himalayas block cold winds from Central Asia and trap monsoon rain on the Indian side.

• Their snow and rain feed perennial rivers (rivers that flow all year round), which support farming and people across the northern plains.

• Old fold mountains still have value (GS-I and GS-III, resources and environment):

• The Aravallis are worn down, but they hold minerals.
• They also act as a barrier that slows the spread of the Thar Desert, so protecting them matters for the environment.

Related concepts

• Convergent boundary
• Subduction zone
• Ocean-continent convergence
• Ocean-ocean convergence
• Island arc
• Continent-continent convergence
• Wadati-Benioff zone

Read more

• Convergent boundaries: subduction, island arcs and fold mountains