Himalayan orogeny

Also called: India-Eurasia collision · Topic: Structure and Physiography of India: Himalayas, Plains, Peninsula, Coasts and Islands · NCERT: Class 7, Ch 1 "Geographical Diversity of India"; Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 4 "Distribution of Oceans and Continents"

Meaning

Himalayan orogeny (orogeny means mountain-building) is the building of the Himalayas. It began when the northward-moving Indian plate closed the Tethys sea and hit the Eurasian plate about 40–50 million years ago, in the Cenozoic era [2]. The collision squeezed and folded the thick Tethys sediments and pushed them up, in phases.

It matters because the collision has not stopped. India still pushes north, so the Himalayas are still rising. This is why they are young, weak and full of earthquakes. It also produced the trough that became the Indo-Ganga-Brahmaputra plain.

Explanation

How it happened: cause → process → result

• Cause: India moved north.
• The Indian plate once lay south of the equator. It was joined to the Australian plate, inside the southern supercontinent Gondwana.
• The two plates split. Australia moved south-east and India moved north.

• As India moved north, it closed the Tethys sea, an old ocean between Gondwana and Eurasia.

• Collision:

• India hit Eurasia about 40–50 million years ago [2]. Class 7 NCERT rounds this to about 50 million years ago.

• Folding:

• Thick sediments on the Tethys floor were caught between the two plates.

• They were squeezed, folded and pushed upward. So the Himalayas are made of folded sedimentary rocks and are fold mountains of tectonic origin (made by plate movement).

• Uplift in phases, from north to south: 1. Greater Himalaya first 2. Lesser Himalaya next 3. Shiwalik last

• Downwarp (the result in front of the mountains):

• The rising mountains were very heavy, and their weight bent the Indian plate downward just south of the range.
• This made a long trough called a foreland basin (old term: geosynclinal depression) [4].
• Rivers filled the trough with 1,000–2,000 m of alluvium. This became the Indo-Ganga-Brahmaputra plain.

Parts of the system: the great thrusts

• A thrust is a low-angle fault where one rock mass is pushed up and over another.
• Main Himalayan Thrust (MHT): the master fault under the whole range. Here the Indian plate slides beneath the Himalaya. All the other thrusts branch upward from it [2].

Thrust (north → south)	Separates
South Tibet Detachment (STD)	Tethys Himalaya / Greater Himalaya. It is a normal fault on the northern side [2]
Main Central Thrust (MCT)	Greater (Higher) Himalaya / Lesser Himalaya [2]
Main Boundary Thrust (MBT)	Lesser Himalaya / Shiwalik [2]
Main Frontal Thrust (MFT)	Shiwalik / Indo-Gangetic plain. It is the youngest and reaches the surface [2][3]

• Pattern: the thrusts get younger from north to south. The mountain front keeps creeping south over the plain.

Why it is still going on

• India still pushes north:
• India moves north about 5 cm a year.

• The Himalayas rise about 5 mm a year (NCERT).

• Not all of India's movement goes into the Himalayas:

• About 15–20 mm a year of north–south squeezing is taken up across the Himalayan belt, mostly on the Main Himalayan Thrust [3].

• Signs of a young, active range:

• The rocks are still being bent (folds), broken (faults) and pushed over each other (thrusts).
• The land keeps rising, so rivers keep cutting down. This makes gorges, V-shaped valleys, rapids and waterfalls.
• Strain builds up on the thrusts and is released as earthquakes.

Timing note

• NCERT says the trough was largest in the third phase of Himalayan formation, "about 64 million years ago".
• Most geologists place this third phase much later, in the Pliocene–Pleistocene (roughly the last 5 million to 10,000 years). This fits the Quaternary age of the alluvium at the surface.

Where it occurs

• The Himalayan arc: three parallel ranges, the Greater Himalaya, the Lesser Himalaya and the Shiwalik, with the Tethys Himalaya to the north of the STD.
• Gorges cut by rivers into the rising land: the Indus, the Sutlej and the Brahmaputra (Dihang).
• Central seismic gap: about 78°–82°E, in Uttarakhand and western Nepal. Strain has built up here for a long time without a great earthquake, so a big future quake is possible [3].
• Foreland basin in front of the range: the Indo-Gangetic plain. It covers about 700,000 km² and stretches more than 2,400 km from the Arabian Sea to the Bay of Bengal, across Pakistan, India and Bangladesh [5].
• Seismic zones: the whole Himalayan belt lies in zones V and IV, the zones with the highest earthquake risk.
• Problem sites in the news: Joshimath and the Char Dham road in Uttarakhand.

Don't confuse with

• Block faulting (Peninsular Block): the Peninsula has not been folded since the Cambrian. It only had vertical movement and blocks breaking along faults, which made the Narmada and Tapi rift valleys and the Satpura block mountain. The Himalayas were made by folding and thrusting.
• Aravalli (relict fold mountains): the Aravalli are among the world's oldest fold mountains, worn down to stumps. The Himalayas are young (Tertiary) fold mountains that are still rising.
• Rift valley (graben) vs foreland basin: a rift valley is a strip that sinks between two faults, like the Narmada. The foreland basin of the Indo-Gangetic plain formed because the weight of the Himalaya bent the plate down [4].
• MCT vs MBT: the MCT separates the Greater and Lesser Himalaya (the northern limit of the Lesser Himalaya). The MBT separates the Lesser Himalaya and the Shiwalik (its southern limit) [2].

Prelims Hooks

• Collision date: India hit Eurasia about 40–50 million years ago, in the Cenozoic era [2]. The rocks are folded Tethys sediments.
• Order of uplift: Greater Himalaya → Lesser Himalaya → Shiwalik (oldest to youngest, north to south).
• Thrusts from north to south: STD → MCT → MBT → MFT. All branch from the Main Himalayan Thrust. The MFT is the youngest [2].
• Rates: India moves about 5 cm a year and the Himalayas rise about 5 mm a year (NCERT). About 15–20 mm a year of squeezing is taken up across the Himalaya [3].
• Central seismic gap: 78°–82°E, Uttarakhand–western Nepal [3].
• Trap: the Indo-Gangetic plain is a foreland basin (old name: geosynclinal depression) made by the Himalayan load. It is not a rift valley, and it is not part of the Peninsular Block.

Mains Points

• Hazards come from an orogeny that is still going on (GS-III, disaster management):
• The MCT, MBT and MFT are active thrusts, and the central seismic gap (78°–82°E) has a long-delayed great earthquake [3].

• Policy responses:

  • Strict building codes in zones V and IV.
  • Fault-zone mapping before roads, tunnels and dams are approved (Joshimath, Char Dham road).
  • Early warning for landslides.

• The orogeny shaped North India's land and resources (GS-I):

• Himalaya: glacier-fed rivers that flow all year, hydropower and forests, along with steep, weak slopes that slide.
• Foreland basin: 1,000–2,000 m of alluvium gives fertile soil and plenty of groundwater [5]. This supports dense population.

• Over-pumping now lowers groundwater in Punjab, Haryana and western Uttar Pradesh.

• Young rivers vs old rivers:

• Himalayan rivers still cut gorges and carry heavy sediment loads, because the land keeps rising.
• This sediment silts up rivers and deltas in the plain and raises flood risk.
• It affects where dams are built, river-linking plans and flood management in the Ganga-Brahmaputra basin.

Related concepts

• Geological divisions of India
• Peninsular Block

Read more

• Geological structure: the Peninsular Block, the Himalayas and the Indo-Ganga-Brahmaputra plain