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

Detail

1. The starting point: an old earth and a moving plate

• The earth is about 4,600 million years old.
• Two kinds of force have shaped it:
• Endogenic forces (forces from inside the earth): heat inside the earth moves plates, builds mountains and causes earthquakes and volcanoes.

• Exogenic forces (forces at the surface): rivers, wind, ice, waves and weathering wear the land down and lay down new material.

• The Indian plate's journey (Class 11):

• The Indian plate once lay south of the equator. It was joined to the Australian plate.
• The two plates split apart. Australia moved south-east and India moved north.

• India is still moving north. This keeps pushing the Himalayas up and causes earthquakes.

• Gondwana version (Class 7):

• Gondwana was a huge southern supercontinent. It included India, Africa, South America, Australia and Antarctica.
• India broke away from Gondwana and hit Eurasia about 50 million years ago.
• India still pushes north about 5 cm a year, and the Himalayas rise about 5 mm a year.

• Not all of India's movement goes into the Himalayas. About 15–20 mm a year of the north–south squeezing is taken up across the Himalayan belt, mostly on the Main Himalayan Thrust [3].

• Result: three geological divisions of India. These divisions are made by rock type, age and how the land formed, and they broadly match India's physical features: 1. The Peninsular Block 2. The Himalayas and other Peninsular (extra-peninsular) mountains 3. The Indo-Ganga-Brahmaputra plain

Feature	Peninsular Block	Himalayas	Indo-Ganga-Brahmaputra plain
Age	Oldest (Archaean, over 2,500 million years)	Young (Tertiary, from about 40–50 million years ago)	Youngest (Quaternary alluvium)
Rocks	Gneisses and granites	Folded sedimentary rocks from the Tethys sea	Loose alluvium (river-laid sand, silt and clay)
Nature	Rigid and stable	Weak and flexible, still active	Filled-up trough (a long, low hollow in the earth's crust)
Main movement	Vertical movement, block faulting	Folding, faulting, thrusting	Sinking and filling with sediment
Rivers	Old rivers in shallow valleys	Young rivers in gorges and V-shaped valleys	Slow, winding rivers
Main value	Minerals	Water, forests, hydropower	Fertile soil, dense population

2. The Peninsular Block

• Definition: the oldest and most stable part of India. It is a shield, which means a large, hard, ancient block of the earth's crust that has not been folded for a very long time.
• Age and build-up:
• The Indian shield is a patchwork of several Archaean cratons (very old, stable cores of a continent), joined by younger Proterozoic mobile belts (zones where those cores were welded together) [6].
• The Dharwar craton in Karnataka is one of the oldest and largest Archaean blocks in the world. Its history goes back about 3.5 billion years [6].

Northern boundary: an irregular line

• From west to east:
• Kachchh (Gujarat)
• → along the western flank of the Aravalli Range near Delhi
• → roughly parallel to the Yamuna and Ganga
• → to the Rajmahal hills (Jharkhand) and the Ganga delta (West Bengal).

Extensions outside the main block

• North-east: the Karbi Anglong plateau (Assam) and the Meghalaya plateau (Garo, Khasi and Jaintia hills).

• These are cut off from the main block by the Malda gap (Garo-Rajmahal gap), a sunken stretch that later filled with Ganga-Brahmaputra sediment.

• West: Rajasthan. Here the old rock is buried under desert sand and other deposits.

Rocks and stability

• The block is made mostly of very ancient gneisses (rocks banded by heat and pressure) and granites (hard rocks that cooled slowly from magma underground).
• It has stayed rigid since the Cambrian period (about 540 million years ago).

• Exception: the west coast sank under the Arabian Sea (it was submerged). This is why the western coastal plain is narrow.

• It has not been folded. Instead it has gone through:

• Vertical movements: whole parts went up or down.
• Block faulting: the crust broke along cracks called faults, and blocks moved up or down along them.

How block faulting makes landforms

• Rift valleys:
• Two parallel faults form in the crust.
• The strip between them sinks.
• A long, narrow valley forms, called a rift valley or graben, and a river later flows along it.

• Examples: the Narmada, Tapi and Mahanadi rift valleys. The Narmada and Tapi flow west into the Arabian Sea through these rifts. This is why they do not build deltas and form estuaries instead.

• Block mountains:

• The crust breaks along faults.
• A block is pushed up between them, or stays high while the land around it sinks.
• A raised block is left, called a block mountain or horst.
• Example: the Satpura range, which lies between the Narmada and Tapi rifts.

Landforms of an old, worn-down block

• Relict hills and residual hills: stumps of very old mountains, worn down by weathering and erosion over hundreds of millions of years.

Hill	Location
Aravalli	Gujarat–Rajasthan–Haryana–Delhi; among the world's oldest fold mountains
Nallamala	Andhra Pradesh (Eastern Ghats)
Velikonda (Veliconda)	Andhra Pradesh (Eastern Ghats)
Palkonda	Andhra Pradesh (Eastern Ghats)
Javadi	Tamil Nadu (Eastern Ghats)
Mahendragiri	Odisha (Eastern Ghats)

• Valleys are broad and shallow, with gentle slopes (low gradient). The rivers have worked on this land for a very long time and are close to their base level (the lowest level a river can cut down to, usually sea level).
• East-flowing rivers build deltas in the Bay of Bengal. Examples are the Mahanadi, Godavari, Krishna and Kaveri.

3. The Himalayas and other Peninsular (extra-peninsular) mountains

• Nature: young, weak and flexible. The rocks are still being bent and broken.
• Ongoing deformation:
• Folds: rock layers bent like a wave.
• Faults: rock broken, with the two sides moving apart.

• Thrusts: a low-angle fault where one rock mass is pushed up and over another.

• Origin: tectonic, meaning made by plate movement. This makes them fold mountains.

• Young rivers and landforms (Figure 2.1):
• The land is still rising, so the rivers keep cutting downward.
• The result is gorges, V-shaped valleys, rapids and waterfalls.
• Examples: the gorges of the Indus, Sutlej and Brahmaputra (Dihang).

Himalayan orogeny: how the mountains were built

• Orogeny means mountain-building.

• Step by step: 1. Cause: India moved north and closed the Tethys sea, an old ocean between Gondwana and Eurasia. 2. Collision: India hit Eurasia about 40–50 million years ago, in the Cenozoic era [2]. 3. Folding: thick sediments from the Tethys floor were squeezed, folded and pushed upward. 4. Phases: uplift came in several phases. The ranges formed from north to south: Greater Himalaya first, then the Lesser Himalaya, then the Shiwalik last. 5. Downwarp: the weight of the rising mountains bent the crust in front of them. This made a trough, which later became the plain (see Section 4).

• Main Himalayan Thrust (MHT): the master fault under the whole range. Here the Indian plate slides beneath the Himalaya. The other thrusts branch upward from it [2].

The great thrust faults (north to south)

Thrust	Separates	Note
South Tibet Detachment (STD)	Tethys Himalaya / Greater Himalaya	A normal fault on the northern side [2]
Main Central Thrust (MCT)	Greater (Higher) Himalaya / Lesser Himalaya	Northern limit of the Lesser Himalaya [2]
Main Boundary Thrust (MBT)	Lesser Himalaya / Shiwalik	Southern limit of the Lesser Himalaya [2]
Main Frontal Thrust (MFT)	Shiwalik / Indo-Gangetic plain	The youngest and southernmost thrust; it reaches the surface [2][3]

• Pattern: the thrusts get younger from north to south. The mountain front keeps moving south over the plain.
• Seismic gap: in the central seismic gap (about 78°–82°E, Uttarakhand and western Nepal), strain has built up for a long time without a great earthquake. A big future quake is possible here [3].
• Hazard link: these active thrusts are why the Himalayan belt lies in seismic zones V and IV, the zones with the highest earthquake risk (link: natural-hazards-disasters).

4. The Indo-Ganga-Brahmaputra plain

• Origin: a geosynclinal depression. A geosyncline is a long trough in the crust that keeps sinking as sediment piles up in it.
• Modern term: foreland basin
• Cause: the huge weight of the Himalaya presses down on the Indian plate.
• Process: the plate bends downward just south of the mountains.

• Result: a long trough forms in front of the range. This is called a foreland basin. The Indo-Gangetic plain is a classic example [4].

• Timing: the trough reached its greatest size during the third phase of Himalayan formation.

• (NCERT: "about 64 million years ago".)

• Most geologists place the third phase much later, in the Pliocene–Pleistocene (roughly the last 5 million to 10,000 years). That fits the Quaternary age of the alluvium at the surface.

• Filling:

• Rivers from the Himalayas (Indus, Ganga, Brahmaputra and their tributaries) and from the Peninsula (Chambal, Betwa, Ken, Son) dropped sediment into the trough.
• The average depth of the alluvium is 1,000–2,000 m.

• Peninsular rivers were a major source of sediment and water for the Ganga plains and the Bay of Bengal until as late as about 60,000 years before present [7].

• Size:

• About 700,000 km², stretching more than 2,400 km from the Arabian Sea to the Bay of Bengal across Pakistan, India and Bangladesh [5].
• It is the world's most extensive tract of uninterrupted alluvium, which means the largest unbroken stretch of river-laid sediment [5].
• The thick alluvium gives fertile soil and holds a lot of groundwater for well irrigation [5].

5. Why structure matters

Division	What the structure gives	Reason
Peninsular Block	Mineral wealth: iron ore, manganese, mica, gold (Kolar), coal (Gondwana basins)	Old igneous and metamorphic rocks; coal formed in sunken rift basins
Indo-Ganga-Brahmaputra plain	Fertile soils, dense population, groundwater, the "food bowl"	Deep alluvium renewed by floods; flat land; perennial rivers
Himalayas	Earthquakes, landslides, hydropower, glacier-fed rivers	Active thrusts; steep, young, weak slopes

Prelims Hooks

• Thrusts from north to south: STD → MCT (Greater/Lesser Himalaya) → MBT (Lesser Himalaya/Shiwalik) → MFT (Shiwalik/plain). All branch from the Main Himalayan Thrust.
• Northern edge of the Peninsular Block, west to east: Kachchh → western flank of the Aravallis near Delhi → parallel to the Yamuna and Ganga → Rajmahal hills → Ganga delta.
• Detached parts of the Peninsular Block: Karbi Anglong and the Meghalaya plateau (north-east) and Rajasthan (west). Trap: the Shiwalik and the Purvanchal are not part of it.
• Process–landform pairs: block faulting → Narmada, Tapi and Mahanadi rift valleys; uplifted block → Satpura block mountain.
• Relict hills: Aravalli, Nallamala, Javadi, Veliconda, Palkonda, Mahendragiri. Trap: these are worn-down remains of old mountains, not young fold mountains.
• Rigid since the Cambrian. The one exception is the submerged west coast.
• Plain depth: alluvium 1,000–2,000 m deep. The plain is a foreland basin, once called a geosynclinal depression.
• Rates: India moves north 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].
• Oldest to youngest: Peninsular Block (Archaean; Dharwar craton about 3.5 billion years [6]) → Himalaya (Tertiary) → Indo-Gangetic alluvium (Quaternary).

Mains Points

• Structure controls where resources are:
• Minerals and Gondwana coal lie in the Peninsular Block.
• Alluvial soil, groundwater and dense population lie in the plain.
• Hydropower and glacier-fed rivers come from the Himalaya.

• Together this explains India's settlement pattern and why industry grew in the Chota Nagpur–Odisha belt (GS-I).

• Himalayan hazards:

• The MCT, MBT and MFT are active, and the central seismic gap (78°–82°E) has a long-delayed great earthquake.

• Policy responses should include strict building codes in zones V and IV, mapping of fault zones before roads, tunnels and dams are approved (Joshimath, Char Dham road), and landslide early warning (GS-III, disaster management).

• Peninsula against Himalaya:

• The rigid Peninsular Block has old rivers with deltas, gentle valleys and rain-fed flow.
• The young Himalaya has perennial rivers in gorges with high sediment loads.

• This difference shapes river-linking plans, the siting of dams and the risk of floods in the plain.

• Alluvial plain under stress:

• Deep alluvium supports intensive well irrigation.
• Over-pumping has caused groundwater decline in Punjab, Haryana and western Uttar Pradesh.
• River sediment keeps silting rivers and deltas, so flood and sediment plans for the Ganga-Brahmaputra basin must be built around how the basin was formed.