Peninsular Block

Also called: Peninsular shield · Topic: Structure and Physiography of India: Himalayas, Plains, Peninsula, Coasts and Islands · NCERT: Class 11, Ch 2 "Structure and Physiography"

Meaning

The Peninsular Block (also called the Peninsular shield) is the oldest and most stable part of India. It is made mostly of very old gneisses and granites and has stayed rigid since the Cambrian period (about 540 million years ago). Its northern edge is an uneven line. It runs from Kachchh, along the western flank of the Aravalli Range near Delhi, roughly parallel to the Yamuna and Ganga, to the Rajmahal hills and the Ganga delta.

It is one of India's three geological divisions. Its old rocks hold most of India's mineral wealth. Its stable, worn-down surface also explains the gentle valleys and the old rivers of the Peninsula.

Explanation

What it is made of and how old it is

• A shield: a shield is a large, hard, ancient block of the earth's crust that has not been folded for a very long time.
• Age: it is the oldest of India's three divisions. Its rocks are Archaean, which means more than 2,500 million years old.
• How it was built:
• It is a patchwork of several Archaean cratons. A craton is a very old, stable core of a continent [2].
• These cores are joined by younger Proterozoic mobile belts. These are the zones where the old cores were welded together [2].

• 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 [2].

• Rocks:

• Gneisses are rocks that heat and pressure have pressed into bands.
• Granites are hard rocks that cooled slowly from magma deep underground.

How it has moved: no folding, only up-and-down movement

• Rigid since the Cambrian. Since then, the block has not been folded.

• The one exception: part of the west coast sank under the Arabian Sea (it was submerged). This is why the western coastal plain is narrow.

• Two kinds of movement have shaped it instead of folding:

• Vertical movements: whole parts of the block went up or down.

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

• Block faulting → rift valley (graben):

• Two parallel faults form in the crust.
• The strip between them sinks.
• A long, narrow valley is left. A river later flows along it.

• Examples: the Narmada, Tapi and Mahanadi rift valleys.

• Block faulting → block mountain (horst):

• The crust breaks along faults.
• One block is pushed up, or it stays high while the land around it sinks.
• Example: the Satpura range, which stands between the Narmada and Tapi rifts.

Landforms of an old, worn-down block

• Relict hills and residual hills:
• These are the stumps of very old mountains.
• Weathering and erosion have worn them down over hundreds of millions of years.

• Examples: Aravalli, Nallamala, Velikonda (Veliconda), Palkonda, Javadi and Mahendragiri.

• Broad, shallow valleys with gentle slopes (low gradient):

• Rivers have worked on this land for a very long time.

• They are now close to their base level, the lowest level a river can cut down to (usually sea level).

• Two types of river mouth:

• West-flowing Narmada and Tapi run through rift valleys into the Arabian Sea. They form estuaries (wide, funnel-shaped river mouths), not deltas.
• East-flowing Mahanadi, Godavari, Krishna and Kaveri build deltas in the Bay of Bengal.

Its boundaries and the parts that lie outside it

• Northern boundary (west to east): Kachchh (Gujarat) → western flank of the Aravalli Range near Delhi → roughly parallel to the Yamuna and Ganga → Rajmahal hills (Jharkhand) → Ganga delta (West Bengal).
• Outer parts in the north-east:
• The Karbi Anglong plateau (Assam) and the Meghalaya plateau (Garo, Khasi and Jaintia hills).

• The Malda gap (Garo-Rajmahal gap) cuts them off from the main block. This sunken stretch was later filled with Ganga-Brahmaputra sediment.

• Outer part in the west: Rajasthan. Here the old rock lies buried under desert sand and other deposits.

Where it occurs

• Main block: the whole of peninsular India, south of the line Kachchh–Aravallis–Delhi–Yamuna-Ganga–Rajmahal hills–Ganga delta.
• Old core: the Dharwar craton in Karnataka [2].
• Rift valleys: Narmada, Tapi and Mahanadi.
• Block mountain: Satpura, between the Narmada and Tapi.
• Relict hills:
• Aravalli in Gujarat, Rajasthan, Haryana and Delhi.
• Nallamala, Velikonda and Palkonda in Andhra Pradesh (Eastern Ghats).
• Javadi in Tamil Nadu (Eastern Ghats).

• Mahendragiri in Odisha (Eastern Ghats).

• Outer parts: the Karbi Anglong plateau (Assam), the Meghalaya plateau (Garo, Khasi and Jaintia hills) and the buried rocks under Rajasthan.

• Coasts: the narrow western coastal plain, along the coast that sank. Deltas of the Mahanadi, Godavari, Krishna and Kaveri on the east coast.
• Minerals: iron ore, manganese, mica, gold (Kolar), and coal in the Gondwana basins.
• In the world: the block was once part of Gondwana. This southern supercontinent also included Africa, South America, Australia and Antarctica.

Don't confuse with

• Himalayas: young (Tertiary) fold mountains made of folded sedimentary rock from the Tethys sea. They are still being folded, faulted and thrust. The Peninsular Block is Archaean, rigid and has not been folded since the Cambrian.
• Indo-Ganga-Brahmaputra plain: the youngest division (Quaternary alluvium). It is a foreland basin, a trough pushed down by the weight of the Himalaya and filled with loose river sediment. The Peninsular Block is hard crystalline rock.
• Block mountain (Satpura) vs relict fold mountain (Aravalli): the Satpura was raised by block faulting (a horst). The Aravalli is the worn-down stump of a very old fold mountain. Both lie inside the Peninsular Block.
• Shiwalik and Purvanchal: these are young Himalayan-system ranges and are not part of the block. Karbi Anglong and the Meghalaya plateau are part of it.

Prelims Hooks

• Northern edge, west to east: Kachchh → western flank of the Aravallis near Delhi → parallel to the Yamuna and Ganga → Rajmahal hills → Ganga delta.
• Detached parts: Karbi Anglong and the Meghalaya plateau (separated by the Malda gap, also called the Garo-Rajmahal gap), plus Rajasthan. Trap: the Shiwalik and the Purvanchal are not part of it.
• Rigid since the Cambrian. The only exception is the submerged west coast, which is why the western coastal plain is narrow.
• Process and landform pairs: block faulting → Narmada, Tapi and Mahanadi rift valleys (graben). Uplifted block → Satpura (horst).
• Relict hills: Aravalli, Nallamala, Velikonda, Palkonda, Javadi, Mahendragiri. Trap: these are worn-down old mountains, not young fold mountains.
• Oldest to youngest: Peninsular Block (Archaean; Dharwar craton about 3.5 billion years [2]) → Himalaya (Tertiary) → Indo-Gangetic alluvium (Quaternary).

Mains Points

• The block's structure decides where minerals are found (GS-I/GS-III):
• Old igneous and metamorphic rocks hold iron ore, manganese, mica and gold (Kolar).
• Coal formed in sunken rift basins (the Gondwana basins).

• This explains why industry grew in the Chota Nagpur–Odisha belt.

• Peninsular rivers vs Himalayan rivers:

• Peninsular rivers: old, rain-fed rivers in broad, gentle valleys. East-flowing rivers build deltas; the Narmada and Tapi form estuaries.
• Himalayan rivers: perennial rivers in gorges, carrying heavy sediment.

• This difference shapes river-linking plans, where dams are built, and the flood risk in the plain.

• Stability is a strength:

• The block is rigid and changes only through vertical movement and block faulting.
• The Himalayan belt has active thrusts and lies in seismic zones V and IV.
• Planners should look at this structural difference when choosing sites for large dams, industry and infrastructure.

Related concepts

• Geological divisions of India
• Himalayan orogeny

Read more

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