Major Landforms of the Earth

Step-by-step formation of fold mountains:

1. Two continental tectonic plates approach each other (convergent plate boundary)
2. Sedimentary rocks accumulated at the bottom of ancient seas between the plates (geosynclines) are subjected to immense lateral compressional forces
3. These rocks buckle, fold, and thrust upward over millions of years, building mountain ranges
4. Rock strata show visible folding — synclines (downward folds) and anticlines (upward folds)

Case Study — The Himalayas:

• ~80 million years ago (mya), India was ~6,400 km south of Asia, moving north at ~9 m per century (~9 cm/year)
• The Tethys Sea (an ancient ocean) lay between the Indian and Eurasian plates
• ~50 mya: The Indian plate collided with the Eurasian plate (Eocene); Tethys Sea sediments were compressed and folded upward → the Himalayas began rising; the Tethys Sea closed ~50 mya as the two plates met
• Himalayan building continued after the initial collision; northward speed dropped by ~half (~4–5 cm/year today)
• The Indian plate is still moving north at ~4–5 cm per year → the Himalayas are still rising at 5–6 mm per year (GPS-measured; some sections up to 10 mm/year)
• Marine fossils (sea shells, ammonites) are found in Himalayan rocks — direct evidence of the Tethys Sea floor origin

Divisions of fold mountains — old vs young:

Other young fold mountain ranges worldwide:

• Alps (Europe): African plate + Eurasian plate collision; ~30–40 mya
• Andes (South America): Nazca oceanic plate subducting under South American plate; highly active volcanically
• Rockies (North America): Pacific plate subduction; forms the continental divide

Formation mechanism:

• When tensional (pulling apart) forces act on the Earth's crust, parallel faults develop
• The land between two faults may be uplifted → forms a horst (block mountain)
• The land between two faults may subside/sink → forms a graben (rift valley)
• The horst has a relatively flat top and steep escarpment (scarp) faces

India's classic horst–graben system:

• Satpura Range = horst (block mountain); forms an east-west wall
• Narmada Valley (north of Satpura) = graben (rift valley); Narmada flows west through it
• Tapi Valley (south of Satpura) = graben (rift valley); Tapi flows west through it
• Both Narmada and Tapi are west-flowing rivers because they follow the graben topography — they do NOT form deltas (unlike most peninsular rivers); they form estuaries
• Seismic refraction data confirms the horst nature of Satpura and the graben character of the Narmada and Tapi valleys

Western Ghats: The escarpment of the Western Ghats (steep west-facing cliff) has a block mountain character — formed partly by faulting when the Indian subcontinent separated from the African continent (~130–150 mya, Gondwana breakup). The abrupt western face and gentle eastern slope (towards the Deccan Plateau) is characteristic of block-faulted topography.

Global examples:

• Rhine Valley (Germany/France) = graben; Vosges Mountains and Black Forest = horsts on either side
• East African Rift System = series of grabens; Lake Tanganyika, Lake Malawi, Lake Turkana lie in these grabens
• Jordan Valley / Dead Sea = classic graben; Dead Sea is ~430–440 m below sea level (dropping ~1 m/year due to water extraction; ~440 m as of 2025) — the lowest land surface on Earth

Formation: Magma rises from the mantle through vents; successive eruptions accumulate lava and ash layers building conical mountains ("mountains of accumulation").

Types by activity:

India: Barren Island (Andaman & Nicobar) is India's only confirmed active volcano; it has repeated eruptive phases — a major eruption occurred in 2022–2023 (ash plumes up to 15,000 ft) with continued thermal and eruptive activity documented through 2024–2025 (Smithsonian Global Volcanism Program).

Deccan Traps ≠ volcanic mountain: The Deccan Plateau is a lava plateau (flood basalt), not a volcanic mountain. It formed from massive, relatively low-viscosity lava flows over ~600,000–800,000 years, ~66 mya. The link to the Chicxulub asteroid impact remains scientifically debated — one hypothesis is that the impact seismic shockwaves re-energized the Deccan eruptions.

Formation: These are remnants of ancient mountains (originally fold or other types) that have been worn down by millions of years of erosion by wind, water, glaciers, and weathering. Only the hardest, most resistant rocks survive.

Characteristics: Low elevation; rounded summits; ancient crystalline (Precambrian) rock; geologically stable; no seismic risk.

India Examples:

Aravalli — Key UPSC Facts:

• India's oldest mountain range; one of the world's oldest fold mountain systems (by age of rocks: ~3.3–2.5 billion years; by mountain-building event: ~1.8 bya — Precambrian Aravalli Orogeny)
• Longest in India: ~692 km (Gujarat to Delhi)
• Ecological function: Acts as a natural barrier preventing the Thar Desert from expanding eastward; recharge zone for Delhi's groundwater
• Average elevation today: ~300–900 m (originally comparable to the Alps — heavily eroded)