- Geomorphology studies the origin and evolution of landforms. It is the base of physical geography in NCERT Class 11 (Fundamentals of Physical Geography) and supplies a steady stream of UPSC and SSC questions.
- Landforms are shaped by two sets of forces: endogenic (from within the Earth — volcanoes, earthquakes, mountain building) and exogenic (on the surface — weathering and erosion by rivers, glaciers, wind and waves).
- Most of what we know about the Earth's interior comes indirectly, from how seismic waves travel through it.
Layers of the Earth
| Layer | Depth | Features |
|---|---|---|
| Crust | About 5 km under oceans; about 30 km under continents, up to 70 km under mountains | Continental crust (granitic, "SIAL") is thicker and lighter; oceanic crust (basaltic, "SIMA") is thinner and denser |
| Mantle | To about 2,900 km | Its upper part, the asthenosphere, is weak and partly molten — the main source of magma |
| Outer core | 2,900–5,150 km | Liquid iron and nickel ("NIFE"); its movement generates the Earth's magnetic field |
| Inner core | 5,150–6,371 km | Solid because of immense pressure |
Discontinuities: Conrad (upper and lower crust), Mohorovičić or Moho (crust and mantle), Repetti (upper and lower mantle), Gutenberg (mantle and core) and Lehmann (outer and inner core). The lithosphere is the rigid crust plus the uppermost mantle.
Seismic Waves
| Wave | Type | Travels through |
|---|---|---|
| P (primary) | Longitudinal (push–pull); fastest, arrives first | Solids, liquids and gases |
| S (secondary) | Transverse; slower | Solids only |
| Surface waves (Love, Rayleigh) | Move along the surface; slowest | Cause the most destruction |
S-waves are not recorded beyond about 103° from the epicentre, which shows that the outer core is liquid. P-waves have a shadow zone between about 103° and 142° because they are refracted by the core.
Earth's radius≈6,371 km
Crust–mantle boundaryMoho
Liquid layerOuter core
Continental driftWegener, 1912
Only active volcano in IndiaBarren Island
S-wave shadowBeyond ~103°
From Continental Drift to Plate Tectonics
- Alfred Wegener proposed continental drift in 1912: all continents were once joined in a supercontinent, Pangaea, surrounded by an ocean, Panthalassa. His evidence included the jigsaw fit of the continents, matching fossils (such as Mesosaurus) and matching rocks and glacial deposits (tillite).
- In the 1960s, Harry Hess proposed sea-floor spreading: new crust forms at mid-ocean ridges and is consumed at trenches. Ocean-floor rocks are youngest near the ridges.
- Plate tectonics combines these ideas: the lithosphere is broken into seven major and several minor plates moving over the asthenosphere.
| Boundary | Movement | Examples |
|---|---|---|
| Divergent (constructive) | Plates move apart | Mid-Atlantic Ridge; East African Rift Valley |
| Convergent (destructive) | Plates collide | Continent–continent: Himalayas; ocean–continent: Andes; ocean–ocean: Mariana Trench and island arcs |
| Transform (conservative) | Plates slide past each other | San Andreas Fault, California |
The Indian plate broke away from Gondwana and collided with the Eurasian plate roughly 40–50 million years ago, raising the Himalayas. It is still moving north by a few centimetres a year, which is why the Himalayan belt is earthquake-prone.
Rocks and the Rock Cycle
| Type | How formed | Examples |
|---|---|---|
| Igneous ("primary") | Cooling of magma or lava | Granite (intrusive, coarse); basalt (extrusive, fine — the Deccan Traps) |
| Sedimentary | Deposition and compaction of sediments, in layers | Sandstone, shale, limestone, coal; the only rocks that contain fossils. They cover about 75% of the surface but only about 5% of the crust by volume |
| Metamorphic | Heat and pressure change existing rocks | Limestone → marble; sandstone → quartzite; shale → slate; granite → gneiss |
Volcanoes and Earthquakes
- Shield volcanoes (Hawaii) have gentle slopes of fluid basaltic lava; composite or strato volcanoes (Mount Fuji) are steep and explosive; a caldera is a huge collapsed crater; flood basalts form plateaus such as the Deccan Traps.
- About two-thirds of the world's active volcanoes lie on the Pacific "Ring of Fire".
- India's only active volcano is Barren Island in the Andaman Sea; Narcondam is dormant.
- An earthquake starts at the focus (hypocentre); the point on the surface directly above it is the epicentre. Magnitude (energy) is measured on the Richter or moment-magnitude scale, which is logarithmic; intensity (damage) on the Mercalli scale.
Landforms by Agent
| Agent | Erosional landforms | Depositional landforms |
|---|---|---|
| Running water | V-shaped valley, gorge, canyon, waterfall, potholes | Alluvial fan, flood plain, levees, meanders and oxbow lakes, delta |
| Glaciers | U-shaped valley, cirque, arête, horn, hanging valley | Moraine, drumlin, esker, outwash plain |
| Wind | Mushroom rock, yardang, deflation hollow | Sand dunes (crescent-shaped barchans), loess |
| Groundwater (limestone/karst) | Sinkholes, caves | Stalactites (hang from the roof) and stalagmites (rise from the floor) |
| Sea waves | Cliffs, sea caves, arches, stacks | Beaches, spits, bars, lagoons (e.g., Chilika) |
- Discontinuity ↔ layers — Moho (crust–mantle) and Gutenberg (mantle–core) are the most asked.
- Seismic-wave reasoning: why the S-wave shadow proves a liquid outer core.
- Landform ↔ agent matching — a U-shaped valley is glacial, a barchan is wind-made, an oxbow lake is fluvial.
Q1. The boundary between the Earth's crust and mantle is called the:
✅ Answer: (b) Mohorovičić discontinuity
💡 Explanation: The Moho separates the crust from the mantle; Gutenberg separates the mantle from the core.
Q2. S-waves cannot pass through the Earth's outer core because it is:
✅ Answer: (a) Liquid
💡 Explanation: S-waves travel only through solids, so the S-wave shadow zone shows the outer core is liquid.
Q3. The Himalayas were formed at which type of plate boundary?
✅ Answer: (d) Continent–continent convergent
💡 Explanation: They rose when the Indian plate collided with the Eurasian plate.
Q4. Marble is the metamorphic form of:
✅ Answer: (c) Limestone
💡 Explanation: Limestone becomes marble; sandstone becomes quartzite; shale becomes slate.
Q5. A U-shaped valley is formed mainly by:
✅ Answer: (a) Glaciers
💡 Explanation: Glaciers carve broad U-shaped valleys; rivers carve V-shaped valleys.
🔁 Revise in 60 Seconds
- Crust → Moho → mantle (asthenosphere) → Gutenberg (2,900 km) → liquid outer core → Lehmann → solid inner core.
- P: all media, fastest; S: solids only (shadow beyond 103°); surface waves most destructive.
- Wegener 1912 (Pangaea); Hess sea-floor spreading; boundaries: divergent, convergent, transform.
- Rocks: igneous (granite, basalt), sedimentary (fossils), metamorphic (marble, quartzite, slate, gneiss). Barren Island — only active volcano.
📌 Prelims Traps to Avoid
- ✘ The inner core is solid; the outer core is liquid.
- ✘ Stalactites hang down; stalagmites grow up.
- ✘ Richter measures magnitude; Mercalli measures intensity.
How do we know the Earth's outer core is liquid?
S-waves travel only through solids. They disappear beyond about 103° from an earthquake's epicentre, creating a shadow zone, which shows they cannot cross the outer core — so the outer core must be liquid.
What are the three types of plate boundaries?
Divergent boundaries, where plates move apart (Mid-Atlantic Ridge); convergent boundaries, where plates collide (the Himalayas, the Andes, the Mariana Trench); and transform boundaries, where plates slide past each other (the San Andreas Fault).