Geomorphology, at a glance
How Earth’s structure and internal-external forces build and erode every landform
Three main layers
Crust, mantle, and core.
Seven major plates
Constantly drifting on the mantle.
105° shadow zone
Proves the outer core is liquid.
India crossed the equator
On its 200-million-year drift north.
What Are Earth’s Layers, and How Do We Even Know?
What are Earth’s layers, and what shapes the land above them?
- Earth is made of three main layers: the crust, mantle, and core. The thin crust is where every landform takes shape. The mantle beneath it drives plate tectonics. The core is Earth’s innermost layer.
- The crust runs about 5 km deep under the oceans, but 30 km deep under continents, and up to 70 km under the Himalayas.
- The mantle runs from the crust down to about 2,900 km. Its upper part, the asthenosphere, reaches about 400 km down and is the main source of magma.
- The crust plus the solid uppermost mantle together make the lithosphere. It is 10-200 km thick, and this is where all natural earthquakes happen.
- The core starts at 2,900 km depth. Its outer part is liquid, but its inner part is solid, and both are made mostly of nickel and iron.
How do scientists actually know what’s down there?
- Mining, deep drilling, and volcanic material thrown up during eruptions give scientists direct rock samples from below the surface.
- Gravity anomalies, magnetic surveys, meteorite samples, and seismic waves all reveal what’s inside indirectly, without ever touching it.
- P-waves arrive first and pass through solids, liquids, and gases. S-waves arrive later and pass only through solids.
- S-waves never reach seismographs more than 105° from the earthquake’s epicentre. This gap, the shadow zone, proves the outer core is liquid, since S-waves cannot pass through liquid.
Q1. True or False: the crust plus the solid uppermost mantle together make up the lithosphere, where all natural earthquakes happen.
Q2. True or False: S-waves can pass through the Earth’s liquid outer core, just like P-waves do.
How Did Wegener’s Wild Idea Become Plate Tectonics?
What did Wegener actually propose, and why did it take decades to accept?
- Alfred Wegener’s 1912 continental drift theory proposed that continents once formed a single landmass and slowly drifted apart. He named it Pangaea, meaning “all earth,” surrounded by one mega-ocean, Panthalassa.
- Pangaea first split into a northern Laurasia and a southern Gondwanaland. Evidence includes matching ancient rock belts on Brazil’s coast and Western Africa, and Gondwana sediments from India matching counterparts across six Southern Hemisphere landmasses.
- Wegener suggested two driving forces: a pole-fleeing force from Earth’s equatorial bulge, and a tidal force from the Sun and Moon’s pull. Most scientists at the time found both far too weak to actually work.
- Harry Hess proposed in 1961 that new crust erupts at mid-ocean ridges and pushes older crust outward. This sea floor spreading finally explained the missing mechanism, and is why oceanic crust is never older than 200 million years.
How did the Indian plate actually get here?
- The lithosphere is broken into seven major plates (Pacific, North American, South American, Eurasian, African, Indo-Australian, Antarctic), plus several minor ones like Nazca, Cocos, and Arabian.
- India began as an island off Australia’s coast, separated from Asia by the Tethys Sea, and started moving north about 200 million years ago when Pangaea broke apart.
- Around 60 million years ago, while India was still near the equator and still travelling, huge lava flows built the Deccan Traps.
- India collided with Asia about 40-50 million years ago. This continent-continent collision pushed the Himalayas upward, and they are still rising today.
Q3. True or False: Alfred Wegener put forth his continental drift theory in 1912, naming the single supercontinent Pangaea.
Think it through, before reading on. Chalk and clay are both porous rocks, yet water flows through chalk easily and barely moves through clay. What actually explains the difference?
Show the explanation
Porosity and permeability are different properties. Porosity is how much open space a rock holds; permeability is whether those spaces connect enough to let water flow through. Chalk’s pores connect well, so it is both porous and permeable. Clay’s pores are tiny and poorly connected, so it holds water but stays nearly impermeable.
Test Yourself
- UPSC CSP 2025Asked which statements are genuine evidence of continental drift. Matching rock belts between Brazil and West Africa, and Ghana’s gold deposits tracing to the Brazil plateau, are both correct evidence.
- UPSC CSP 2025Asked whether chalk is permeable and clay is impermeable. Both statements are correct: it comes down to pore connectivity, not just pore volume.
- UPSC CSP 2024Asked whether rainfall itself is a genuine weathering agent. It is: rainwater dissolves carbon dioxide and oxygen, driving chemical weathering through carbonation and oxidation.
- CAPF (ACs) 2025Asked which mass movements count as rapid. Mudflow and slump are rapid; creep and solifluction are slow, a genuine and recurring exam trap.
Remember the trap. Creep and solifluction are slow mass movements. Mudflow and slump are rapid ones. Exams test this exact split often.
Why Do Rocks Weather Differently, and How Does Soil Form?
What actually shapes a volcano’s form, and how do rocks break down?
- Shield volcanoes are built from very fluid basalt lava, which spreads flat instead of piling steep; Hawaii’s volcanoes are the classic example. Composite volcanoes erupt cooler, thicker lava with ash, building the classic steep cone.
- Flood basalt volcanoes pour out huge, thin sheets of runny lava over vast areas, and India’s own Deccan Traps are a giant example. Magma that cools underground, instead of erupting, forms batholiths, laccoliths, sills, and dykes.
- Chemical weathering decomposes rock through reactions like oxidation and carbonation. Physical weathering breaks rock apart without changing its chemistry. Biological weathering comes from plant roots, burrowing animals, and decaying matter.
- Exfoliation is a result, not a process. Repeated expansion and contraction flakes curved rock sheets off a surface, leaving it smooth and rounded.
How does soil actually form, and what happens underground?
- Soil formation depends on five factors together: parent material, topography, climate, biological activity, and time.
- Heavy rain washes soil components downward, called eluviation, and deposits them lower down, called illuviation. Removing silica this way is called desilication.
- In dry climates, evaporation pulls groundwater up, leaving salts that harden into a crust called hardpan. In moderate-rainfall tropical areas, calcium carbonate instead forms nodules called kanker.
- Bacteria like Rhizobium, living in the root nodules of legumes, convert nitrogen gas from the air into a form plants can actually use.
Q4. True or False: composite volcanoes erupt explosively and eject pyroclastic material, unlike the fluid lava of shield volcanoes.
Q5. True or False: mudflow and slump are classified as slow mass movements, while creep and solifluction are rapid.
Q6. True or False: Rhizobium bacteria, living in legume root nodules, convert nitrogen gas into a form plants can use.
Quick Q&A
What proves Earth’s outer core is liquid?
The seismic shadow zone: S-waves never reach stations more than 105° from an earthquake’s epicentre.
What replaced Wegener’s weak driving forces?
Harry Hess’s 1961 sea floor spreading theory, based on new crust erupting at mid-ocean ridges.
Is exfoliation a weathering process?
No, it is a result of unloading or thermal expansion, not a process itself.
Which mass movements are rapid?
Mudflow and slump; creep and solifluction are slow.
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