Geomorphology

Both the outer core (liquid) and inner core (solid) are composed of iron and nickel. The liquid outer core generates Earth's magnetic field through convection currents (dynamo effect). The continental crust is SiAl (silica + aluminium); oceanic crust is SiMa (silica + magnesium). The mantle is composed of silicate rocks rich in magnesium and iron. The Gutenberg Discontinuity separates the mantle from the outer core.

S-waves (Secondary/transverse/shear waves) can only travel through solids. They cannot pass through the liquid outer core — creating an S-wave shadow zone on the opposite side of Earth. This was the key evidence that proved the outer core is liquid. P-waves (Primary/compressional) travel through solids, liquids, and gases. L-waves travel only on the surface. P-waves arrive first at seismographs (that is why they are 'primary').

The Himalayas formed ~50 million years ago when the Indian Plate collided with the Eurasian Plate (convergent boundary), folding sedimentary rocks upward — classic fold mountains. They are young fold mountains and still rising at ~5 mm/year. Block mountains (e.g., Vindhya, Black Forest) form by faulting. Aravallis are old fold mountains. Volcanic mountains form by lava accumulation (e.g., Kilimanjaro, Fuji).

Granite = intrusive igneous rock (magma cooled slowly underground → coarse-grained). Marble = metamorphic (formed from limestone under heat and pressure — Taj Mahal is made of marble). Limestone = sedimentary (formed from marine shells/organisms). Coal = organic sedimentary (compressed plant material over millions of years). Key pairs: Limestone→Marble, Shale→Slate, Sandstone→Quartzite, Coal→Graphite→Diamond.

In block mountain formation, tensional forces create parallel faults. The central uplifted block = horst (e.g., Black Forest, Vosges Mountains). The down-dropped blocks = graben (forming rift valleys, e.g., East African Rift Valley, Rhine Graben). The Rhine Valley is a graben flanked by the Black Forest (horst) on the east and Vosges (horst) on the west. This is a directly tested CDS geomorphology term.

Lonar Lake is a saline, soda lake formed by a meteorite impact ~52,000 years ago — one of only four known hyper-velocity impact craters in basaltic rock worldwide. It is situated in the Deccan Trap basalt of Maharashtra. Frequently tested CDS trap because it looks like a volcanic crater. Crater Lake (Oregon, USA) = volcanic caldera. Lonar = impact crater (meteorite).

Stalactites hang from cave ceilings; stalagmites grow upward from the cave floor. Both are speleothems formed by deposition of calcium carbonate (CaCO₃) from dripping groundwater in limestone (karst) regions. Memory trick: stalacTITE has a 'T' for 'top'; stalagMITE has 'M' for 'middle/mound.' Karst topography (limestone dissolution features) also includes dolines, sinkholes, and limestone pavements.

The Ring of Fire is a horseshoe-shaped zone encircling the Pacific Ocean containing ~90% of the world's earthquakes and ~75% of volcanoes. It coincides with Pacific Plate boundaries. Major events: 2004 Sumatra earthquake, 2011 Japan earthquake, 1906 San Francisco earthquake. Countries on Ring of Fire: Japan, Philippines, Indonesia, Chile, Peru, USA (Pacific coast). Critical for military strategic awareness.

Mid-oceanic ridges form at divergent boundaries where plates move apart — magma rises to fill the gap, creating new oceanic crust (seafloor spreading). The Mid-Atlantic Ridge is the world's longest mountain chain (~65,000 km). Ocean trenches form at convergent/subduction boundaries. Transform faults are where plates slide horizontally. The Atlantic Ocean is widening as the Mid-Atlantic Ridge expands.

The Deccan Trap (covering ~500,000 km² in the Deccan Plateau) consists of massive basaltic lava flows erupted ~66 million years ago — coinciding with the K-Pg mass extinction (end of dinosaurs). The weathering of this basalt over millions of years produced India's black cotton soil (Regur). Maharashtra, Goa, parts of Karnataka and AP overlie Deccan Trap basalt. 'Trap' comes from Swedish 'trappa' = stairs (step-like formations).

Earth's layers: Crust (0–35 km) → Mantle (35–2,900 km) → Outer Core (2,900–5,100 km, liquid iron-nickel) → Inner Core (5,100–6,371 km, solid iron-nickel). Boundaries: Mohorovičić Discontinuity (Moho) = crust-mantle; Gutenberg Discontinuity = mantle-outer core; Lehmann Discontinuity = outer core-inner core. The outer core's convection generates Earth's magnetic field.

Fold mountains form at convergent plate boundaries where collision compresses rock layers like a crumpled tablecloth. Young fold mountains: Himalayas (India-Eurasian), Alps (African-Eurasian), Andes (Nazca-South American), Rockies. Old fold mountains: Aravallis (India), Appalachians (USA), Urals (Russia). Block mountains form by faulting. Volcanic mountains by eruption. Residual mountains by erosion.

P-waves (Primary/compressional waves) travel fastest (~6–8 km/s in crust) and arrive first at seismographs. S-waves (Secondary/shear waves) arrive second (~3.5 km/s). L-waves (Love and Rayleigh surface waves) arrive last but cause the most damage to buildings. The time gap between P and S wave arrivals is used to calculate the distance to the epicentre. P-waves travel through all materials (solid, liquid, gas).

The P-wave shadow zone is the region 103°–180° from the epicentre where no P-waves are detected (because they are refracted by the liquid outer core). The S-wave shadow zone is 103°–180° where no S-waves arrive (they cannot pass through the liquid outer core). The existence of these shadow zones is the primary evidence for Earth's internal structure and the liquid state of the outer core.

Granite = intrusive (plutonic) igneous rock — magma cooled SLOWLY deep underground, allowing large crystals to form (coarse-grained texture). Basalt = extrusive (volcanic) igneous rock — lava cooled RAPIDLY at the surface, forming fine-grained texture. Granite forms the core of continents (continental crust = granitic). Basalt forms the oceanic crust and the Deccan Traps. Both are igneous — the difference is whether cooling happened underground or at surface.

The East African Rift Valley is a graben — a down-dropped block of land between parallel faults created by tensional (divergent) forces. Rift valleys are the early stage of continental splitting — the East African Rift is slowly splitting Africa apart. Other examples: Rhine Graben (Europe), Dead Sea Rift. Great Rift Valley lakes: Tanganyika, Malawi, Turkana. The Rhine Valley in Europe = graben flanked by Black Forest and Vosges horsts.

Karst features: Sinkhole/doline = circular depression formed when cave roof collapses or limestone dissolves at surface. Stalactite = hangs DOWN from ceiling. Stalagmite = grows UP from floor. Natural bridges, limestone caves (e.g., Borra Caves, AP; Mawsmai Caves, Meghalaya). Moraine is glacial, not karst. Karst regions in India: Meghalaya, Andamans, parts of Rajasthan.

Ocean trenches form at convergent boundaries where oceanic crust subducts under another plate — the descending plate is destroyed (melts in the mantle). Mariana Trench (deepest, 11,034 m) formed where Pacific Plate subducts under the Mariana Plate. Java Trench (Indian Ocean's deepest) formed where Indo-Australian Plate subducts. Divergent boundaries create mid-ocean ridges. Transform boundaries create lateral faults (e.g., San Andreas Fault).

The Aravallis are among the world's oldest fold mountains (~3.5 billion years old) — so heavily eroded that they are now low, discontinuous ridges (highest peak: Guru Shikhar, 1,722 m, Rajasthan). They run for ~800 km from Gujarat to Delhi. They formed during the Precambrian era — far older than the Himalayas (50 million years old). The Aravallis act as a climatic divide between the Thar Desert and the Gangetic Plain.

Coral polyps host zooxanthellae algae in a symbiotic relationship — the algae photosynthesize and provide energy for the coral. Since photosynthesis requires sunlight, coral reefs only develop in shallow (0–25 m), clear, warm (23–29°C) water. Found between 30°N and 30°S. Cold current regions on western coasts of continents have no coral reefs. Rising ocean temperatures cause coral bleaching (algae expelled, coral turns white and dies).