Pariksha SaathiLesson 4 of 10 · Physical and World Geography
Landforms and Their Makers: Volcanoes, Earthquakes, Mountains, Rivers, Glaciers, Wind and Waves
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Building up and wearing down
The Earth's relief is shaped by two sets of forces. Endogenic forces, powered by heat inside the Earth, build relief through folding, faulting, volcanoes and earthquakes; slow movements that bend or break the crust are called diastrophism. Exogenic forces, driven by the Sun's energy and gravity, wear relief down. Weathering breaks rocks where they lie: physically, as in exfoliation (onion-like peeling of rock in hot deserts) and frost wedging, and chemically, as in oxidation and carbonation, by which rainwater carrying carbon dioxide dissolves limestone. Erosion by rivers, glaciers, wind, waves and groundwater then carries the material away, and this levelling of the land is called gradation.
Volcanoes
A volcano is an opening in the crust through which molten rock, gases and ash escape; molten rock is called magma underground and lava once it erupts. Volcanoes are active (erupting from time to time, like Etna and Stromboli in Italy, Stromboli being called the Lighthouse of the Mediterranean), dormant (quiet for a long time but able to erupt) or extinct. Shield volcanoes such as Mauna Loa in Hawaii are broad and gently sloping, built of fluid basalt lava; composite or strato volcanoes such as Mount Fuji are steep cones of alternating lava and ash with explosive eruptions; a caldera is a huge crater left when a volcano collapses; and flood basalts, like the Deccan Traps, pour out of long fissures. Vesuvius in Italy buried the Roman town of Pompeii in AD 79. Magma that cools underground forms batholiths (large granite masses), laccoliths (domes), sills (horizontal sheets) and dykes (vertical walls). Most of the world's active volcanoes lie around the Pacific in the Ring of Fire, along the subduction zones of the Andes, Japan, the Philippines, Indonesia and New Zealand. In India, Barren Island in the Andaman Sea is an active volcano and Narcondam a dormant one.
Earthquakes
An earthquake is the shaking of the ground when stress built up along a fault is suddenly released. The point where the rupture starts is the focus or hypocentre, and the point on the surface directly above it is the epicentre. P (primary) waves are the fastest; they push and pull the rock like sound waves and pass through solids, liquids and gases. S (secondary) waves shake the rock at right angles to their path and pass only through solids. Surface waves (Love and Rayleigh waves) arrive last but do the most damage. The Richter scale, devised by Charles Richter in 1935, measures magnitude on a logarithmic scale, where each whole number means 10 times the ground motion and about 32 times the energy; for large quakes seismologists now mostly use the moment magnitude scale. The Mercalli scale grades intensity, the damage observed, from I to XII. The strongest earthquake ever recorded struck Chile in 1960 (magnitude 9.5), and the Sumatra earthquake of 26 December 2004 raised a tsunami across the Indian Ocean.
Mountains
Fold mountains form where compression crumples rock layers into upfolds (anticlines) and downfolds (synclines). Young fold mountains, the Himalayas, Alps, Andes, Rockies and Atlas, are high with sharp peaks; old fold mountains such as the Aravallis, the Appalachians and the Urals are worn down. Block mountains form by faulting: a block raised between faults is a horst, and one that sinks is a graben or rift valley. The Black Forest (Germany) and the Vosges (France) are block mountains with the Rhine rift valley between them, and the Great Rift Valley of East Africa is the best-known rift; in India the Narmada and Tapi flow west through rift valleys. Volcanic mountains such as Kilimanjaro and Fuji are built by eruptions.
Rivers and groundwater
In its upper course a river cuts V-shaped valleys, gorges, canyons, rapids and waterfalls; the Grand Canyon of the Colorado River (USA) is the classic canyon. In the middle course it swings in meanders across a flood plain, and at the foot of mountains it drops alluvial fans. In the lower course meanders are cut off as ox-bow lakes, natural levees line the banks, and the river ends either in a delta, a fan of sediment at the mouth, or in an estuary, where tides sweep the sediment away. The Ganga-Brahmaputra delta is the largest in the world, while the Narmada and Tapi form estuaries. In limestone (karst) areas, named after a region of Slovenia, groundwater dissolves rock into sinkholes and caves, where stalactites hang from the roof and stalagmites rise from the floor.
Glaciers, wind and waves
Glaciers carve bowl-shaped cirques, knife-edge ridges called aretes, pyramid peaks (horns) such as the Matterhorn, U-shaped valleys and hanging valleys; drowned glacial valleys on the coast are fjords, typical of Norway. They deposit unsorted debris (till) as moraines and drumlins, while their meltwater streams leave winding ridges of sand and gravel called eskers. In deserts the wind scoops out deflation hollows, carves mushroom rocks, yardangs and zeugen, and piles sand into crescent-shaped barchans and long seif dunes; fine wind-blown dust settles as loess, as on the Loess Plateau of China. Sea waves cut cliffs, wave-cut platforms, caves, arches and stacks, and deposit beaches, spits and bars; a bar can enclose a lagoon, as at Chilika Lake in Odisha.
Key facts
- Endogenic forces (internal heat) build relief by folding, faulting, volcanoes and earthquakes; exogenic forces (Sun and gravity) wear it down by weathering and erosion (gradation).
- Shield volcano: gentle slopes of fluid basalt (Mauna Loa, Hawaii). Composite volcano: steep explosive cone of lava and ash layers (Fuji). Caldera: collapsed crater.
- Stromboli (Italy) is the Lighthouse of the Mediterranean; Vesuvius buried Pompeii in AD 79; Barren Island is India's active volcano and Narcondam a dormant one.
- Intrusive forms: batholith (huge granite mass), laccolith (dome), sill (horizontal sheet), dyke (vertical wall).
- P waves: fastest, push-pull, through solids, liquids and gases. S waves: sideways shaking, solids only. Surface (Love and Rayleigh) waves: slowest and most destructive.
- Richter scale (1935): magnitude, each step 10 times the ground motion and about 32 times the energy. Mercalli: intensity, I to XII. Strongest recorded: Chile, 1960, magnitude 9.5.
- Horst = raised block, graben = rift valley. Black Forest and Vosges are block mountains with the Rhine rift valley between them; the Narmada and Tapi flow through rift valleys.
- River landforms: V-shaped valley, gorge and waterfall (upper course); meander and flood plain (middle); ox-bow lake, levee and delta (lower). The Ganga-Brahmaputra delta is the world's largest.
- Glacial: cirque, arete, horn (Matterhorn), U-shaped valley, fjord (Norway), moraine. Wind: barchan, seif, yardang, zeugen, loess. Waves: cliff, arch, stack, spit, lagoon (Chilika).
Quick revision
Endogenic forces build, exogenic forces wear down; exfoliation in deserts, carbonation dissolves limestone; magma underground, lava at the surface; shield (Mauna Loa), composite (Fuji), caldera; Ring of Fire around the Pacific; Barren Island active, Narcondam dormant; focus below, epicentre above; P fastest, S solids only, surface waves most destructive; Richter 1935 magnitude, Mercalli intensity; Chile 1960 9.5; horst and graben; V-valley, meander, ox-bow lake, delta; cirque, U-valley, fjord, moraine; barchan, loess; stack, spit, lagoon.