Plateaus, Plains, Folds, Faults, Earthquakes, Volcanoes and Landform Types: OAS Notes Part 2

These geomorphology notes for OPSC OAS Part 2 cover the landform families that follow plate tectonics: plateaus and plains, folds and faults, earthquakes and volcanoes, rocks, weathering and mass movement, and then the erosional and depositional landforms carved by rivers, glaciers, waves, groundwater and wind. Each section is written around the exact questions OPSC, UPSC and other state PCS ask, with previous year practice questions at the end.

Plateaus: Formation, Types and World Examples

A plateau is a tabular upland with a relatively flat top and steep sides, standing above the surrounding land. The standard textbook threshold is a relief of more than 300 feet above the adjacent lowland.

NDA and OPSC-style questions test the 300-feet threshold, the Tibetan Plateau's elevation and the Pamir as the 'Roof of the World'.

How plateaus form

• Upwarping: vertical uplift of a broad crustal block, as with the Tibetan Plateau.
• Downwarping and denudation: a basin is filled and the surrounding upland is worn down, leaving a plateau of denudation.
• Lava accumulation: successive basaltic flows build a lava plateau, as with the Deccan Traps.
• Dissection: an uplifted tableland is cut by rivers into interfluves, mesas and buttes.

Plateau cities include Madrid on the Meseta Central, Bengaluru on the Deccan and Nairobi on the East African Plateau. Mesas and buttes are residual flat-topped remnants of a dissected plateau, with mesas larger than buttes.

Plains and Valleys

Plains are extensive, low-relief surfaces. They form by three main routes: the uplift or emergence of a submerged sea floor; the filling of depressions with sediment; and prolonged denudation of a plateau, producing a peneplain.

OPSC and UPSC frequently test the distinction between structural, depositional and erosional plains, and the identification of peneplain with river action.

Valleys

Valleys are primarily erosional features, though deposition later modifies them. The shape of a valley is a diagnostic clue to the agent that cut it.

Folds and Faults: Landforms of Crustal Deformation

Folding occurs under compression; faulting occurs under tension or compression when rocks fracture and move.

NDA 2015 asked for the upfold — anticline. NDA 2019 asked for the fold with both limbs horizontal — recumbent fold.

Death Valley in California is a rift valley. The Vosges and Black Forest are horsts flanking the Rhine graben. The Damodar valley in India flows through a rift trough.

Earthquakes: Focus, Waves and Measurement

An earthquake is the sudden release of accumulated tectonic strain along a fault. The point of origin inside the Earth is the focus or hypocentre; the point on the surface directly above it is the epicentre, where shaking is strongest.

The Richter scale (Charles F. Richter, 1935) measures magnitude on a base-10 logarithmic scale. The Mercalli scale measures observed intensity. Homoseismal lines connect places that felt the shock at the same time; isoseismal lines connect places of equal intensity.

UPSC 2018 asked about the Barren Island volcano. CDS 2014 tested focus versus epicentre and homoseismal lines. WBCS 2020 asked the scale used for earthquake intensity.

Seismic zoning of India

Major Indian earthquakes: Bhuj (Gujarat, 26 January 2001), Latur (Maharashtra, 30 September 1993), Uttarkashi (1991) and the Great Assam earthquake (1950).

Volcanoes: Types, Distribution and Indian Examples

Volcanoes occur at convergent margins (Andes, Japan, Cascades), divergent margins (Mid-Atlantic Ridge, Iceland), intra-plate hotspots (Hawaii, Reunion) and continental rift zones (East African Rift). The Ring of Fire is the Circum-Pacific belt of convergent margins, the world's most active seismic and volcanic zone.

Products of eruptions include lava, pyroclastic debris, ash and dust, and gases such as sulphur compounds and nitrogen compounds, as UPSC 2024 asked. Volcanic soils are fertile; the black cotton soil of India is derived from the weathering of fissure volcanic rock, a fact UPSC 2021 tested.

Barren Island in the Andaman Sea is India's only active volcano; it lies about 140 km northeast of Port Blair and has erupted repeatedly after 1991, including in 2017 and later. Narcondam is a dormant volcano in the Andamans. The Deccan Traps are a Cretaceous-Palaeogene flood basalt province covering about 500,000 km², a classic example of fissure eruption.

UPSC 2018 tested the Barren Island statements. UPSC 2001 asked where volcanic eruptions do not occur — the Caspian Sea. UPSC 2005 asked the location of Mount St. Helens — the United States.

Rocks: Igneous, Sedimentary and Metamorphic

Granite is an acid, plutonic (intrusive) rock, lighter in colour and less dense than basic rocks. Basalt is basic, extrusive and dark. Gneiss is metamorphic, not igneous; dolomite is sedimentary, not igneous — two frequently tested traps.

UPSC 2001 asked what quartzite is metamorphosed from — sandstone. NDA 2017 and Astt. Comm. 2018 tested non-igneous rocks (dolomite, gneiss). NDA 2012 tested granite as acid and plutonic, not extrusive.

Weathering and Mass Movement

Weathering is the decay and disintegration of rock in situ. Erosion involves removal and transport. Mass movement is the downslope transfer of material under gravity. Together they constitute denudation.

Exfoliation is physical, not chemical — a frequent trap. Chemical weathering is greatest in hot and humid climates. Frost action dominates in Arctic and periglacial regions.

CDS 2015 asked which is not chemical weathering — exfoliation. NDA 2018 asked where chemical weathering is greatest — hot and humid climate. NDA 2011 tested the definition of weathering.

Fluvial Landforms: Rivers at Work

Fluvial erosion depends on discharge, channel gradient and the nature of the load. The erosional landforms of rivers include V-shaped valleys, gorges, canyons, waterfalls, rapids, potholes, meanders, incised meanders and peneplains.

Incised meanders form when a meandering river is rejuvenated by uplift and cuts down vertically, producing steep-sided meander loops. They indicate rejuvenation, not the mature stage. A peneplain is the end product of prolonged fluvial erosion, an almost featureless surface with residual monadnocks.

UPSC 2022 asked which river created the Gandikota canyon — the Pennar. MPPSC 2017 asked what peneplain is related to — a river. UPPSC 2018 asked which river has a bird's-foot delta — the Mississippi.

Glacial Landforms

Glaciers erode by plucking (lifting blocks of rock) and abrasion (grinding the bed with embedded debris). They deposit material as till and outwash.

Glacial troughs often have remarkably flat floors because the ice grinds the bed uniformly. Glacial retreat may form moraine-dammed lakes, as UPSC has noted for the Himalaya. Siachen is the second-longest glacier outside the polar regions and the largest in the Himalaya-Karakoram region.

NDA 2019 asked the fold with horizontal limbs — recumbent. Bank questions test that tombolo and stack are not glacial, while roches moutonnées and eskers are.

Coastal Landforms: Waves at Work

When waves erode a headland from two opposite sides, an arch forms; when the arch roof collapses, a stack remains. A tombolo ties an island to the mainland. The erosional sequence is cave → arch → stack.

NDA 2018 asked which is not a coastal depositional feature — stack. Astt. Comm. 2019 matched loess, tombolo, point bars and moraines to their agents.

Karst Topography: Groundwater Landforms

Karst develops on soluble rocks, chiefly limestone, dolomite and gypsum, through carbonation and solution. Surface features include sinkholes, dolines, uvalas, poljes, lapies and karst windows; underground features include caves, caverns, stalactites, stalagmites, pillars and travertine.

Springs are common in well-jointed rocks, karst terrain and tilted strata where permeable beds meet the surface. Doline, uvala and lapies are erosional karst forms, a distinction OPSC and UPSC often test.

CDS 2016 and NDA 2019 asked where stalactites and stalagmites belong — karst topography, underground water. NDA 2015 tested the hanging versus rising distinction.

Aeolian Landforms: Wind at Work

Deflation is the lifting and removal of loose material. Abrasion wears rock by impact of transported grains; attrition wears the grains themselves. Yardangs form by wind abrasion of alternating hard and soft beds, not by fluvial erosion. Barchans are crescent-shaped dunes with horns pointing downwind; seifs are long ridges parallel to the wind.

OPSC 2022 matched barchan–aeolian, meander–fluvial, saddle–glacial, stack–coastal. Bank questions test deflation and zeugen as wind features.

Odisha Angle: Landforms and Hazards

Odisha's landform mosaic is shaped by the Eastern Ghats, the Mahanadi delta and the coastal lagoon of Chilika, formed by a sand bar separating it from the Bay of Bengal. The Mahanadi builds one of India's largest deltas, and the Bhitarkanika mangroves in Kendrapara district are a product of repeated falls in sea level leaving extensive marshland, a fact UPSC 2023 tested directly.

Odisha is cyclone-prone because of its concave coast and shallow continental shelf; the 1999 super cyclone is the benchmark hazard event. Seismically, the state lies mostly in Zone III, with parts of the coast in Zone II. The Sukinda chromite deposits of Jajpur are hosted in ultramafic rocks of the Eastern Ghats belt, and the state's laterite and black cotton soils are products of weathering of the Deccan and Eastern Ghats rocks.

Practice Questions