Physical Geography Essentials

Physical geography questions reward precision over general familiarity. A handful of clusters come up again and again: what lies beneath the crust, the evidence behind continental drift and the plate tectonic theory that eventually explained it, how mountains are classified, how ocean currents and the atmosphere behave, and rock properties that sound backwards until you understand the mechanism.

The Earth's interior: crust, mantle and core

Everything else in this chapter, why continents drift, why mountains rise where they do, why earthquakes cluster along certain lines, follows from what actually lies beneath the surface. No one has drilled anywhere close to the centre (the deepest borehole ever sunk, at Kola in Russia, reached only about 12 km, a scratch on a planet with a radius of roughly 6,378 km), so almost everything known about the interior is inferred indirectly, chiefly from how seismic waves generated by earthquakes speed up, slow down, bend and disappear as they pass through the different layers below.

The interior is divided into three broad layers: the crust, the mantle and the core.

The crust is the outermost, brittle, solid shell, and it is strikingly thin compared with the planet as a whole. Oceanic crust averages only about 5 km thick and is basalt-rich, denser material sometimes called SIMA (silica and magnesium); continental crust averages around 30 km but thickens under major mountain systems, reaching as much as 70 km beneath the Himalayas, and is granite-rich, lighter material called SIAL (silica and aluminium). The line where the crust ends and the mantle begins is the Mohorovicic discontinuity, usually shortened to the Moho, marked by a sudden jump in seismic wave velocity as the waves cross from crustal rock into denser mantle rock.

The mantle runs from the Moho down to a depth of about 2,900 km, making it by far the thickest of the three layers. Its uppermost part, extending to roughly 400 km, is the asthenosphere, a comparatively weak, partially molten zone that supplies most of the magma reaching the surface in volcanic eruptions. The crust together with this uppermost, still-solid part of the mantle forms the rigid lithosphere, the layer that actually moves during plate tectonics: typically about 5 km to 100 km thick under the oceans, thickening to around 200 km under the continents. Below the asthenosphere, the lower mantle is denser and fully solid.

The core begins at that 2,900 km mantle-core boundary and continues to the Earth's centre at about 6,378 km. It is made overwhelmingly of iron and nickel, occasionally called the nife layer for that reason, and comes in two parts: a liquid outer core, running to about 5,100 km, and a solid inner core below that. The inner core stays solid despite its extreme temperature because the pressure at that depth is high enough to keep the iron-nickel alloy from melting.

Wegener's Continental Drift theory: the evidence, and why it was rejected

In 1912, the German meteorologist Alfred Wegener proposed that all of today's continents were once joined into a single supercontinent, which he named Pangaea ("all earth"), surrounded by a single vast ocean he called Panthalassa. Wegener argued that Pangaea began breaking apart around 200 million years ago, splitting first into a northern landmass, Laurasia, and a southern one, Gondwanaland, which then split further into the continents recognisable today.

Wegener backed the theory with evidence you can list, not just assert:

• Matching coastlines and rock formations: the Brazilian and West African coastlines fit together like puzzle pieces, and the rock strata along them match.
• Matching gold-bearing strata: similar mineral-bearing rock layers appear in Ghana and Brazil, continents now separated by an ocean.
• Gondwana-age glacial sediments: matching glacial deposits and scratch marks (striations) appear on continents now widely separated (South America, Africa, India, Australia, Antarctica), evidence they were once joined and glaciated together, then drifted apart.
• Matching fossils: identical species turn up on continents now separated by oceans, too far apart to have swum or flown across. The small freshwater reptile Mesosaurus is known only from South Africa and Brazil, localities now roughly 4,800 km apart with an ocean between them; the seed fern Glossopteris turns up across India, Africa, South America, Australia and Antarctica, the very continents that made up Gondwanaland.

All four point to the same conclusion: today's separated continents were once a single landmass.

Despite this evidence, most geologists rejected the theory for decades. The gap was never the evidence itself but the mechanism: Wegener could only suggest that continents ploughed through solid ocean floor, driven by a pole-fleeing force (from the bulge the Earth's rotation creates at the equator) and a tidal force (from the pull of the sun and moon), and physicists soon showed both forces were far too weak to shift landmasses of that size. Without a credible explanation for how it could happen, continental drift sat on the margins of geology until the 1960s.

From continental drift to plate tectonics: the missing mechanism arrives

The mechanism Wegener could not supply came from three separate advances, spread across nearly forty years, only pulled together after his death.

In the 1930s, the geologist Arthur Holmes proposed that heat from radioactive decay deep inside the Earth sets up slow convection currents within the mantle, currents strong enough to drag the crust sitting above them. This gave continental drift a plausible engine for the first time, though it remained an unproven hypothesis.

Proof came from the ocean floor. Post-war mapping showed the seabed was no featureless plain: it carried its own mountain systems, the mid-ocean ridges, and its own deep trenches, and rock samples showed the ocean floor was consistently far younger than continental rock, nowhere older than about 200 million years, against continental formations dated to as much as 3,200 million years in places. In 1961, the geologist Harry Hess used these findings to propose seafloor spreading: molten rock rises at the crest of a mid-ocean ridge, solidifies into new oceanic crust, and pushes the older crust away on either side, crust that eventually sinks back into the mantle at a deep ocean trench.

In 1967, three geophysicists working independently, McKenzie, Parker and Morgan, combined seafloor spreading with the idea of a rigid, plate-divided lithosphere into a single framework: Plate Tectonic theory. Where Wegener could only describe continents drifting, plate tectonics supplied the engine driving them (mantle convection) and the process doing the actual work at the surface (seafloor spreading), finally giving the original theory the credible mechanism it had always lacked.

Types of plate boundaries and what each one produces

The lithosphere is broken into a set of rigid plates moving relative to one another, and what happens at a plate's edge depends entirely on which of three ways the neighbouring plates are moving.

Convergent boundaries are not all alike, and UPSC tests the distinction between them:

• Oceanic plate meets continental plate: the denser oceanic plate is forced down, or subducted, beneath the lighter continental plate, producing a deep trench offshore and a chain of fold mountains on the continental side. The Nazca Plate subducting beneath the South American Plate builds the Andes this way.
• Continental plate meets continental plate: neither plate is dense enough to subduct, so the crust between them buckles and is forced upward instead. The ongoing collision of the Indian Plate into the Eurasian Plate is what continues to raise the Himalayas.
• Oceanic plate meets oceanic plate: the older, denser plate subducts beneath the younger one, producing a deep ocean trench and, above the subduction zone, a curved chain of volcanic islands called an island arc. Japan and the Mariana Islands are both built this way.

The world's major tectonic plates

The lithosphere is divided into seven major plates: Pacific, North American, South American, Eurasian, African, Antarctic, and the Indo-Australian plate (India, Australia and the surrounding ocean floor, sometimes treated as two separate plates, Indian and Australian, since they have begun drifting at slightly different rates). The Pacific Plate is almost entirely oceanic and is ringed by the seismically and volcanically active belt known as the Ring of Fire; the Eurasian Plate, by contrast, is largely continental. A handful of smaller, named plates sit between the major ones, including the Nazca, Cocos, Caribbean, Arabian, Philippine, Juan de Fuca and Scotia plates, several of which turn up by name in questions about a specific subduction zone or fault.

Fold, block, residual and volcanic mountains: a classic mix-up

UPSC regularly asks which mountain belongs to which type, and each type forms by a genuinely different mechanism:

The Vosges/Black Forest pair is a favourite distractor precisely because their location (Western Europe, near other fold ranges) tempts students to call them fold mountains. They are not: they are classic block mountains, formed by rift faulting during the same tectonic episode that folded the Alps, not by folding itself.

Volcanic mountains form differently again, building entirely new material up from below rather than folding or faulting existing rock. Shield volcanoes, built from thin, fast-flowing basaltic lava, have gentle slopes and low explosivity; the Hawaiian volcanoes are the classic example. Composite volcanoes (also called stratovolcanoes) erupt cooler, thicker lava together with ash and other pyroclastic debris, which piles up in alternating layers to build the tall, steep-sided cone seen in Mount Fuji and Mount Kilimanjaro. A third form, the flood basalt province, releases highly fluid lava that spreads across enormous areas rather than building a single cone; India's own Deccan Traps, covering most of the Maharashtra plateau today, formed this way and are regularly tested alongside the plateau's volcanic origins.

Ocean currents: warm and cold, and what they do to climate

Ocean currents move heat around the planet, and a coastline's climate often depends on which type of current runs past it:

• Warm currents (moving from the equator toward the poles) raise coastal temperatures and add moisture to onshore winds. The Gulf Stream / North Atlantic Drift is why north-west Europe (including Britain and Norway) is far milder than its latitude would otherwise suggest.
• Cold currents (moving from the poles toward the equator) cool coastal air and suppress rainfall. The Peru (Humboldt) Current off South America is a major reason the Atacama Desert is one of the driest places on Earth, despite sitting right on the coast.

The general rule UPSC tests: a warm current on the western side of a continent, or a cold current on the western side, changes rainfall patterns enough to create deserts or unusually mild, wet regions that would not be expected from latitude alone.

Isotherms: why they bend the way they do

An isotherm joins places of equal temperature. In January (Northern Hemisphere winter), isotherms bend equatorward as they cross large landmasses like Asia and North America. The mechanism: land has much lower specific heat capacity than water, so it cools faster than the ocean in winter, making the interior of a continent colder than the sea at the same latitude, which pulls the isotherm line southward as it crosses land.

Atmospheric dust: more land than ocean, more mid-latitude than either extreme

Dust particles are overwhelmingly land-sourced, from dry, sparsely vegetated regions and human/industrial activity, so they are far less abundant over open ocean than over continents. Within land areas, dust is most concentrated over the sub-tropics and temperate latitudes (deserts, industrial belts), not the moist equatorial zone or the ice-covered poles.

Chalk vs clay: the counter-intuitive pairing

This is a favourite "which is which" trap:

Because clay is porous (holds water) but impermeable (won't let it pass through), clay layers commonly act as an aquiclude, a barrier that traps groundwater in the aquifer above or below it. This is why clay-lined reservoirs hold water while sandy or chalky ground lets it drain away.

The greenhouse effect, in three facts

1. The atmosphere absorbs and re-radiates outgoing longwave (infrared) radiation from the Earth's surface, keeping it warmer than it would otherwise be.
2. Water vapour and carbon dioxide are among the gases most responsible for this effect.
3. Without an atmosphere, Earth's average surface temperature would be dramatically lower, closer to the Moon's extremes.

Quick revision points

• The Earth's interior has three layers: crust (oceanic floor about 5 km thick, continental crust about 30 km, up to 70 km under the Himalayas), mantle (down to 2,900 km, its upper asthenosphere is the main magma source), and core (liquid outer core, solid inner core, overwhelmingly iron and nickel). The Moho marks the crust-mantle boundary.
• Wegener proposed Pangaea in 1912; the theory was rejected for decades for lacking a credible mechanism, not for lacking evidence. Continental drift evidence: matching coastlines/rock strata, matching gold-bearing strata (Ghana, Brazil), shared Gondwana-age glacial sediments, and matching fossils (Mesosaurus, Glossopteris).
• Plate Tectonic theory (1967) supplied the missing mechanism: Holmes's mantle convection currents (1930s) plus Hess's seafloor spreading (1961).
• Three boundary types: divergent (mid-ocean ridges, rift valleys), convergent (subduction and fold mountains, e.g. India colliding into Eurasia to raise the Himalayas), transform (fault zones, e.g. the San Andreas Fault).
• Seven major plates: Pacific, North American, South American, Eurasian, African, Antarctic, Indo-Australian.
• Fold mountains form by compression (Himalayas); block mountains by faulting (Vosges, Black Forest); residual mountains by erosion (Aravallis); volcanic mountains by eruption (Kilimanjaro, Fuji, the Hawaiian shield volcanoes; the Deccan Traps are a flood basalt province).
• Warm currents warm and moisten coasts (Gulf Stream, north-west Europe); cold currents cool and dry them (Peru Current, Atacama Desert).
• January isotherms bend equatorward over land because land cools faster than the ocean.
• Clay is porous but impermeable, making it an aquiclude; chalk is both porous and permeable.

These facts show up as statement-based and matching questions far more than essay-style ones, so practise them in that exact format.