Rocks, Landforms and Geomorphic Processes

Walk on any hillside and you are walking on rocks; dig a little and you find soil that was once rock. The whole solid crust of the Earth is built of rocks, and rocks are built of minerals. A mineral is a naturally occurring, inorganic substance with a definite chemical composition and an orderly atomic structure (quartz, calcite, mica). A rock is an aggregate of one or more minerals (granite is a mixture of quartz, feldspar and mica). The study of rocks is called petrology. On the basis of how they are formed, rocks fall into three families: igneous, sedimentary and metamorphic.

The word metamorphic means 'change of form'. Remember it this way: igneous = born from fire (melt), sedimentary = built from pieces, metamorphic = baked and squeezed. Only sedimentary rocks carry fossils; igneous rocks never do, and in metamorphic rocks fossils are mostly destroyed. Fossil fuels (coal, petroleum) are found in sedimentary rocks.

Sedimentary rocks are named after how they form. Mechanically formed rocks are made of broken fragments: sandstone (sand grains), conglomerate (rounded pebbles cemented together), shale (compressed clay and mud), loess (wind-blown silt). Chemically formed rocks come out of water: rock salt, gypsum, some limestone. Organically formed rocks come from living things: coal (plants), chalk and much limestone (shells and corals).

Rocks are not permanent. The rock cycle is the endless recycling of rock material. It is not a fixed one-way loop: igneous rock can be metamorphosed directly, a sedimentary rock can be melted, a metamorphic rock can be weathered and re-deposited. The word to remember is that every rock is a temporary stage, and the Earth's heat (from inside) and the Sun's energy (from outside) keep the cycle running.

Oxygen and silicon together make up nearly three-fourths of the crust, so the commonest minerals are silicates. Among them, feldspar is the most abundant mineral group of the crust (roughly half of it), followed by quartz (silica). Other important rock-forming minerals are mica, pyroxene, amphibole and olivine. Minerals are also grouped as metallic (iron ore, bauxite, copper) and non-metallic (mica, limestone, gypsum); how they are mined and used belongs to the economic geography chapters.

The surface of the Earth is under constant attack. Forces from inside (endogenic: folding, faulting, volcanoes) build the relief up, while forces from outside (exogenic: driven by the Sun's energy, gravity and the atmosphere) wear it down. The wearing down is called denudation, which means 'stripping off'. Denudation has four parts: weathering (breaking the rock where it stands), mass movement (the material slides down under gravity), erosion (an agent picks up and scrapes) and transportation (the agent carries the load away). Deposition then builds new surfaces. The agents of erosion are running water, glaciers, wind, waves and groundwater.

Weathering is not erosion. In weathering the rock only breaks or decays where it is, and no agent moves it. Erosion needs a moving agent (water, ice, wind, waves) to pick the material up. If rocks never weathered, there would be no loose material to carry, and erosion would be almost nil. That is why weathering is called the first step of denudation.

Weathering is quietly important. It makes the regolith from which soil forms, and so supports agriculture. It concentrates valuable ores: bauxite (aluminium ore) and iron ore enrich as the surrounding material is leached out. It also weakens rock on slopes, preparing it for mass movement. Mass wasting means the downslope movement of rock and soil under the direct pull of gravity, with no other agent (water may only lubricate it). The word to remember is 'gravity is the only driver'.

Erosion is the wearing away of the land surface by the moving agents, and the removal of the loosened material. The agents scrape the surface using the rock fragments they carry (this is called abrasion), dissolve rock (solution) or simply lift loose grains. The same agent then transports its load: by rolling and sliding along the bed, by bouncing (saltation), or by carrying it in suspension and solution. When the agent loses energy, it drops the load: this is deposition. So every agent has an erosional landform and a depositional landform, which we shall study agent by agent in the next topics. Erosion that makes the land lower and flatter is degradation; deposition that builds up the surface is aggradation.

Running water is the most powerful agent that shapes the land. Rain falls, gathers in streams and rivers, and the river does three jobs in order: it erodes (cuts the bed and banks), transports (carries sand, silt and pebbles) and deposits (drops the load where its speed falls). A river is like a person's life. In its youth (upper course, mountains) it is fast, steep and cuts downward. In maturity (middle course) it begins to swing sideways. In old age (lower course, plains) it is slow, wide and mostly deposits. So the landforms of a river depend on which stage we are looking at.

A gorge forms in hard rock because hard rock resists sideways collapse. The river cuts down quickly, but the hard walls do not crumble back, so they stay almost vertical. In soft sediment, the walls would slump and the valley would become wide and gently sloping, not a gorge. The answer to 'where do gorges develop' is therefore hard rock.

Now the second half of the river's work: deposition. When a river leaves the mountains and the slope eases, it drops its load, starting with the heaviest pebbles. At the foot of the hills it builds an alluvial fan (a fan-shaped heap). On the plain it swings in loops called meanders, because the water flows faster on the outside of a bend and cuts the bank there (a cut-off or concave bank), while on the inside of the bend it is slow and drops sand (a point bar). Over time a loop is so tight that the river cuts through its neck and abandons the loop, which becomes a crescent-shaped ox-bow lake. During a flood, the river overflows and the coarsest silt falls first, right beside the channel, building raised banks called natural levees. The finer silt spreads far and builds the flood plain.

A delta forms when a river enters a sea or lake, slows suddenly and drops its entire load. The name comes from the Greek letter delta. The Ganga-Brahmaputra delta (Sundarbans) is the largest in the world. In India, the rivers that flow east into the Bay of Bengal (Mahanadi, Godavari, Krishna, Kaveri) build deltas, while the west-flowing Narmada and Tapi flow in rift valleys and enter the Arabian Sea through estuaries. Paradip port lies in the Mahanadi delta.

Two more ideas help in exams. Rejuvenation: if the land rises or sea level falls, the river becomes steep again and starts cutting downward; the old flood plain is left as stepped platforms called river terraces, and incised meanders may form. Peneplain: after a very long time the land is worn down to an almost featureless plain; isolated hard remnants standing above it are monadnocks (in India, the Aravalli is a classic worn-down range). Finally, rivers form drainage patterns that reflect the rock below: dendritic (tree-like, on uniform rock, e.g. the Northern Plains), trellis (rectangular, parallel valleys with short cross-streams, on folded land), radial (streams flow outward from a central hill, e.g. Amarkantak), rectangular (on jointed rock) and parallel (on a steep, uniform slope, e.g. Western Ghats' west-flowing streams).

A glacier is a huge, slow river of ice. It forms where more snow falls than melts, so the snow is buried, squeezed into ice and, under its own weight, begins to creep downhill. Glaciers are of two main kinds: continental ice sheets that bury whole landmasses (Antarctica, Greenland) and valley (mountain) glaciers that fill mountain valleys (Gangotri, Siachen in the Himalaya-Karakoram). Ice is a far more powerful sculptor than water, because it drags rock fragments embedded in its base. It erodes by plucking (tearing blocks of rock loose as water freezes to them) and by abrasion (scraping and polishing the bed). The special thing to remember: rivers make V-shaped valleys, glaciers make U-shaped valleys.

Where the water ran, the deposit is sorted; where the ice dropped it, it is not. Meltwater sorts material by size and rounds it (outwash, eskers, kames). Ice itself dumps everything mixed, angular and unsorted (till, moraines, drumlins). This single idea decides most options. Eskers are therefore meltwater ridges, and outwash is sorted, rounded sediment.

Wind works mainly in hot deserts, where there is little vegetation and dry, loose sand. It erodes by deflation (blowing away fine loose particles, leaving hollows called deflation hollows or blowouts), by abrasion (sandblasting rock with grains it carries) and by attrition (grains wearing each other down). The sand is mostly carried close to the ground, so the base of an obstruction is sandblasted harder than its top.

Wind-blown deposits also include loess, a fine, yellowish, fertile, wind-borne silt blown out of deserts and glacial outwash plains. Loess forms thick, fertile plains in northern China (the Yellow River's yellow colour comes from it). Large sand seas are called ergs, stony deserts hamada and pebbly deserts reg. In India the Thar Desert, west of the Aravallis, has dunes (mainly barchans) built by the south-west monsoon winds and by dry desert winds.

Sea waves shape coasts using the same ideas. Waves hurl water and pebbles at a cliff (hydraulic action, abrasion). Where a headland is attacked from both sides the story is: a crack becomes a cave, two caves meet to make an arch, the roof collapses leaving a stack, and the stack wears down to a stump. Material removed from cliffs and from rivers is moved along the shore by waves and settles in sheltered spots, building beaches, spits and bars.

Not all rain runs off; a lot sinks into the ground and becomes groundwater. In most rocks this water changes little. But where the rock is limestone or dolomite (rocks made of calcium carbonate, called soluble rocks), groundwater slowly dissolves the rock and carves a strange underground world of caves, holes and vanishing streams. The landscape so formed is called karst topography, named after the Karst (Kras) plateau on the Slovenia-Italy border. Remember the three conditions: soluble rock, plenty of joints and cracks for water to enter, and enough rainfall.

The big landforms of the world are mountains, plateaus and plains. The simplest way to remember them is by height and shape. A mountain is a steep, high landform that rises well above its surroundings, usually with a narrow summit. A plateau is an elevated flat-topped land, a 'tableland', with at least one steep side. A plain is a low, level or gently rolling land. Because the Earth's own forces build them, each type has a clear origin, and the origin is what exams ask.

Fold mountains are young and high; block and residual mountains are old. Young fold mountains are still rising and still sharp-peaked (Himalaya). Old fold mountains such as the Aravalli have been eroded down to low hills. The Aravalli is the oldest fold mountain of India and one of the oldest in the world, rich in minerals such as copper, zinc and lead, but it is not a high range today.

The Deccan Plateau of India is a part of the ancient Gondwanaland, which once joined India with Africa, Australia, South America and Antarctica. It is one of the oldest and most stable land blocks, made of hard igneous and metamorphic rocks, and much of its north-west is covered by Deccan lava (basalt). Landforms found on the peninsular plateau include tors (isolated rock masses left by weathering of granite), block mountains, rift valleys and spurs. Among the best known residual hills are the Shevaroy Hills (Tamil Nadu, an outlying range of the Eastern Ghats near Salem).

Lakes are hollows on land filled with water. Most are classified by how the hollow was made. Tectonic lakes fill basins created by faulting or folding (Lake Baikal, the deepest lake in the world; Wular Lake, Jammu and Kashmir, India's largest freshwater lake). Glacial lakes fill hollows scooped or dammed by ice (Great Lakes of North America; tarns; the high lakes of Ladakh and the Himalaya). Crater lakes fill volcanic craters (Crater Lake, USA) or meteorite craters (Lonar Lake, Maharashtra). Fluvial lakes include ox-bow lakes and floodplain lakes. Lagoons are coastal lakes behind sand bars (Chilika, Pulicat). Salt lakes form in dry regions where water cannot flow out and evaporates (Sambhar in Rajasthan, Great Salt Lake, Dead Sea). Artificial lakes are made by dams (Gobind Sagar).