Rocks and landforms

The first principle

Every landform is the result of a contest between two sets of forces. Endogenic forces, powered by the Earth's internal heat, build relief by lifting, folding and pouring out lava, while exogenic forces, powered by the sun through wind, water and ice and assisted by gravity, wear that relief down. Rocks decide how this contest plays out, because a hard granite resists the same rain that quickly dissolves a limestone, so the study of landforms begins with the study of rocks.

The core

The rock cycle

• Rocks are of three kinds by origin. Igneous rocks form when molten magma or lava cools, sedimentary rocks form when fragments and dissolved material settle in layers and are compacted and cemented, and metamorphic rocks form when existing rocks are changed by heat and pressure without melting.
• The three are linked in a loop. Plate movement buries, heats and lifts rock, while weathering and erosion break it down at the surface, so any rock can become any other given time.

Primary rocks: characteristics and types

• Igneous rocks are called primary rocks because they are the first to form from molten material, and all other rocks derive ultimately from them. They are crystalline, usually hard and compact, contain no fossils because life cannot survive in magma, are not layered, and resist erosion unless jointed.
• By place of cooling. Intrusive (plutonic) rocks cool slowly deep underground, so crystals have time to grow large, as in granite and gabbro; they appear as batholiths, laccoliths, sills and dykes. Extrusive (volcanic) rocks cool quickly at the surface, so they are fine grained or glassy, as in basalt, pumice and obsidian.
• By chemistry. Acid (felsic) rocks have more than about 65 per cent silica, are light coloured and less dense, and come from viscous, explosive magma (granite). Basic (mafic) rocks have less silica and more iron and magnesium, so they are dark and dense, and come from fluid lava (basalt, gabbro).
• Economic importance. Granite is a durable building and monument stone; basalt is crushed for road and rail ballast; weathered basalt forms the fertile black cotton soil (regur) of the Deccan; and many metallic ores, such as those of copper, gold and tin, are found in or near igneous intrusions because hot fluids from cooling magma deposit them.

Sedimentary and metamorphic rocks in brief

• Sedimentary rocks are layered and often carry fossils. They are mechanically formed (sandstone, shale), organically formed (limestone, coal) or chemically formed (rock salt, gypsum), and they hold almost all coal, petroleum and natural gas.
• Metamorphism reorganises minerals under heat and pressure, so limestone becomes marble, sandstone becomes quartzite, shale becomes slate and granite becomes gneiss.

Dimensions

Endogenic and exogenic processes

• Endogenic processes are diastrophism (folding, faulting and the slow rise or fall of crust) and volcanism, both driven by mantle heat and plate movement (see the note on plate tectonics).
• Exogenic processes work through stress on surface material. Weathering breaks rock in place, physically by repeated expansion and contraction, frost wedging and salt crystal growth, or chemically by solution, oxidation and hydration; chemical weathering is fastest where it is hot and wet, physical weathering where temperatures swing widely. Mass wasting is the downslope movement of weathered material under gravity, from slow soil creep to sudden landslides. Erosion then removes and carries material by running water, glaciers, wind and waves.

Running water

• Rivers cut steep V shaped valleys and gorges in their youth, where the gradient is high; as the slope eases they swing sideways into meanders, and in the lowlands they deposit floodplains, levees and deltas.

Glaciers and fjords

• A glacier erodes by plucking blocks from its bed and by abrasion, as rocks frozen into its base scrape the floor. This produces armchair shaped cirques, sharp arêtes and horns, and U shaped valleys with steep sides and flat floors; tributary glaciers that were smaller cut shallower valleys, which are left hanging high on the main valley wall.
• How fjords form. During the ice ages, thick glaciers moved down pre existing river valleys towards the coast. Because a glacier keeps eroding its bed until the ice becomes thin enough to float, a valley glacier several hundred metres thick could deepen its trough far below the sea level of the time. When the ice melted at the end of the last glacial period, sea level rose by about 120 m, the sea flooded these deep troughs, and long, narrow, steep sided inlets were left. A rock sill often survives near the mouth, where the glacier thinned and lost erosive power, so fjords are often deeper inland than at the sea.
• Where fjords occur. Fjords are found where glaciated mountains meet the sea at high latitudes on west coasts, which received heavy snowfall from westerly winds: Norway, southern Chile, the South Island of New Zealand (Fiordland), Greenland, Alaska and British Columbia in Canada.
• Why they are picturesque. Fjords combine near vertical rock walls rising hundreds of metres straight from deep, calm water, mountains and glaciers meeting the sea, and waterfalls pouring from hanging valleys, all within a short distance. This scenery is why the West Norwegian Fjords, Geirangerfjord and Naerøyfjord, were inscribed as a World Heritage Site by the United Nations Educational, Scientific and Cultural Organization (UNESCO) in 2005, and why fjord coasts draw large numbers of cruise visitors.

Wind (aeolian processes)

• Wind is a weak agent except where the ground is dry, loose and bare, which is why it shapes deserts. It erodes by deflation, lifting loose particles and leaving hollows and stony desert pavements, and by abrasion, blasting rock with sand carried within a metre or two of the ground, which undercuts rocks into mushroom shapes and carves ridges called yardangs parallel to the wind.
• It deposits sand as dunes. A barchan is crescent shaped with horns pointing downwind and forms where sand is limited and wind direction steady; a seif (longitudinal) dune runs parallel to the wind. Fine silt carried far away settles as loess, which forms fertile plains in northern China. The role of wind in spreading deserts is taken up in the note on desertification.

Waves

• Waves erode cliffs, wave cut platforms, arches and stacks on exposed coasts and build beaches, spits, bars and lagoons where energy is lower, as at Chilika in Odisha.

In practice

• The Deccan Traps. About 66 million years ago, fluid basaltic lava poured out through fissures in repeated flows over a vast area of western and central India. Each flow weathered at its top before the next one covered it, giving the step like (trap, from the Swedish word for stairs) flat topped hills of the Western Ghats and Maharashtra plateau. Its mineral and soil resources are covered in the note on minerals.
• Granite landscapes. Rounded granite domes and tors such as those around Hampi and Bengaluru show how jointed granite weathers along cracks into boulders, a result of physical and chemical weathering on a hard rock.
• Himalayan mass wasting. Steep slopes, weak and fractured rock and heavy monsoon rain make the Himalaya one of the most landslide prone regions in the world; the Geological Survey of India prepares landslide susceptibility maps for these states.

Issues and way forward

• Human activity accelerates mass wasting. Road cutting, quarrying and construction on steep slopes remove support from weathered rock, which is why landslide disasters cluster along new hill roads. Slope stabilisation, drainage and land use zoning based on Geological Survey of India susceptibility maps, as the National Disaster Management Authority guidelines on landslides (2009) recommend, are the remedy.
• Unregulated quarrying of stone. Mining of granite, basalt and sand scars landscapes and removes aquifers and hills. Environmental clearance for minor minerals, which the Supreme Court required in Deepak Kumar v State of Haryana (2012), and recycled construction material reduce the pressure.
• Fragile fjord and glacier landscapes. Retreating glaciers and heavy tourism threaten the ecology of fjords; Norway has decided to phase in zero emission requirements for ships in its World Heritage fjords, which shows how such places can be protected while remaining open.

Common confusions

• Weathering and erosion. Weathering breaks rock where it lies; erosion removes and transports the material.
• Intrusive and extrusive. The difference is where the magma cooled, not its chemistry, so the same basic magma gives gabbro underground and basalt at the surface.
• Fjord and ria. A fjord is a drowned glacial valley, deep and U shaped; a ria is a drowned river valley, shallower and V shaped, as on the coast of Galicia in Spain.
• Acid and basic. These terms describe silica content, not acidity in the chemical sense of pH.

For the exam

• Keyword bank: rock cycle, primary rocks, intrusive, extrusive, felsic, mafic, batholith, dyke, weathering, mass wasting, cirque, U shaped valley, hanging valley, fjord, threshold or sill, deflation, abrasion, yardang, barchan, loess.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction (data): Igneous and metamorphic rocks make up about 95 per cent of the Earth's crust, yet sedimentary rocks cover about three quarters of its land surface, a contrast that the rock cycle explains.
  • Introduction (report): UNESCO inscribed Geirangerfjord and Naerøyfjord as World Heritage in 2005 as classic examples of fjord landscapes shaped by glaciers.
  • Conclusion: Landforms record the long contest between internal forces that build relief and external agents that wear it down, with rock type deciding the result.
  • Conclusion: Understanding rocks and slopes is the basis of safe roads, dams and towns, especially in the young and fragile Himalaya.