Rocks, Weathering and Landforms

What you need to know

• Igneous rocks form from the cooling and solidification of magma or lava. Intrusive or plutonic types cool slowly deep underground and so have large crystals, for example granite and gabbro. Extrusive or volcanic types cool quickly at the surface and so have tiny crystals or none, for example basalt, obsidian and pumice.
• Sedimentary rocks form from compacted and cemented sediments laid down in layers, so they are stratified and may contain fossils. They are grouped as mechanically formed, such as sandstone, shale and conglomerate; organically formed, such as limestone, chalk and coal; and chemically formed, such as rock salt and gypsum.
• Metamorphic rocks form when existing rocks are changed in place by great heat, pressure or both, without melting. Granite becomes gneiss, limestone becomes marble, sandstone becomes quartzite, shale becomes slate, and coal becomes graphite.
• Physical or mechanical weathering breaks rock into smaller pieces without changing its chemical make-up. In Nigeria the chief agent is temperature change: fierce daytime heating and rapid night cooling make the outer rock layers expand and peel away in curved sheets, a process called exfoliation or onion weathering, clearly seen on Olumo Rock and on the Idanre and Zuma inselbergs.
• Other physical processes are block disintegration along joints, granular disintegration where individual mineral grains fall out, frost action or freeze and thaw in cold mountains, and pressure release when overlying rock is removed and the rock below expands and cracks.
• Chemical weathering changes the composition of the rock. Carbonation occurs when rain water dissolves carbon (IV) oxide to form a weak carbonic acid which attacks limestone; oxidation reddens and crumbles iron-rich rocks; hydrolysis turns the feldspar in granite into clay; hydration makes minerals swell as they take in water; and solution removes soluble salts outright.
• Chemical weathering is strongest in the hot wet south of Nigeria, where it produces deep red lateritic soils, while physical weathering dominates in the hot dry north. Biological weathering adds to both, through tree roots prising joints apart, burrowing animals, lichens secreting acids and human quarrying and excavation.
• A river erodes by hydraulic action, the sheer force of moving water; by corrasion or abrasion, where the load it carries grinds the bed and banks; by attrition, where the load particles knock against each other and become smaller and rounder; and by corrosion or solution, where the water dissolves soluble rock.
• River landforms of erosion are the V-shaped valley, gorge, rapids, waterfall, pothole and interlocking spur, typically in the upper course. A waterfall forms where a band of hard rock lies across a stream above softer rock: the soft rock is cut back faster, a step develops, plunge pool erosion undercuts the hard band and it collapses, so the fall retreats upstream.
• River landforms of deposition belong to the lower course: meanders, ox-bow lakes, flood plains, natural levees, braided channels, deltas and estuaries. The Niger Delta is an arcuate delta built where the river deposits its load faster than the sea can remove it.
• Wind erodes by deflation, lifting loose fine material away; by abrasion, sandblasting rocks near the ground; and by attrition. Its landforms are deflation hollows and oases, mushroom or pedestal rocks, zeugen, yardangs and ventifacts. Wind deposition builds crescent-shaped barchans, long seif dunes aligned with the prevailing wind, and sheets of fine loess.
• Waves erode by hydraulic action, corrasion, attrition and solution, producing cliffs, wave-cut platforms, caves, arches, stacks and stumps on headlands. Wave deposition and longshore drift build beaches, spits, bars, tombolos and the lagoons that line the Nigerian coast from Lagos to Lekki.
• Glaciers erode by plucking and abrasion, producing cirques or corries, aretes, pyramidal peaks, U-shaped valleys, hanging valleys, roches moutonnees and fjords, and deposit moraines, drumlins, erratics, eskers and outwash plains. Nigeria has no glaciers, so these come up as pure recall or diagram questions.
• Rock type controls the landform. Resistant granite and quartzite stand up as inselbergs and ridges, limestone weathers into karst features with sinks and caves, and soft shale and sandstone form lowlands and broad valleys. This is why the Jos Plateau and the Idanre Hills stand above the plains around them.