Cameroon GCE O-Level Geology cheat sheet

Earth's internal structure: crust (5–70 km, silicate), mantle (to ~2,900 km, peridotite), outer core (liquid Fe–Ni, source of the magnetic field), inner core (solid Fe–Ni).

The Earth's crust is either continental (thick, granitic, mean density ~2.7 g/cm³) or oceanic (thin, basaltic, mean density ~3.0 g/cm³); continental crust floats higher (isostasy).

Rock cycle: igneous forms from cooling magma/lava; sedimentary forms by weathering, transport, deposition, compaction and cementation; metamorphic forms from heat and pressure acting on pre-existing rock.

Igneous classification by silica: felsic/granitic (light, quartz-rich, viscous, explosive eruptions) versus mafic/basaltic (dark, low silica, fluid, gentle eruptions).

Sedimentary rocks: clastic (sandstone, shale, conglomerate) classified by grain size; chemical/biochemical (limestone, rock salt, chert) formed by precipitation or organisms.

Metamorphic grade indicators: slate→schist→gneiss with increasing grade; index minerals (chlorite, biotite, garnet, staurolite) mark zones; foliation develops under directed pressure.

Mohs hardness scale: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10; a mineral scratches only lower numbers.

Mineral identification uses lustre, colour, streak, cleavage/fracture, hardness and specific gravity; streak is more reliable than colour because colour varies with impurities.

Plate tectonics: lithospheric plates move over the asthenosphere; divergent boundaries build ridges and new oceanic crust, convergent boundaries give subduction, trenches, volcanoes and fold mountains, transform boundaries give earthquakes only.

Earthquake focus is the underground origin; epicentre is the point on the surface directly above it. Seismic waves: P (primary, compressional, fastest, travel through solids and liquids), S (secondary, shear, slower, cannot pass liquids), L (surface, slowest, most damaging).

Seismograph records arrival times; the S–P interval gives distance to the epicentre, and three station circles locate the epicentre. Magnitude (Richter/moment) measures energy; intensity (Mercalli) measures observed damage.

Volcano types: shield volcanoes from fluid basaltic lava (gentle slopes); composite/strato volcanoes from alternating ash and viscous lava (steep, explosive); calderas form by collapse after eruption.

Folding and faulting: anticlines arch upward, synclines sag downward; normal faults form under tension (hanging wall drops), reverse faults under compression, strike-slip faults under lateral shear.

Weathering: mechanical (freeze–thaw, exfoliation, biological) breaks rock without chemical change; chemical (carbonation, oxidation, hydrolysis) alters minerals; erosion transports the debris, mass wasting moves it downslope under gravity.

River processes: erosion (hydraulic action, abrasion, attrition, solution), transport (traction, saltation, suspension, solution), deposition when velocity falls; V-shaped valleys in uplands, meanders, floodplains and deltas in lowlands.

Groundwater and karst: water percolates through permeable rock to the water table; carbonation of limestone produces caverns, stalactites (hang down), stalagmites (grow up) and sinkholes; aquifers are porous and permeable, aquicludes are not.

Relative dating laws: superposition (younger layers above older), original horizontality, lateral continuity, cross-cutting relationships (a fault or intrusion is younger than what it cuts), and inclusions (fragments are older than the host rock).

Fossils used as index markers (ammonites, trilobites, graptolites) identify relative ages because they are distinctive and short-lived in time; unconformity marks a gap in the record caused by erosion or non-deposition.

Geological time scale order: Precambrian, Palaeozoic (Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian), Mesozoic (Triassic, Jurassic, Cretaceous), Cenozoic (Palaeogene, Neogene, Quaternary).

Absolute dating uses radioactive decay: half-life is the time for half the parent atoms to decay; after n half-lives the fraction remaining is (1/2)^n. Carbon-14 (half-life ~5,730 years) dates organic material up to ~50,000 years; uranium-lead dates old rocks.

Economic deposits: ores are minerals mined profitably; bauxite gives aluminium, haematite and magnetite give iron, chalcopyrite gives copper, galena gives lead, sphalerite gives zinc, cassiterite gives tin.

Fossil fuels: coal forms from compressed peat in swamps (increasing rank: peat, lignite, bituminous, anthracite); oil and gas form from marine plankton buried in source rock, migrate into porous reservoir rock trapped beneath a cap rock.

Cameroon geology: the Cameroon Volcanic Line runs from the Atlantic islands through Mount Cameroon (active, last major eruptions 1999 and 2000) to the Adamawa Plateau; the country also has Precambrian basement and coastal sedimentary basins.

Hydrogeology and hazards: porosity is the capacity to hold water, permeability the ability to transmit it; slope failure risk rises with steep slopes, weak or saturated rock, deforestation and seismic shaking.

Soil is the weathered mineral and organic layer supporting plant growth; profile horizons: O (organic litter), A (topsoil, humus-rich), B (subsoil, leached accumulations), C (weathered parent rock), R (bedrock).

Soil texture depends on particle size: sand (coarse, well drained), silt (intermediate), clay (fine, retains water and nutrients); loam is a balanced mixture best for agriculture.

Soil pH affects nutrient availability; acidic soils are corrected with lime (calcium carbonate), alkaline soils with organic matter or gypsum.

Mass movement types: soil creep (slow, imperceptible), earthflow, mudflow, landslide and rockfall (fast); triggered by heavy rain, steep slopes, undercutting and loss of vegetation.

Structural geology mapping: strike is the compass direction of a horizontal line on a bedding plane; dip is the angle of steepest descent perpendicular to strike, measured with a clinometer.

Maps and scale: a scale of 1:50,000 means 1 cm on the map equals 50,000 cm (500 m) on the ground; contour lines join equal elevations, and closely spaced contours indicate steep ground.

Rock properties for engineering: strength, porosity, permeability and jointing determine suitability; limestone dissolves and is prone to sinkholes, clay swells when wet and shrinks when dry.

Natural hazards in Cameroon: Mount Cameroon eruptions and gas release from Lake Nyos (1986 carbon dioxide disaster) show that volcanic and limnic hazards require monitoring and evacuation planning.

Magma differentiation: cooling magma crystallises minerals in sequence (Bowen's reaction series), so early-formed mafic minerals settle and the remaining melt becomes more silica-rich.

Texture of igneous rocks records cooling rate: coarse-grained (phaneritic, intrusive) means slow cooling underground; fine-grained (aphanitic, extrusive) means rapid cooling at the surface; glassy or vesicular texture indicates very rapid cooling or gas escape.

Sedimentary structures as palaeoenvironment clues: cross-bedding indicates currents (dunes, rivers), graded bedding indicates waning flow, ripple marks indicate shallow water, mud cracks indicate drying.

Fossilisation requires rapid burial, hard parts and absence of oxygen; most fossils are found in sedimentary rock, never in igneous rock and rarely in metamorphic rock.

Metamorphism types: contact (thermal) metamorphism around intrusions produces hornfels; regional metamorphism during mountain building produces foliated rocks such as schist and gneiss.

Plate driving mechanisms: mantle convection, slab pull (dense subducting plate) and ridge push; sea-floor spreading creates symmetrical magnetic stripes on either side of mid-ocean ridges.

Mineral resources and sustainability: reserves are known, economically extractable deposits; recycling, substitution and efficient use extend resource life, and mining causes land degradation and acid mine drainage.

Geological fieldwork skills: record rock type, colour, grain size, texture, bedding attitude and structures in a field notebook; use a hand lens, hardness kit, dilute acid test for calcite, and a compass clinometer.

The geological column and life: Precambrian life was mainly bacteria and algae; trilobites and brachiopods dominate the Palaeozoic; ammonites and dinosaurs dominate the Mesozoic; mammals and grasses dominate the Cenozoic.

Mass extinctions mark era boundaries: the Permian–Triassic event removed most marine species, and the Cretaceous–Palaeogene event (impact and volcanism) ended the dinosaurs.

Fault terminology: the hanging wall lies above the fault plane, the footwall below; in a normal fault the hanging wall moves down relative to the footwall, in a reverse fault it moves up.

Rock deformation behaviour: brittle rocks fracture (faults, joints) under low temperature and pressure; ductile rocks fold under high temperature and pressure, producing anticlines, synclines and monoclines.

Erosion agents compared: rivers cut valleys and carry sediment to the sea; glaciers erode U-shaped valleys and deposit moraine and till; wind erodes and deposits dunes in arid regions; waves produce cliffs, beaches and spits.

The rock cycle links to plate tectonics: igneous activity occurs at ridges, subduction zones and hotspots; metamorphism occurs at collision zones; sedimentary basins form at passive margins and in subsiding areas.

Hydrocarbon and coal formation conditions: rapid burial, absence of oxygen, and sufficient heat and pressure over geological time convert organic matter into kerogen, then oil and gas.

Environmental geology: groundwater contamination spreads through permeable rock, so landfill sites need impermeable clay or synthetic liners; quarrying and mining require rehabilitation plans.

Crystal systems and symmetry: minerals are classified by crystal form into seven systems (cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, trigonal); cleavage planes reflect internal atomic structure.

Silicate structure controls properties: isolated tetrahedra (olivine) versus framework silicates (quartz, feldspar); sheet silicates (mica, clay) split easily into thin flakes.

Sedimentary rock classification by grain size: gravel/conglomerate > 2 mm, sand/sandstone 0.06–2 mm, silt/siltstone 0.004–0.06 mm, clay/shale < 0.004 mm.

Limestone tests: dilute hydrochloric acid produces effervescence (carbon dioxide); this distinguishes calcite and limestone from dolomite (weak reaction) and from sandstone.

Volcanic hazard assessment: monitoring uses seismographs (tremor), tiltmeters (ground swelling), gas emission measurements and thermal imaging; evacuation warnings follow increased activity.

Earthquake hazard reduction: building codes, reinforced structures, base isolation, education and drills reduce casualties; seismic risk maps identify high-hazard zones.

Geological maps show outcrop patterns: a resistant bed forms a ridge, a soft bed a valley; a V-shaped outcrop pattern pointing upstream in a valley indicates the bed dips upstream.

Ore formation processes: magmatic segregation (chromite), hydrothermal veins (gold, quartz), sedimentary precipitation (ironstone, rock salt) and residual weathering (bauxite).