GeologyQ&A by dot point

Earth structure: the layered internal structure of the Earth (crust, mantle, outer core and inner core) and the mechanical layers (lithosphere and asthenosphere); the seismic evidence for the layering from changes in P and S wave velocity at boundaries such as the Moho; the P and S wave shadow zones as evidence for a liquid outer core; the use of meteorites and density as evidence for the composition of the core and mantle.

Earthquakes and seismic waves: the focus and epicentre; the elastic rebound mechanism; the P, S and surface waves and their properties; the difference between magnitude (the logarithmic Richter scale and its saturation, and the moment magnitude scale) and intensity (the Modified Mercalli scale); the use of P and S wave arrival times and travel-time graphs to locate an epicentre by triangulation.

Plate margins and their features: the processes and characteristic features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-ocean island arcs, ocean-continent margins and continent-continent collision); the Benioff zone, subduction and decompression melting; the diagnostic rocks, structures, earthquakes and volcanoes of each margin type.

Plate tectonics theory and evidence: the development of the theory from Wegener's continental drift, through Hess's sea-floor spreading, to plate tectonics; the evidence of palaeomagnetism and the symmetrical magnetic striping of the ocean floor; the increasing age of oceanic crust away from mid-ocean ridges; the driving mechanisms of mantle convection, ridge push and slab pull.

The lithosphere, mantle plumes and hotspots: the structure and composition of oceanic and continental lithosphere; the principle of isostasy and isostatic adjustment; mantle plumes and hotspots as a cause of intraplate volcanism; hotspot tracks and the age progression of volcanic island chains (for example Hawaii); the basis of the Component 3 geology of the lithosphere option.

Volcanic activity and eruption styles: the control of silica content, viscosity and dissolved gas on eruption style; the contrast between basaltic effusive eruptions (shield volcanoes and fissures) and andesitic or rhyolitic explosive eruptions (stratovolcanoes, pyroclastic flows and calderas); the volcanic products (lava, tephra and pyroclastic material); the link between eruption style and plate setting.

Elements, atomic structure and bonding: the major rock-forming elements; atomic structure (protons, neutrons and electrons) and isotopes; ionic, covalent and metallic bonding; how the type of bonding and the arrangement of atoms control the physical properties of minerals such as hardness and cleavage.

Identifying minerals by physical properties: hardness (the Mohs scale), cleavage and fracture, lustre, colour and streak, habit, density (specific gravity) and special properties (magnetism, reaction with acid); and the use of these diagnostic properties to identify the common rock-forming and ore minerals in hand specimen.

Mineral and rock tests and field skills: the practical tests used to identify minerals and rocks (hardness, acid, magnet, streak, density); the recording of field observations through field sketches, annotated specimens and sampling; and the fieldwork requirement (a minimum of four days for the A-level) and how practical and fieldwork skills are assessed within the written components.

Silicate minerals and mineral classification: the silicon-oxygen tetrahedron as the building block of silicates; the polymerisation series from isolated tetrahedra (olivine) through chains (pyroxenes, amphiboles) and sheets (micas, clays) to frameworks (quartz, feldspars); and the classification of non-silicate minerals into carbonates, oxides, sulphides, halides and native elements.

The rock cycle and rock classification: the threefold classification of rocks into igneous, sedimentary and metamorphic; the processes that link them (crystallisation, weathering, erosion, transport, deposition, compaction and cementation, burial, metamorphism, melting and uplift); and the role of the surface (external) and internal processes driven by solar energy and the Earth's internal heat.

Earthquake hazards, risk and mitigation: the primary and secondary hazards of earthquakes (ground shaking, liquefaction, landslides, tsunami, fire); the distinction between hazard, vulnerability, exposure and risk; the factors controlling the severity of impact; and the prediction, monitoring and mitigation strategies (building design, hazard mapping, early warning and planning).

Groundwater, aquifers and hydrogeology: the storage and movement of water in rocks; porosity and permeability; aquifers, aquicludes and the water table; artesian conditions; Darcy's law for groundwater flow; and the practical issues of abstraction, recharge, over-abstraction and groundwater pollution.

Hydrocarbons and petroleum systems: the formation of oil and gas from organic-rich source rocks by burial and maturation; migration into porous and permeable reservoir rocks; the role of impermeable cap (seal) rocks and trap structures (anticline, fault, stratigraphic and salt traps); and the elements that must coincide for an accumulation to form.

Mass movement and landslide hazards: the types of mass movement (rockfall, slide, slump, flow and creep); the factors controlling slope stability (slope angle, rock and soil strength, water, bedding orientation, vegetation and undercutting); the triggers of slope failure; and the engineering and planning methods used to reduce landslide hazards.

Ore deposits and economic minerals: the processes that concentrate metals into economic ore deposits (magmatic segregation, hydrothermal and vein deposits, placer deposits, secondary enrichment and sedimentary deposits); the concepts of ore grade, cut-off grade and reserves; and the calculation of the tonnage of metal from grade and tonnage data.

Volcanic hazards and monitoring: the primary and secondary hazards of volcanic eruptions (lava flows, pyroclastic flows, tephra and ash fall, lahars, gases, sector collapse); the control of magma composition on eruption style and hazard; and the monitoring and prediction methods (seismicity, ground deformation, gas emissions, thermal and historical records) used to forecast eruptions.

Dip, strike and true thickness: the definition and measurement of true dip, apparent dip, dip direction and strike with a compass-clinometer; structure contours; the calculation of the true (perpendicular) and vertical thickness of a bed from its outcrop width and dip using trigonometry; the distinction between vertical and true thickness; and the rule of Vs for outcrops crossing valleys.

Folds, faults and joints: fold elements (limb, axial plane, hinge) and types (anticline and syncline, symmetric, asymmetric, overturned, recumbent, monocline); fault types and the stress they record (normal from tension, reverse and thrust from compression, strike-slip and tear from shear); dip-slip versus strike-slip movement; throw, heave and the fault plane; joints as fractures with no displacement; and reading these structures on geological maps and cross-sections.

Geological maps and cross-sections: reading outcrop patterns, reading dip from outcrop width and topography, and the younging direction; constructing a cross-section from a map; deducing the geological history (the order of events) using superposition, cross-cutting relationships, unconformities and included fragments; the difference between simplified map extracts (Component 1) and real published map extracts (Component 3); and three-point problems in outline.

Stress and strain: the three stress regimes (compression, tension and shear) and the strain (deformation) they produce; elastic, ductile and brittle behaviour; the factors that control deformation style (temperature, confining pressure, strain rate, rock type and pore fluid pressure); competent and incompetent rocks; and why rocks deform ductilely at depth but brittlely near the surface.

Unconformities and the geological record: the angular unconformity (tilted or folded beds overlain at a different angle), the disconformity (parallel beds separated by an erosion surface) and the nonconformity (sediments on eroded igneous or metamorphic basement); the ordered sequence of events each records (deposition, uplift, tilting, erosion, renewed deposition); the gap (hiatus) in the record; and the use of unconformities to reconstruct geological history on maps and cross-sections.

Igneous intrusions and volcanic forms: concordant intrusions (sills and laccoliths) versus discordant intrusions (dykes, batholiths and stocks); chilled margins, and baked margins and contact aureoles around intrusions, as way-up and relative-age evidence; cross-cutting relationships; and volcanic forms (shield volcanoes, stratovolcanoes or composite cones, cinder cones, calderas and lava plateaux).

Igneous rock classification and textures: the classification of igneous rocks by silica content and composition (ultramafic peridotite, mafic basalt and gabbro, intermediate andesite and diorite, felsic rhyolite and granite) and by grain size and cooling rate (glassy, aphanitic, phaneritic, porphyritic, vesicular and pyroclastic textures); and the relationship between cooling rate and crystal size.

Magma differentiation and Bowen's reaction series: the order of crystallisation of silicate minerals from a cooling magma (the discontinuous ferromagnesian branch olivine to pyroxene to amphibole to biotite, and the continuous plagioclase branch from calcium-rich to sodium-rich, then potassium feldspar, muscovite and quartz); fractional crystallisation and partial melting; and how differentiation evolves a magma from mafic to felsic.

Metamorphism, grade and facies: contact (thermal) metamorphism producing hornfels within an aureole versus regional metamorphism producing foliated rocks; the agents of metamorphism (heat, pressure and chemically active fluids); metamorphic grade and the prograde sequence from mudstone (slate, phyllite, schist, gneiss) with index minerals (chlorite, biotite, garnet, kyanite, sillimanite); foliated rocks (slate, schist, gneiss) versus non-foliated rocks (marble from limestone, quartzite from sandstone); protoliths; and metamorphic facies in outline.

Sedimentary rocks and depositional environments: the classification of clastic rocks by grain size (conglomerate and breccia, sandstone including arkose, greywacke and orthoquartzite, siltstone, mudstone and shale) and of chemical and biogenic rocks (limestone including oolitic, micritic and fossiliferous, chalk, the evaporites rock salt and gypsum, and coal); sedimentary structures (cross-bedding, graded bedding, ripple marks, desiccation cracks) as way-up and environment indicators; depositional environments (fluvial, deltaic, shallow marine, deep marine, desert); and diagenesis and lithification.

Weathering, erosion and sediment transport: physical weathering (freeze-thaw and exfoliation), chemical weathering (hydrolysis of feldspar to clay, carbonation of limestone, oxidation) and biological weathering; the distinction between weathering and erosion; transport by traction, saltation, suspension and solution, and how transport rounds and sorts grains to determine the maturity of a sediment.

Evolution and the fossil record: the evidence for evolution preserved in successive strata; modes of evolutionary change (gradualism and punctuated equilibrium); the use of evolutionary trends in lineages for dating; the major mass extinctions and their possible causes; and the broad pattern of the history of life through the geological time scale.

Fossils, preservation and index fossils: the modes of fossil preservation (unaltered hard parts, recrystallisation, replacement, moulds and casts, carbonisation, trace fossils); the conditions that favour preservation; and the characteristics that make a good index (zone) fossil for biostratigraphic correlation.

Palaeoenvironments and palaeoclimate proxies: the use of fossils, sedimentary structures and lithology to reconstruct past environments; palaeoclimate proxies (for example coal, evaporites, tillites, reef limestones, oxygen isotopes and fossil leaf shape); the use of facies and Walther's law; and the evidence for past climate change recorded in the rocks.

Quaternary glacial and periglacial geology (Component 3 option): glacial and periglacial processes and their deposits and landforms (till, moraines, drumlins, eskers, outwash, periglacial features); the evidence for Quaternary climate change (glacial-interglacial cycles, oxygen isotopes, ice cores); sea-level change; and the methods used to date and reconstruct Quaternary environments.

Radiometric dating and half-life: radioactive decay and the concept of half-life; the use of parent-to-daughter ratios to calculate absolute ages; the main dating methods and their suitable age ranges (for example uranium-lead, potassium-argon, rubidium-strontium and carbon-14); the assumptions and limitations of radiometric dating; and the construction of the absolute geological time scale.

Relative dating and stratigraphic principles: the principles of superposition, original horizontality, lateral continuity, cross-cutting relationships and included fragments; way-up (younging) indicators; and the use of these principles to reconstruct the sequence of geological events from a section or map.