Geology and environmental science: Earth systems, materials, time, resources, hazards and evidence
Earth as a system: structure, composition and dynamics
This independent teaching chapter develops earth as a system: structure, composition and dynamics. Apply each concept to observable evidence, state the assumptions of calculations, and distinguish a plausible interpretation from a conclusion demonstrated by the data. Regional examples are illustrative subject context, not a prescribed regional paper, official weighting or claim about current local measurements.
Chemical and mechanical layering
- Chemical layers: crust (felsic to mafic silicates), mantle (peridotite, Mg–Fe silicates) and core (Fe–Ni alloy).
- Mechanical layers: rigid lithosphere (crust plus uppermost mantle, with thickness varying by age and tectonic setting), ductile asthenosphere with a seismic low-velocity zone, mesosphere, liquid outer core and solid inner core.
- Discontinuities: Mohorovičić (crust/mantle), Gutenberg at about 2900 km (mantle/outer core) and Lehmann (outer/inner core).
Indirect evidence for the interior
- P waves cross solids and fluids; S waves cross solids only. S waves disappear beyond 103° from the epicentre and P waves are refracted into a shadow between 103° and 142°, which proves a liquid outer core.
- Mean density of the Earth is 5.5 g/cm³ against about 2.7 g/cm³ for surface rocks, so the interior must be denser.
- Iron meteorites serve as compositional analogues, and the geomagnetic field requires a convecting, electrically conducting fluid.
- Isostasy: Airy compensation uses crustal roots, Pratt compensation uses lateral density changes; post-glacial rebound proves that the asthenosphere flows.
Feedbacks
A classic example is the silicate-weathering thermostat: rising temperature speeds chemical weathering, which consumes CO₂, weakens the greenhouse effect and cools the planet.
Worked example
A student writes that the mantle must be molten because lava comes from it. Reasoning: S waves traverse the whole mantle, so the bulk mantle behaves as a solid; local partial melting can occur in suitable tectonic settings, and lava is a segregated melt rather than representative of the bulk mantle. The low-velocity zone is not a universal layer of liquid magma. The correct conclusion is a solid mantle with local partial melting.
Traps that cost marks
- Equating the lithosphere with the crust; the lithosphere includes the uppermost mantle.
- Confusing Mohorovičić and Gutenberg, or placing the mantle/core boundary at 6371 km instead of about 2900 km.
- Claiming the inner core is liquid; it is solid because pressure raises the melting point faster than temperature rises.
- Ignoring that the S-wave shadow begins at 103°, not at 180°.
Internal geodynamics: plate tectonics, magmatism and metamorphism
This independent teaching chapter develops internal geodynamics: plate tectonics, magmatism and metamorphism. Apply each concept to observable evidence, state the assumptions of calculations, and distinguish a plausible interpretation from a conclusion demonstrated by the data. Regional examples are illustrative subject context, not a prescribed regional paper, official weighting or claim about current local measurements.
Plate boundaries and driving forces
- Divergent: mid-ocean ridges with normal faults, shallow seismicity and tholeiitic MORB; continental rifts add alkaline volcanism.
- Convergent: ocean–ocean gives island arcs and back-arc basins; ocean–continent gives Andean-type margins with calc-alkaline andesites and granodiorites; continent-continent convergence produces collision, thrusts and folding; it does not by itself imply an active oceanic-subduction volcanic arc, and some collision regions also have magmatism.
- Transform: strike-slip motion, shallow earthquakes, with lateral displacement as the defining motion; associated magmatism or fault-rock alteration depends on the broader setting.
- Driving forces: slab pull dominates, ridge push is secondary, and mantle convection with basal drag acts as either a passive or a resisting force.
Magmatism
Magma series run from tholeiitic (low potassium, ridges and ocean islands) through calc-alkaline (commonly associated with subduction settings) to alkaline (intraplate). In a magma chamber, fractional crystallisation follows Bowen's reaction series, so early removal of olivine and calcium plagioclase leaves a silica-rich residual melt.
Metamorphism
Metamorphism is solid-state recrystallisation driven by temperature, pressure and fluids. Regional orogenic metamorphism produces the slate–schist–gneiss sequence with index minerals chlorite, biotite, garnet, staurolite, kyanite and sillimanite. Contact metamorphism creates hornfels aureoles and marble. Blueschist and eclogite appear where the geothermal gradient is low, as in subduction zones, whereas granulite records high temperature.
Worked example
A belt contains blueschist passing into eclogite and, twenty kilometres inland, greenschist. The high-pressure assemblages support investigation of a subduction-related history. Compare mineral equilibria, ages and structural relationships before reconstructing a paired belt: two rock names and their present-day separation do not by themselves locate an ancient trench or prove that the belts formed together.
Traps that cost marks
- Treating granite as the direct product of mantle melting instead of crustal melting or fractionation.
- Confusing contact and regional metamorphism, or assuming that marble is exclusive to contact metamorphism: it can also form during regional metamorphism, whereas hornfels commonly indicates a contact aureole.
- Saying plates float on magma; they move over ductile peridotite.
- Placing Iceland or the Canaries on a destructive boundary.
External geodynamics: weathering, erosion, transport and sedimentation
This independent teaching chapter develops external geodynamics: weathering, erosion, transport and sedimentation. Apply each concept to observable evidence, state the assumptions of calculations, and distinguish a plausible interpretation from a conclusion demonstrated by the data. Regional examples are illustrative subject context, not a prescribed regional paper, official weighting or claim about current local measurements.
Weathering
- Physical: freeze–thaw shattering, salt crystallisation, thermoclasty and exfoliation, root wedging. Clasts form without mineralogical change.
- Chemical: dissolution (limestone in carbonated water), hydrolysis (feldspar to clay), oxidation (Fe²⁺ to Fe³⁺, giving red soils) and carbonation. Hot humid climates favour chemical work.
- Soil develops through interacting mineral, organic, biological and transport processes; it is a dynamic system, not an inevitable final state of every weathered rock.
Erosion, transport and deposition
- Gravity, water, wind, ice and waves remove material; mass movement (creep, slides, flows) is a distinct step.
- Rivers carry bed load (traction, saltation), suspended load and dissolved load. The Hjulström curve shows that clay needs higher velocities to be eroded than sand, yet stays in suspension once disturbed.
- Rounding increases with distance and abrasion; sorting reflects how consistent the agent is. Glacial till is angular and unsorted, dune sand is well rounded and well sorted, fluvial gravel is rounded and moderately sorted.
- Structures: graded bedding (turbidites, fining upward), cross-bedding (dune migration, palaeocurrents), ripples, mud cracks (emergence) and bioturbation.
- Diagenesis - compaction, cementation and recrystallisation - turns sediment into rock.
Worked example
A sandstone body fines upward, shows parallel then ripple lamination and is capped by mudstone with marine fossils. The grain-size trend and structures are consistent with waning flow. A turbidite is one interpretation to investigate, but fining-upward sediment alone does not uniquely establish a deep-water density current; compare bed geometry, basal contacts, fossil assemblages and the regional succession before excluding other marine settings.
Traps that cost marks
- Calling weathering erosion; erosion implies transport.
- Reading well-rounded grains as long transport without checking sorting.
- Confusing cross-bedding with graded bedding.
- Assuming karst is only underground: lapiaz, dolines and simas are expressions of the same limestone dissolution.
Historical geology, paleontology and geological time
This independent teaching chapter develops historical geology, paleontology and geological time. Apply each concept to observable evidence, state the assumptions of calculations, and distinguish a plausible interpretation from a conclusion demonstrated by the data. Regional examples are illustrative subject context, not a prescribed regional paper, official weighting or claim about current local measurements.
Relative dating principles
- Superposition, original horizontality, lateral continuity, cross-cutting relationships and inclusions.
- Faunal succession: fossil assemblages change in a consistent order, allowing correlation of distant outcrops.
- Unconformities record gaps: angular (tilted beds truncated below horizontal ones), disconformity (parallel beds with an erosional gap) and nonconformity (sediments over igneous or metamorphic basement).
- Uniformitarianism: present processes explain the past, without excluding catastrophic events.
Absolute dating
Radiometric methods use the decay equation N = N₀e^(−λt) and the half-life T½ = ln 2/λ. U–Pb suits old zircons, K–Ar suits volcanic rocks, Rb–Sr requires closed systems, and radiocarbon is commonly used for suitable relatively recent carbon-bearing material, often organic remains, with a practical range of about 50,000 years depending on method and sample; it is not a general method for dating an ancient rock formation.
The time scale and life
Eons: Hadean, Archean, Proterozoic, Phanerozoic; eras: Palaeozoic, Mesozoic, Cenozoic. Milestones include the Great Oxidation Event, the Cambrian explosion, the end-Permian extinction (the largest) and the K–Pg impact. Suitable species or assemblages within groups such as trilobites, graptolites, ammonites and nummulites can support biostratigraphy. The chosen taxon must have an appropriate time range and distribution; an entire long-lived fossil group is not automatically a short-ranging index fossil.
Worked example
A granite intrudes folded limestones but is cut by an unconformity below horizontal sandstones. Reasoning: folding predates the granite, the granite predates erosion and the unconformity, and the sandstones are the youngest unit. If suitable zircon dating and supporting evidence establish a granite crystallization age of 300 Ma, the folding predates that intrusion and the overlying sandstone postdates it. A mineral cooling age must not automatically be substituted for the time of intrusion.
Number work
A sample retains 12.5 % of its parent isotope, so three half-lives have passed; with a half-life of 40 Ma the rock is 120 Ma old.
Traps that cost marks
- Dating a sedimentary rock from a clast inside it.
- Treating radiocarbon as a general crystallization clock for ancient limestone, or ignoring sample type, inherited carbon and analytical age limits.
- Reading an angular unconformity as a fault.
- Mixing the order of eras, or forgetting that the Precambrian covers roughly 88 % of Earth history.
Minerals and rocks: identification, classification and uses
This independent teaching chapter develops minerals and rocks: identification, classification and uses. Apply each concept to observable evidence, state the assumptions of calculations, and distinguish a plausible interpretation from a conclusion demonstrated by the data. Regional examples are illustrative subject context, not a prescribed regional paper, official weighting or claim about current local measurements.
Minerals
A mineral is natural, inorganic, solid, crystalline, with a definite chemical composition. Silicates dominate and are classified by tetrahedron arrangement: nesosilicates (olivine, garnet, kyanite), sorosilicates (epidote), cyclosilicates (beryl), inosilicates (pyroxene single chain, amphibole double chain), phyllosilicates (mica, clay) and tectosilicates (quartz, feldspar). Key properties: Mohs hardness from talc to diamond, cleavage versus fracture, streak, lustre, colour, density, magnetism and effervescence with HCl (calcite). Polymorphs such as calcite-aragonite or kyanite-sillimanite have different stability relations; interpret them with temperature, composition and the rock history rather than pressure alone.
Rock classification
- Igneous: plutonic (coarse; granite, diorite, gabbro) versus volcanic (fine; rhyolite, andesite, basalt); felsic to mafic by silica content and colour index.
- Sedimentary: clastic (conglomerate, sandstone, lutite), chemical (rock salt, gypsum, chert) and biochemical (limestone, coal).
- Metamorphic: foliated (slate, schist, gneiss) and non-foliated (marble, quartzite, hornfels).
Uses
Limestone and clay for cement, gypsum for plaster, clays for ceramics, granite and marble as ornamental stone, evaporites for salt and potash, silica sand for glass, and sulphide ores such as those of the Iberian Pyrite Belt.
Worked example
A coarse, pale rock shows interlocking grains of quartz, pink potassium feldspar, white plagioclase and black biotite; it scratches glass and does not react with acid. Reasoning: coarse interlocking crystals mean plutonic; quartz plus alkali feldspar means felsic; the absence of foliation excludes gneiss; the acid test excludes marble; the rock is granite.
Traps that cost marks
- Confusing cleavage with crystal faces; quartz has fracture but no cleavage.
- Calling a rock a mineral, as in "quartzite mineral".
- Classifying by colour alone: dark colour may come from heavy minerals rather than a mafic bulk composition.
- Forgetting that marble effervesces in acid and quartzite does not.
The atmosphere, hydrosphere and climate system
This independent teaching chapter develops the atmosphere, hydrosphere and climate system. Apply each concept to observable evidence, state the assumptions of calculations, and distinguish a plausible interpretation from a conclusion demonstrated by the data. Regional examples are illustrative subject context, not a prescribed regional paper, official weighting or claim about current local measurements.
Atmosphere
Composition by volume: N₂ 78 %, O₂ 21 %, Ar 0.93 %, CO₂ about 0.04 %, plus variable water vapour. Layers: troposphere (average lapse rate 6.5 °C/km, most ordinary weather), stratosphere (ozone absorbs ultraviolet, temperature rises with height), mesosphere, thermosphere and exosphere.
Energy balance and greenhouse effect
Incoming short-wave radiation balances reflected short-wave radiation (albedo, Earth average about 30 %) plus outgoing long-wave radiation. Greenhouse gases absorb and re-emit long-wave radiation, so the surface stays warmer than the effective radiating temperature. Feedbacks: water vapour and ice–albedo amplify warming; the magnitude and regional expression of cloud feedbacks and ocean carbon uptake depend on the process and conditions studied. Distinguish uncertainty in a response from absence of evidence that the process occurs.
Circulation and hydrosphere
Uneven insolation plus the Coriolis effect produce Hadley, Ferrel and polar cells, trade winds, the westerlies, the polar front and the jet stream. The hydrological cycle moves water between ocean, atmosphere and land through precipitation, evaporation, runoff and infiltration. Thermohaline circulation is driven by temperature and salinity; the North Atlantic Oscillation is relevant to Iberian rainfall variability, while ENSO connections depend on season, region and the wider circulation pattern.
Worked example
Suppose an exercise explicitly prescribes an environmental temperature gradient of 6.5°C/km between 500 m and 2,500 m, starting at 20°C. The height difference is 2 km, so the model gives 20-13=7°C. That calculation samples a prescribed environmental profile; it does not model an unsaturated rising parcel. The dry adiabatic parcel rate is about 9.8°C/km, and a saturated parcel rate varies with conditions. Without moisture and pressure information, do not invent a more realistic summit temperature.
Traps that cost marks
- Saying temperature falls with height in every layer; it rises in the stratosphere.
- Confusing the greenhouse effect with the ozone hole: different gases, altitudes and mechanisms.
- Treating weather and climate as synonyms, or citing one cold winter as evidence against warming.
- Confusing El Niño with La Niña, or placing the North Atlantic Oscillation in the Pacific.
Environmental systems: biodiversity, soils, water and energy resources
This independent teaching chapter develops environmental systems: biodiversity, soils, water and energy resources. Apply each concept to observable evidence, state the assumptions of calculations, and distinguish a plausible interpretation from a conclusion demonstrated by the data. Regional examples are illustrative subject context, not a prescribed regional paper, official weighting or claim about current local measurements.
Biodiversity
Levels are genetic, species and ecosystem. Diversity is measured by richness and evenness (Shannon index). Mediterranean Iberia is a hotspot of endemism with distinctive steppe birds; the Natura 2000 network protects habitats. Main threats are habitat fragmentation, invasive species, fires and climate change.
Soils
A profile runs O, A, B, C, R. Formation depends on climate, organisms, relief, parent material and time. Texture (sand, silt, clay) controls drainage and fertility; pH, organic matter and cation exchange capacity are the key chemical variables. Water erosion is estimated with the Universal Soil Loss Equation (R × K × LS × C × P); salinisation follows irrigation with poor drainage, as in parts of the Ebro and La Mancha depressions.
Water
Renewable resources depend on recharge; aquifers are free (unconfined) or confined, and fossil water is non-renewable. Overexploitation lowers the water table, dries springs such as those feeding Daimiel, causes subsidence and allows saline intrusion. Pollution is dominated by agricultural nitrates and diffuse pesticides, and demand is dominated by irrigation.
Energy
Non-renewable sources (coal, oil, gas, nuclear) versus renewable (wind, solar, hydro, biomass, geothermal). Spain's wind and photovoltaic capacity is large, but intermittency requires storage and grid management. Energy return on investment and life-cycle emissions are better criteria than a simple "clean" label.
Worked example
An aquifer receives 45 mm/yr of recharge over 500 km². Reasoning: 45 mm = 0.045 m, so recharge = 0.045 × 5 × 10⁸ m² = 2.25 × 10⁷ m³/yr, that is 22.5 hm³/yr. If abstraction is 40 hm³/yr, pumping exceeds the stated recharge by 17.5 hm³/yr. In a deliberately simplified balance with no other inflows or outflows, storage decreases by that amount. A real aquifer also requires natural discharge, boundary flows, storage properties and measurement uncertainty.
Traps that cost marks
- Confusing recharge with total precipitation.
- Claiming all aquifers are renewable.
- Confusing biodiversity with abundance of individuals rather than species richness.
- Describing soil as inert dirt and omitting organic matter and soil biota.
Natural hazards, environmental impacts and risk management
This independent teaching chapter develops natural hazards, environmental impacts and risk management. Apply each concept to observable evidence, state the assumptions of calculations, and distinguish a plausible interpretation from a conclusion demonstrated by the data. Regional examples are illustrative subject context, not a prescribed regional paper, official weighting or claim about current local measurements.
Core concepts
A common conceptual model links risk to hazard, vulnerability and exposure. A literal product requires compatible, explicitly defined quantities; hazard includes event characteristics and likelihood, not one universally interchangeable number. Vulnerability covers physical, social and economic fragility; exposure is the value located in the hazardous zone. A hazard becomes a disaster only when it meets vulnerable, exposed populations.
Hazards in Spain
- Seismic: the Betic cordillera and the Pyrenees; the 2011 Lorca earthquake (magnitude 5.1) caused nine deaths through vulnerable masonry.
- Volcanic: the Canary Islands, including the 2021 Tajogaite eruption on La Palma.
- Floods: Mediterranean torrents and ramblas, flash floods in the Júcar and Segura basins.
- Mass movements: landslides in steep marl and flysch terrain.
- Karst collapse and sinkholes in evaporite and limestone areas of the Ebro and La Mancha basins.
- Drought, coastal erosion and desertification in the southeast.
Impacts and management
Environmental impact assessment predicts effects and proposes corrective measures, often using Leopold-type matrices. Mitigation includes land-use zoning, building codes, early-warning networks, reforestation of headwaters, check dams and the flood-risk maps required by the EU Floods Directive.
Worked example
Two towns face the same 1-in-100-year flood. Town A has floodplain housing and no warning system; Town B is built above flood level with a warning plan. Reasoning: the hazard is identical, but exposure and vulnerability differ, so risk is far higher in A; reducing exposure through zoning cuts risk more than any measure aimed at changing the hazard itself.
Traps that cost marks
- Using hazard and risk as synonyms.
- Claiming zero risk is achievable, or that a 100-year event occurs exactly once a century.
- Blaming a hazard for a disaster without discussing vulnerability.
- Confusing prevention (avoid exposure), prediction (forecast) and mitigation (reduce damage).
Geological and environmental field interpretation: maps, cross-sections and data
This independent teaching chapter develops geological and environmental field interpretation: maps, cross-sections and data. Apply each concept to observable evidence, state the assumptions of calculations, and distinguish a plausible interpretation from a conclusion demonstrated by the data. Regional examples are illustrative subject context, not a prescribed regional paper, official weighting or claim about current local measurements.
Topographic maps
- Scale: 1:25,000 means 1 cm = 250 m. Always convert units before computing.
- Contour lines: equidistance is the vertical spacing; steeper slopes give closer contours.
- Gradient (%) = (vertical difference / horizontal distance) × 100.
- Profiles: plot every contour crossing at its true altitude and join smoothly; vertical exaggeration is the vertical-to-horizontal representative-fraction ratio. For horizontal 1:50,000 and vertical 1:5,000, it is(1/5,000)/(1/50,000)=10.
Geological maps and sections
- Strike and dip symbols give bed attitude; dip direction is perpendicular to strike.
- A contact crossing a valley makes an outcrop pattern controlled by both bed attitude and the topographic slope. Compare the dip with the valley gradient and check a cross-section; a V pointing one way is not a universal dip-direction rule.
- Outcrop width depends on bed thickness, dip and topography.
- Faults: recognised by offset marker beds, repetition or omission of strata, and drag folds. Faults can produce repetition or omission in particular traverses or boreholes; determine hanging-wall motion and section geometry before assigning the fault type.
- Folds: anticlines have the oldest beds in the core, synclines the youngest; plunging folds give converging outcrop traces.
- Borehole logs and geophysics extend interpretation below the surface.
Worked example
On a 1:50,000 map, two points are 6 cm apart and lie at 400 m and 700 m. Reasoning: the real distance is 6 × 50,000 cm = 300,000 cm = 3 km; the gradient is 300 m / 3,000 m = 0.10, that is 10 % or about 5.7°.
Traps that cost marks
- Forgetting to convert centimetres to metres or kilometres.
- Measuring a straight line when the route follows a valley.
- Reversing the sense of a fault, or calling an anticline a syncline because the outcrop closes.
- Quoting vertical exaggeration without units, or treating unusual contour geometry without considering the terrain and map convention.
- Confusing apparent dip on a section with true dip in the field.