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Castilla y León PAU Geology and Environmental Sciences study materials
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Castilla y León PAU Geology and Environmental Sciences practice questions

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Question 1Geology, Earth systems, resources, hazards and environmental evidence

Which type of seismic body wave is compressional (longitudinal), travels fastest through the Earth, and can propagate through both solid and liquid layers?

  1. P waves (Primary waves) Correct answer.
  2. S waves (Secondary waves)
  3. Love waves
  4. Rayleigh waves

Explanation

Primary (P) waves are compressional longitudinal waves that travel fastest through Earth's interior and can pass through both solids and liquids because fluids resist compression. Secondary (S) waves are shear waves that cannot travel through liquids, while Love and Rayleigh waves are surface waves.

Question 2Geology, Earth systems, resources, hazards and environmental evidence

Which seismic discontinuity marks the boundary between Earth's crust and the uppermost mantle?

  1. Gutenberg discontinuity
  2. Mohorovičić (Moho) discontinuity Correct answer.
  3. Lehmann discontinuity
  4. Repetti discontinuity

Explanation

The Mohorovičić (Moho) discontinuity marks the boundary between the crust and mantle, characterized by a sharp increase in seismic wave velocity due to the change from mafic/felsic crustal rocks to ultramafic mantle peridotite. The Gutenberg discontinuity separates the mantle from the liquid outer core, and the Lehmann discontinuity separates the liquid outer core from the solid inner core.

Question 3Geology, Earth systems, resources, hazards and environmental evidence

What key mechanical property distinguishes the lithosphere from the underlying asthenosphere?

  1. The lithosphere is rigid and brittle, whereas the asthenosphere is ductile and capable of slow plastic flow. Correct answer.
  2. The lithosphere is entirely liquid metal, whereas the asthenosphere consists of solid mantle silicates.
  3. The lithosphere undergoes high-grade metamorphism, whereas the asthenosphere has no pressure or temperature gradients.
  4. The lithosphere lacks tectonic plates, whereas the asthenosphere is divided into rigid moving tectonic plates.

Explanation

The lithosphere encompasses the crust and uppermost mantle, behaving as a rigid, brittle solid that breaks into tectonic plates. The asthenosphere lies below in the upper mantle, where higher temperatures permit plastic, ductile flow under stress.

Question 4Geology, Earth systems, resources, hazards and environmental evidence

At mid-ocean ridges, what geological process accounts for the formation of new oceanic lithosphere?

  1. Subduction of continental crust into the lower mantle
  2. Decompressional melting of ascending mantle peridotite generating basaltic magma Correct answer.
  3. High-pressure contact metamorphism of deep-sea sediments
  4. Tectonic collision creating thick continental roots

Explanation

As tectonic plates diverge at mid-ocean ridges, asthenospheric peridotite rises into areas of lower pressure. This decompressional melting generates basaltic magma that erupts or cools intrusively to form new oceanic crust.

Question 5Geology, Earth systems, resources, hazards and environmental evidence

In an oceanic-continental subduction zone, why does the oceanic plate subduct beneath the continental plate?

  1. Oceanic lithosphere is denser and thinner than continental lithosphere. Correct answer.
  2. Oceanic lithosphere is less dense and thicker than continental lithosphere.
  3. Continental lithosphere contains higher proportions of iron and magnesium silicates.
  4. Oceanic lithosphere is preserved from subduction by high geothermal gradients.

Explanation

Oceanic lithosphere is composed of dense mafic rock (basalt and gabbro) and cools as it ages, making it denser than the buoyant, felsic (granitic) continental lithosphere. Consequently, the denser oceanic plate sinks into the asthenosphere.

Question 6Geology, Earth systems, resources, hazards and environmental evidence

Which tectonic feature is produced where two lithospheric plates slide horizontally past one another without creating or destroying lithosphere?

  1. Deep ocean trench
  2. Transform fault Correct answer.
  3. Volcanic island arc
  4. Continental rift valley

Explanation

Transform plate boundaries (or conservative margins) involve strike-slip fault movement where plates slide horizontally past each other. Lithosphere is neither created nor consumed along transform faults.

Question 7Geology, Earth systems, resources, hazards and environmental evidence

Which mechanism is currently considered the dominant gravitational force driving plate motion in subduction zones?

  1. Basal traction from asthenospheric drag
  2. Slab pull generated by the negative buoyancy of cold, dense subducting slabs Correct answer.
  3. Ridge push resulting from gravitational sliding at transform offsets
  4. Tidal forces exerted by the Moon and Sun

Explanation

Slab pull is the primary driving force of plate tectonics. As old oceanic lithosphere cools, it becomes denser than the underlying asthenosphere and metamorphoses into dense eclogite upon subduction, exerting a strong gravitational pull that drags the rest of the plate downward.

Question 8Geology, Earth systems, resources, hazards and environmental evidence

Why does addition of water (flux melting) cause magma generation in subduction zones?

  1. Water increases the melting temperature of mantle silicates.
  2. Water lowers the solidus temperature of mantle peridotite. Correct answer.
  3. Water reacts with iron to produce exothermic nuclear reactions.
  4. Water converts felsic granite into dense ultrabasic cumulates.

Explanation

Dehydration of subducting oceanic crust releases hydrous fluids (water and volatiles) into the overlying mantle wedge. These fluids act as fluxing agents, lowering the solidus melting temperature of mantle peridotite and triggering partial melting.

Question 9Geology, Earth systems, resources, hazards and environmental evidence

Which combination of magma characteristics promotes highly explosive volcanic eruptions?

  1. Low silica content, low viscosity, and low dissolved gas content
  2. High silica content, high viscosity, and high dissolved gas content Correct answer.
  3. High iron-magnesium content, high temperature, and low gas content
  4. Low silica content, high temperature, and high fluid flow

Explanation

Felsic/rhyolitic magmas have high silica content (SiO2 > 65%), making them highly viscous. High viscosity prevents dissolved gases (H2O, CO2) from escaping easily; gas pressure builds up until violent, explosive pyroclastic eruptions occur.

Question 10Geology, Earth systems, resources, hazards and environmental evidence

What is a batholith?

  1. A small tabular discordant pluton that cuts across bedding planes
  2. A massive discordant intrusive igneous body with an exposed surface area exceeding 100 square kilometers Correct answer.
  3. A concordant lens-shaped intrusion that arches the overlying strata upward
  4. A tabular concordant igneous intrusion injected parallel to sedimentary rock layers

Explanation

A batholith is the largest type of pluton, defined as a discordant intrusive igneous body with an surface exposure greater than 100 km2, typically composed of granitic or granodioritic rocks formed in continental arc roots.

Question 11Geology, Earth systems, resources, hazards and environmental evidence

In a simple anticline fold, where are the oldest rock layers located?

  1. Along the outer limbs of the fold
  2. In the core along the fold axis Correct answer.
  3. Exclusively at the topographic trough
  4. Distributed randomly across axial surfaces

Explanation

An anticline is an arch-like fold in which rock layers dip away from the axial plane. By stratigraphic principle, the oldest strata are located in the center (core) of the fold.

Question 12Geology, Earth systems, resources, hazards and environmental evidence

What type of stress and movement defines a normal fault?

  1. Compressional stress where the hanging wall moves upward relative to the footwall
  2. Tensional stress where the hanging wall moves downward relative to the footwall Correct answer.
  3. Shear stress where blocks slide horizontally past each other
  4. Tensional stress where the footwall drops downward while the hanging wall rises

Explanation

Normal faults result from tensional (extensional) tectonic forces. Under tension, gravity causes the hanging wall block to move downward relative to the footwall block.

Question 13Geology, Earth systems, resources, hazards and environmental evidence

How is a thrust fault distinguished from a standard reverse fault?

  1. A thrust fault is a low-angle reverse fault with a dip angle generally less than 45 degrees. Correct answer.
  2. A thrust fault forms under tensional stress rather than compressional stress.
  3. A thrust fault has pure vertical displacement without horizontal movement.
  4. A thrust fault occurs exclusively in basaltic oceanic crust.

Explanation

Both thrust faults and reverse faults are caused by compressional stress that pushes the hanging wall up over the footwall. A thrust fault is specifically a reverse fault with a low dip angle (less than or equal to 45 degrees, often less than 30 degrees), allowing large horizontal transport of rock sheets (nappes).

Question 14Geology, Earth systems, resources, hazards and environmental evidence

Which type of metamorphism affects vast regions of continental crust during orogenic mountain-building events, driven by high temperature and directed pressure?

  1. Contact (thermal) metamorphism
  2. Regional (dynamothermal) metamorphism Correct answer.
  3. Cataclastic (fault-zone) metamorphism
  4. Hydrothermal metamorphism

Explanation

Regional (dynamothermal) metamorphism occurs over large continental areas during plate collisions and orogenies. It is driven by both high temperatures and directed differential pressure, creating foliated rocks such as slate, phyllite, schist, and gneiss.

Question 15Geology, Earth systems, resources, hazards and environmental evidence

Which metamorphic facies forms under conditions of very high pressure but relatively low temperature, characteristic of subduction zone environments?

  1. Greenschist facies
  2. Blueschist facies Correct answer.
  3. Hornfels facies
  4. Granulite facies

Explanation

Blueschist facies metamorphism occurs in subduction zones where cold oceanic slabs sink rapidly, subjecting rocks to high pressures at relatively low temperatures. The characteristic blue color comes from the glaucophane amphibole mineral.

Question 16Geology, Earth systems, resources, hazards and environmental evidence

How do hotspot volcanic chains (such as the Hawaiian-Emperor chain) provide evidence for plate motion?

  1. A stationary mantle plume melts passing lithospheric plates, forming a linear age-progressive chain of volcanoes. Correct answer.
  2. The hotspot moves rapidly across static oceanic lithosphere in random directions.
  3. Hotspots only form along active subduction zone trenches.
  4. Hotspot volcanoes erupt simultaneously along the entire length of the chain.

Explanation

Mantle plumes originate deep in the mantle (D'' layer) and remain relatively stationary. As a lithospheric plate moves over the stationary plume, a chain of volcanic islands forms with ages increasing systematically in the direction of plate motion.

Question 17Geology, Earth systems, resources, hazards and environmental evidence

What paleomagnetic evidence confirmed Fred Vine and Drummond Matthews' hypothesis of seafloor spreading?

  1. Symmetrical bands of normal and reversed magnetic polarities recorded in ocean floor basalts on either side of mid-ocean ridges Correct answer.
  2. Uniform magnetic intensity across all continental landmasses regardless of age
  3. Absence of magnetic minerals in oceanic crust compared to continental crust
  4. Random orientation of magnetic dip angles in basaltic lavas at ridge axes

Explanation

Basaltic magma cooling at mid-ocean ridges locks in Earth's prevailing magnetic field direction. As Earth's magnetic field periodically reverses, symmetrical stripes of normal and reversed magnetic anomalies form parallel to and centered on mid-ocean ridges.

Question 18Geology, Earth systems, resources, hazards and environmental evidence

Which major tectonic event during the Late Paleozoic formed the crystalline basement of the Iberian Massif in central and western Spain?

  1. Alpine Orogeny
  2. Variscan (Hercynian) Orogeny Correct answer.
  3. Caledonian Orogeny
  4. Cadomian Orogeny

Explanation

The Variscan (or Hercynian) Orogeny resulted from the collision of Gondwana and Laurussia in the Late Paleozoic (Devonian-Carboniferous) to form Pangaea. This collision folded, metamorphosed, and intruded granites into the Paleozoic rocks that form the Iberian Massif.

Question 19Geology, Earth systems, resources, hazards and environmental evidence

How did the Alpine Orogeny affect the Mesozoic-Cenozoic relief of Castilla y León?

  1. It uplifted the Cantabrian Mountains and Central System while forming the Cenozoic Duero Basin. Correct answer.
  2. It completely submerged the entire Iberian Peninsula under a permanent deep ocean.
  3. It produced extensive oceanic crust spreading within the interior of the Iberian Massif.
  4. It leveled all Iberian relief into a featureless marine peneplain.

Explanation

The Cenozoic Alpine convergence between the African and Eurasian plates uplifted peripheral ranges (Cantabrian Mountains, Central System, Iberian System) and caused flexural subsidence of the continental interior, forming the massive Duero sedimentary basin in Castilla y León.

Question 20Geology, Earth systems, resources, hazards and environmental evidence

According to the principle of isostasy, what happens to a mountain belt as weathering and erosion remove thousands of meters of surface rock?

  1. The crustal root sinks deeper into the mantle due to loss of surface friction.
  2. The mountain belt experiences post-glacial rebound without crustal modification.
  3. The crustal root rises (isostatic uplift) as weight is removed, maintaining gravitational equilibrium. Correct answer.
  4. The entire mountain belt converts instantaneously into oceanic crust.

Explanation

Isostasy is the buoyant equilibrium between Earth's lithosphere and asthenosphere. As surface erosion removes weight from a mountain range, buoyancy causes the deep crustal root to rebound upward (isostatic uplift), replacing about 80-85% of the eroded thickness.

Question 21Geology, Earth systems, resources, hazards and environmental evidence

Which physical weathering process is most dominant in mountainous regions where temperatures frequently fluctuate above and below 0 degrees Celsius?

  1. Frost wedging (gelifraction) Correct answer.
  2. Carbonation
  3. Hydrolysis
  4. Haloclasty

Explanation

Frost wedging (gelifraction) occurs when water fills rock fractures and expands by roughly 9% upon freezing. Freeze-thaw cycles exert immense pressure against fracture walls, shattering rock in high mountain environments such as the Cantabrian and Central mountains.

Question 22Geology, Earth systems, resources, hazards and environmental evidence

What chemical weathering reaction converts potassium feldspar (KAlSi3O8) into clay minerals (such as kaolinite) in the presence of acidic groundwater?

  1. Oxidation
  2. Hydrolysis Correct answer.
  3. Dissolution
  4. Thermal expansion

Explanation

Hydrolysis is a chemical weathering process in which H+ ions in carbonic acid replace cations in silicate mineral lattices. Potassium feldspar reacts with hydrogen ions and water to yield soluble potassium, silica, and insoluble clay minerals like kaolinite.

Question 23Geology, Earth systems, resources, hazards and environmental evidence

Which soil horizon is typically enriched in organic matter, humus, and microbial life, forming the upper topsoil?

  1. C horizon
  2. A horizon Correct answer.
  3. B horizon
  4. R horizon

Explanation

The A horizon (topsoil) contains accumulated decomposed organic matter (humus) mixed with mineral particles, supporting dense plant root networks and biological activity.

Question 24Geology, Earth systems, resources, hazards and environmental evidence

In the upper reach of a river system, what erosion pattern and valley shape are predominantly developed?

  1. Broad, flat-floored valleys with extensive meanders and oxbow lakes
  2. V-shaped valleys dominated by vertical incision and steep gradients Correct answer.
  3. U-shaped glacial troughs with hanging valleys
  4. Flat deltaic plains with distributary channels

Explanation

In upper reaches (headwaters), steep slopes produce high kinetic stream energy that causes downward vertical erosion (incision), cutting sharp V-shaped valleys with waterfalls and rapids.

Question 25Geology, Earth systems, resources, hazards and environmental evidence

How are sand and gravel particles primarily transported along a riverbed during saltation?

  1. Dissolved as chemical ions within water molecules
  2. Bouncing and skipping along the riverbed in response to fluid lift and turbulent flow Correct answer.
  3. Floating permanently on the water surface due to surface tension
  4. Carried continuously in clay-rich suspension without bed contact

Explanation

Saltation is bedload transport where medium-sized particles (sand and small gravel) bounce and skip along the channel bed as hydraulic lift forces temporarily lift them into the current before gravity pulls them down.

Question 26Geology, Earth systems, resources, hazards and environmental evidence

Which type of rock is most susceptible to karstic chemical dissolution by carbonic acid?

  1. Granite
  2. Limestone Correct answer.
  3. Basalt
  4. Quartzite

Explanation

Limestone (composed mainly of calcite, CaCO3) readily dissolves in acidic water containing dissolved carbon dioxide (H2CO3), forming characteristic karst features like caves, sinkholes, and underground drainage.

Question 27Geology, Earth systems, resources, hazards and environmental evidence

What karstic landform is described as a closed, bowl-shaped surface depression formed by dissolution or collapse of underlying carbonate rock?

  1. Doline (sinkhole) Correct answer.
  2. Polje
  3. Lapiez (grike)
  4. Speleothem

Explanation

A doline (or sinkhole) is a closed circular or elliptical surface depression typical of karst terrain, created by solution of surface carbonate rock or collapse of an underground cave roof.

Question 28Geology, Earth systems, resources, hazards and environmental evidence

Which cross-sectional valley shape is characteristic of erosion carved by valley glaciers?

  1. Sharp V-shape with interlocking spurs
  2. Broad U-shape with steep vertical walls and flat floors Correct answer.
  3. Steep asymmetrical canyon with step terraces
  4. Circular sinkhole depression

Explanation

Glaciers erode valleys through plucking and abrasion across their entire base and sides, widening, deepening, and straightening pre-existing river channels into steep-walled, flat-bottomed U-shaped glacial troughs.

Question 29Geology, Earth systems, resources, hazards and environmental evidence

What is glacial till?

  1. Well-sorted, cross-bedded sand deposited by glacial meltwater streams
  2. Unsorted, unstratified sediment deposited directly by melting glacial ice Correct answer.
  3. Fine wind-blown silt accumulated in periglacial steppe regions
  4. Varved clay layers deposited annually in calm proglacial lakes

Explanation

Till is sediment deposited directly by glacial ice without water sorting. It is unsorted and unstratified, containing a heterogeneous mixture of particle sizes ranging from fine clay to giant boulders (glacial erratics).

Question 30Geology, Earth systems, resources, hazards and environmental evidence

Which dune type is crescent-shaped with horns pointing downwind, forming in areas with limited sand supply and a single dominant wind direction?

  1. Barchan dune Correct answer.
  2. Parabolic dune
  3. Star dune
  4. Transverse dune

Explanation

Barchan dunes are solitary, crescent-shaped sand dunes that form where sand supply is sparse and wind blows consistently from one direction. The tips (horns) point downwind in the direction of sand transport.

Question 31Geology, Earth systems, resources, hazards and environmental evidence

What coastal feature is carved at the base of a rocky cliff by continuous wave impact, leading to cliff collapse and retreat?

  1. Wave-cut notch Correct answer.
  2. Tombolo
  3. Barrier island
  4. Spit

Explanation

Wave action (abrasion and hydraulic action) concentrates energy at high-tide level, undercutting the cliff face to form a wave-cut notch. As the notch deepens, the unsupported cliff above collapses, causing cliff retreat and leaving a wave-cut platform.

Question 32Geology, Earth systems, resources, hazards and environmental evidence

What form of mass movement involves the slow, imperceptible downslope movement of soil and regolith driven by freeze-thaw or wet-dry expansion cycles?

  1. Rock avalanche
  2. Soil creep Correct answer.
  3. Debris flow
  4. Slump

Explanation

Soil creep is the extremely slow (millimeters to centimeters per year) downslope movement of soil. Freeze-thaw or wetting-drying cycles lift particles perpendicular to the slope, while gravity pulls them directly downward upon settling.

Question 33Geology, Earth systems, resources, hazards and environmental evidence

Why is solifluction particularly active in periglacial environments such as tundra regions?

  1. Summer thaw melts the surface soil layer while underlying permafrost remains impermeable, saturating the topsoil. Correct answer.
  2. Extreme wind deflation removes all fine clay particles, leaving dry gravel slopes.
  3. Carbonate dissolution creates subterranean caverns that collapse continuously.
  4. Earthquake shocks trigger liquefaction in dry desert sand dunes.

Explanation

Solifluction occurs in cold climates where the active upper soil layer thaws in summer. Because the deeper subsoil remains permanently frozen permafrost, melted water cannot drain downward, causing the saturated surface soil to flow slowly downslope over the frozen substrate.

Question 34Geology, Earth systems, resources, hazards and environmental evidence

What sedimentological characteristic defines the Cenozoic filling of the Duero Basin in Castilla y León?

  1. Deep-sea abyssal turbidite deposits with oceanic basalts
  2. Continental endorheic to exorheic detrital and evaporitic sediments (clays, sandstones, conglomerates, and gypsums) Correct answer.
  3. Glacial till deposits covering active volcanic shield flows
  4. Pure coral reef limestone formations accumulated in tropical seas

Explanation

The Duero Basin was filled during the Cenozoic by terrestrial sediments eroded from surrounding mountain ranges (Central, Cantabrian, and Iberian systems). Deposits transition from alluvial fan conglomerates and sandstones at basin margins to lacustrine clays, marls, and evaporitic gypsums in the center.

Question 35Geology, Earth systems, resources, hazards and environmental evidence

Which diagenetic process involves the precipitation of dissolved minerals in pore spaces, binding loose sediment grains into hard sedimentary rock?

  1. Compaction
  2. Cementation Correct answer.
  3. Recrystallization
  4. Dissolution

Explanation

Cementation occurs when mineral-rich groundwater percolates through loose sediment pores and precipitates cementing minerals (such as calcite, quartz, or hematite), gluing sediment grains together into solid rock.

Question 36Geology, Earth systems, resources, hazards and environmental evidence

Which agricultural technique helps reduce water-driven soil erosion on steep hillsides?

  1. Contour plowing and terracing Correct answer.
  2. Overgrazing and clear-cutting
  3. Deep vertical plowing parallel to the slope gradient
  4. Removal of field windbreaks and vegetation strips

Explanation

Contour plowing (plowing across slopes along elevation lines) and terracing create ridges and stepped platforms that catch runoff, slowing water velocity and dramatically reducing soil erosion.

Question 37Geology, Earth systems, resources, hazards and environmental evidence

How do granitic tors (such as those in the Central System of Spain) form through two-stage weathering?

  1. Deep subsurface chemical weathering along orthogonal fracture grids, followed by surface erosion stripping the weathered grus. Correct answer.
  2. Direct volcanic lava flows cooling into hexagonal columns at surface high pressure.
  3. Glacial gouging stripping un-fractured basalt plains.
  4. Subduction trench compression pushing limestone blocks up.

Explanation

Tors form in two stages: first, deep subterranean chemical weathering (hydrolysis) alters granite along intersecting vertical/horizontal joints into loose sand (grus); second, erosion strips away the soft grus, exposing rounded corestones (tors) at the surface.

Question 38Geology, Earth systems, resources, hazards and environmental evidence

What drives the longshore transport of beach sediment parallel to the coastline?

  1. Waves approaching the shoreline at an oblique angle, generating a current parallel to the coast Correct answer.
  2. Tidal currents flowing perpendicular to the beach face during high tide
  3. Deep ocean thermohaline circulation upwelling at the shelf break
  4. Wind deflation lifting quartz sand grains into the upper troposphere

Explanation

When waves strike a coast at an angle, swash carries sand up the beach at an angle, while backwash pulls it straight down the slope under gravity. This zig-zag motion produces longshore drift parallel to the shoreline.

Question 39Geology, Earth systems, resources, hazards and environmental evidence

Which fundamental stratigraphic law states that in an undeformed sequence of sedimentary rocks, each layer is older than the one above it and younger than the one below it?

  1. Law of Original Horizontality
  2. Law of Superposition Correct answer.
  3. Principle of Cross-Cutting Relationships
  4. Principle of Faunal Succession

Explanation

Steno's Law of Superposition states that in an undisturbed sequence of sedimentary strata or volcanic flows, the oldest layers reside at the bottom and younger layers lie progressively higher.

Question 40Geology, Earth systems, resources, hazards and environmental evidence

What type of unconformity separates tilted or folded older sedimentary rocks from overlying horizontal younger sedimentary strata?

  1. Disconformity
  2. Angular unconformity Correct answer.
  3. Nonconformity
  4. Paraconformity

Explanation

An angular unconformity occurs when older strata were folded or tilted by deformation, eroded at the surface, and later covered by flat-lying younger sedimentary layers, showing a clear angular mismatch.

Question 41Geology, Earth systems, resources, hazards and environmental evidence

Which natural process acts as a major long-term geological sink for atmospheric carbon dioxide over million-year timescales?

  1. Volcanic degassing at subduction zone arcs
  2. Silicate mineral weathering coupled with marine carbonate deposition Correct answer.
  3. Microbial respiration in topsoil layers
  4. Combustion of fossil fuels

Explanation

Atmospheric CO2 reacts with rainwater to form carbonic acid, which weathers continental silicate rocks (CaSiO3). The resulting calcium and bicarbonate ions wash into oceans, where marine organisms build CaCO3 shells that precipitate into limestone, trapping carbon for millions of years.

Question 42Geology, Earth systems, resources, hazards and environmental evidence

Which specialized group of prokaryotes converts inert atmospheric nitrogen gas (N2) into bioavailable ammonia (NH3)?

  1. Denitrifying bacteria
  2. Nitrogen-fixing bacteria (such as Rhizobium and Azotobacter) Correct answer.
  3. Nitrifying bacteria (Nitrosomonas and Nitrobacter)
  4. Decomposing fungi

Explanation

Nitrogen-fixing bacteria possess the nitrogenase enzyme complex, allowing them to break the triple bond of atmospheric N2 and reduce it to ammonia (NH3/NH4+), which plants can assimilate.

Question 43Geology, Earth systems, resources, hazards and environmental evidence

Unlike the carbon, nitrogen, and sulfur cycles, why does the phosphorus cycle lack a significant atmospheric component?

  1. Phosphorus compounds do not form stable gas phases at Earth's surface temperatures and pressures. Correct answer.
  2. Phosphorus is completely absent from all living organisms.
  3. Phosphorus exists exclusively as high-temperature magma in the outer core.
  4. Phosphorus gas reacts instantly with nitrogen to form heavy solid metals.

Explanation

Phosphorus compounds (primarily phosphate, PO4 3-) do not form volatile gas phases under ambient surface temperatures and pressures. Consequently, phosphorus cycles locally through rocks, soil, water, and biota without a global gaseous atmospheric phase.

Question 44Geology, Earth systems, resources, hazards and environmental evidence

How is Net Primary Productivity (NPP) defined in terms of Gross Primary Productivity (GPP) and autotrophic respiration (Ra)?

  1. NPP = GPP + Ra
  2. NPP = GPP - Ra Correct answer.
  3. NPP = GPP x Ra
  4. NPP = Ra / GPP

Explanation

Gross Primary Productivity (GPP) is the total amount of chemical energy fixed by autotrophs via photosynthesis. Net Primary Productivity (NPP) is the organic matter remaining after plants meet their own metabolic needs through cellular respiration (Ra): NPP = GPP - Ra.

Question 45Geology, Earth systems, resources, hazards and environmental evidence

According to the ecological '10% rule', approximately how much energy stored at one trophic level is transferred to the next higher trophic level?

  1. 100%
  2. 50%
  3. 10% Correct answer.
  4. 1%

Explanation

On average, only about 10% of the energy stored as biomass at one trophic level is incorporated into biomass at the next trophic level. The remaining ~90% is lost as metabolic heat, unconsumed waste, and respiration.

Question 46Geology, Earth systems, resources, hazards and environmental evidence

Under what condition can an ecological pyramid of biomass appear inverted (smaller producer biomass than consumer biomass)?

  1. In open marine ecosystems where microscopic phytoplankton have extremely rapid turnover rates and short lifespans Correct answer.
  2. In temperate forest ecosystems dominated by old-growth oak trees
  3. In desert ecosystems with high carnivore mortality
  4. In agricultural fields treated with high nitrogen fertilizers

Explanation

In open ocean ecosystems, phytoplankton have a very low standing crop biomass at any instant because they are consumed rapidly by zooplankton. However, because phytoplankton reproduce extremely fast (high turnover rate), they generate enough energy to support a larger standing biomass of zooplankton.

Question 47Geology, Earth systems, resources, hazards and environmental evidence

Which scenario triggers primary ecological succession?

  1. Colonization of a newly exposed volcanic lava flow or retreat glacier foreland devoid of pre-existing soil Correct answer.
  2. Regrowth of a forest following a low-intensity wildfire
  3. Abandonment of an agricultural wheat field
  4. Recovery of a meadow after seasonal cattle grazing

Explanation

Primary succession begins on newly exposed mineral substrate completely lacking pre-existing organic soil and biological life, such as cooled lava flows, sand dunes, or retreat glacial forelands.

Question 48Geology, Earth systems, resources, hazards and environmental evidence

What key characteristic distinguishes a climax community from early successional pioneer stages?

  1. High net productivity (NPP/B >> 1) and simple food chains
  2. High species diversity, complex food webs, and dynamic equilibrium where gross production balances total respiration (P/R approx 1) Correct answer.
  3. Dominance of short-lived r-selected opportunistic weeds
  4. Extremely low biomass and open mineral soil horizons

Explanation

A climax community represents the mature, stable endpoint of succession. It features high species diversity, complex trophic networks, large biomass, and metabolic stability where total primary production roughly equals ecosystem respiration (P/R approx 1).

Question 49Geology, Earth systems, resources, hazards and environmental evidence

Which reproductive and life-history traits characterize r-selected species?

  1. Large body size, long lifespan, late maturity, and heavy parental care
  2. Small body size, short lifespan, early sexual maturity, and production of numerous offspring with minimal parental investment Correct answer.
  3. Constant population size maintained near carrying capacity (K)
  4. High competitive capability in crowded, climax communities

Explanation

r-Selected species are adapted to unstable or unpredictable environments. They maximize reproductive rate (r) by producing large numbers of offspring early in life, offering little or no parental care.

Question 50Geology, Earth systems, resources, hazards and environmental evidence

What is meant by the carrying capacity (K) of an ecosystem?

  1. The absolute maximum rate of cellular photosynthesis under ideal laboratory lighting
  2. The maximum population size of a species that a given environment can sustainably support without habitat degradation Correct answer.
  3. The minimum number of individuals required to prevent genetic drift
  4. The total mass of mineral nutrients dissolved in groundwater

Explanation

Carrying capacity (K) is the upper limit of population size that an environment's available resources (food, water, shelter, space) can sustain indefinitely without long-term degradation.