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Galicia PAU Geology and Environmental Sciences study materials
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Galicia PAU Geology and Environmental Sciences practice questions

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

Which seismic wave property explains why S-waves cannot travel through the Earth's outer core?

  1. S-waves are shear transverse waves that require a rigid medium with shear strength to propagate. Correct answer.
  2. S-waves travel faster than P-waves and are deflected by the Mohorovičić discontinuity.
  3. S-waves are longitudinal compressional waves that undergo total internal reflection at the mantle-core boundary.
  4. S-waves suffer extreme thermal attenuation caused by radioactive decay in the lower mantle.

Explanation

S-waves (secondary or shear waves) propagate by displacing particles perpendicular to the direction of wave travel, which requires shear strength. Liquids lack shear resistance and cannot transmit transverse shear stresses, preventing S-waves from passing through the liquid outer core. This absence creates an S-wave shadow zone beyond 103° from an earthquake epicenter.

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

What defines the Mohorovičić discontinuity (Moho) in Earth geophysics?

  1. A compositional boundary separating the silicate crust from the denser peridotitic mantle, marked by a sharp increase in P-wave velocity. Correct answer.
  2. A thermal boundary between the rigid lithosphere and the ductile asthenosphere where partial melting decreases P-wave velocity.
  3. A phase-transition boundary between spinel and perovskite crystal structures within the mantle transition zone.
  4. A liquid-solid phase boundary separating the iron-nickel outer core from the solid inner core.

Explanation

The Mohorovičić discontinuity (Moho) is the seismic and compositional boundary between Earth's crust and upper mantle. Across the Moho, P-wave velocities abruptly increase from approximately 6–7 km/s in the crust to over 8 km/s in the peridotite-rich mantle. Its depth varies from about 5–10 km under oceanic crust to 30–70 km under continental crust.

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

What is the primary driving mechanism of plate tectonics supported by modern geodynamic models?

  1. Mantle thermal convection combined with slab pull at subduction zones and ridge push at mid-ocean ridges. Correct answer.
  2. Gravitational tidal forces exerted by the Moon and Sun pulling the oceanic lithosphere westward.
  3. Centrifugal forces generated by Earth's axial rotation driving continental drift toward the equator.
  4. Convective currents restricted exclusively to the liquid iron-nickel outer core transferring shear stress to the crust.

Explanation

Plate tectonics is driven primarily by mantle thermal convection, where dense, cold subducting lithospheric slabs sink into the mantle under gravity (slab pull), providing the largest driving force. Supplementary forces include ridge push, caused by the gravitational sliding of warm, elevated lithosphere away from mid-ocean ridges, and basal drag from convective mantle flow.

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

During which stage of the Wilson Cycle does continental collision occur, closing an ocean basin and forming mountain belts?

  1. Suture (collision) stage. Correct answer.
  2. Embryonic stage.
  3. Juvenile stage.
  4. Terminal stage.

Explanation

The Wilson Cycle describes the cyclical opening and closing of ocean basins. The suture or continental collision stage occurs when subduction fully consumes the oceanic lithosphere separating two continental masses, causing them to collide, crumple, and form major collisional orogens (such as the Himalayas or the ancient Hercynian belt).

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

How do symmetrical magnetic anomaly stripes on the ocean floor support the theory of seafloor spreading?

  1. They record periodic reversals of Earth's magnetic field frozen into basaltic oceanic crust as it crystallizes and spreads from mid-ocean ridges. Correct answer.
  2. They indicate that continental rocks periodically gain and lose iron content during intense metamorphic episodes.
  3. They demonstrate that solar wind variations induce localized magnetism in marine sedimentary layers near oceanic trenches.
  4. They show that Earth's mantle undergoes magnetic polarity inversions every 100 years.

Explanation

As basaltic magma erupts at mid-ocean ridges, iron-rich minerals like magnetite cool below their Curie temperature (~580°C) and align with Earth's prevailing magnetic field (thermoremanent magnetization). Because Earth's geomagnetic field periodically undergoes polarity reversals, seafloor spreading creates a symmetrical pattern of normal and reversed magnetic anomaly stripes parallel to ridge crests.

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

Why does subduction-zone magmatism typically produce explosive andesitic and rhyolitic eruptions rather than effusive basaltic flows?

  1. Hydration of the mantle wedge lowers the melting point, generating magmas enriched in silica and volatiles that increase viscosity and gas pressure. Correct answer.
  2. Oceanic trenches act as thermal insulators that heat subducting basalt to extreme temperatures without volatile involvement.
  3. Subducting slabs release pure carbon dioxide that solidifies into diamond pipes upon reaching the surface.
  4. Magma in subduction zones is depleted in silica, allowing gases to escape freely without building pressure.

Explanation

In subduction zones, dehydrating oceanic crust releases water and volatiles into the overlying mantle wedge, lowering the mantle peridotite solidus and inducing partial melting. As the resulting magma ascends through thick continental or arc crust, assimilation and fractional crystallization increase its silica content and volatile concentration, producing high-viscosity magmas that trap expanding gases and trigger explosive eruptions.

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

What is the fundamental difference between earthquake magnitude (e.g., Moment Magnitude scale) and earthquake intensity (e.g., Modified Mercalli scale)?

  1. Magnitude quantifies the absolute energy released at the earthquake source, whereas intensity measures observed shaking effects and structural damage at specific surface locations. Correct answer.
  2. Magnitude varies depending on distance from the epicenter, whereas intensity remains constant for a given earthquake event.
  3. Magnitude is assessed using post-event field interviews, whereas intensity is measured exclusively by logarithmic seismograph amplitudes.
  4. Magnitude applies only to volcanic earthquakes, whereas intensity applies strictly to tectonic fault ruptures.

Explanation

Earthquake magnitude measures the physical size and total seismic energy released at the fault rupture source (a single quantitative value per event). In contrast, earthquake intensity describes the localized qualitative effects of ground shaking on humans, structures, and the natural environment at different distances from the epicenter.

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

What is the main source of Earth's internal heat driving geothermal dynamics?

  1. Radiogenic decay of long-lived isotopes (238U, 235U, 232Th, 40K) combined with primordial heat from planetary accretion and differentiation. Correct answer.
  2. Continuous nuclear fusion reactions occurring within Earth's nickel-iron inner core.
  3. Absorption of high-energy cosmic rays and solar radiation penetrating through the mantle.
  4. Frictional heating generated exclusively by oceanic tides dragging across shallow continental shelves.

Explanation

Earth's internal heat budget is sustained primarily by two sources: radiogenic heat generated by the radioactive decay of unstable isotopes (uranium-238, uranium-235, thorium-232, and potassium-40) in the mantle and crust, and primordial heat remaining from planetary accretion and core-mantle differentiation 4.5 billion years ago.

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

Which major Paleozoic tectonic event led to the formation of the Hercynian (Variscan) basement that forms the geological foundation of Galicia?

  1. The collision between Gondwana and Laurussia during the late Paleozoic, forming the supercontinent Pangea. Correct answer.
  2. The opening of the Atlantic Ocean during the early Mesozoic break-up of Pangea.
  3. The Cenozoic collision between the African and Eurasian plates that formed the Pyrenees and Alps.
  4. The Neoproterozoic breakup of Rodinia resulting in extensive basaltic rift volcanism.

Explanation

The Hercynian (Variscan) orogeny occurred during the Devonian and Carboniferous periods (approx. 380–280 Ma) when the supercontinents Gondwana and Laurussia collided to assemble Pangea. This continental collision metamorphosed, folded, and intruded the crust of Iberian Massif, creating the granitic and metamorphic basement characteristic of Galicia.

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

According to the Airy model of isostasy, how do high mountain ranges maintain gravitational equilibrium?

  1. Mountain ranges have deep crustal roots of low-density material projecting down into the denser mantle. Correct answer.
  2. Mountain ranges consist of high-density basaltic columns floating on a uniform liquid mantle layer.
  3. Mountain ranges are supported entirely by lateral compression forces with no vertical buoyancy involved.
  4. Mountain ranges have uniform crustal thickness everywhere, but varying rock densities establish balance.

Explanation

The Airy model of isostasy posits that Earth's crust has a relatively uniform density, so higher topographic elevations (mountains) are compensated by thicker crustal 'roots' extending deeper into the denser mantle, analogous to icebergs floating in water. In contrast, the Pratt model assumes varying crustal densities across blocks of equal depth.

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

How does hotspot volcanism (e.g., Hawaii) provide evidence for plate motion over stationary deep-mantle plumes?

  1. As a tectonic plate moves over a stationary mantle plume, a linear chain of age-progressive volcanoes is formed. Correct answer.
  2. Hotspots move rapidly in opposite directions while tectonic plates remain completely stationary.
  3. Hotspots erupt only during magnetic field reversals, marking historical magnetic pole positions.
  4. Mantle plumes melt the entire lithospheric plate simultaneously, creating wide circular lava plateaus without age progression.

Explanation

A deep-mantle plume rises from near the core-mantle boundary to form a localized thermal anomaly or hotspot. As an overlying lithospheric plate moves continuously across this anchored plume, magma periodically pierces the plate, generating a linear volcanic chain (such as the Hawaiian-Emperor seamount chain) in which volcanoes become progressively older with distance from the active hotspot.

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

Which seismic boundary separates Earth's mantle from the liquid outer core at a depth of approximately 2,900 km?

  1. Gutenberg discontinuity. Correct answer.
  2. Mohorovičić discontinuity.
  3. Lehmann discontinuity.
  4. Conrad discontinuity.

Explanation

The Gutenberg discontinuity lies at a depth of ~2,900 km and marks the sharp boundary between the solid silicate mantle (D'' layer) and the liquid iron-nickel outer core. At this boundary, P-wave velocity drops sharply from ~13 km/s to ~8 km/s, and S-waves terminate completely due to the outer core's fluid state.

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

What type of plate boundary is characterized by horizontal shearing motion where lithosphere is neither created nor destroyed?

  1. Transform (conservative) boundary. Correct answer.
  2. Divergent (constructive) boundary.
  3. Convergent (destructive) boundary.
  4. Subduction zone.

Explanation

Transform plate boundaries (also called conservative boundaries) occur where two tectonic plates slide past one another horizontally along transform faults (such as the San Andreas Fault or oceanic transform offsets). Because plates move laterally, crust is neither created (as at divergent ridges) nor destroyed (as at subduction zones).

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

What process triggers partial melting of mantle mantle peridotite at mid-ocean ridges?

  1. Decompression melting caused by the upward movement of hot mantle rock into lower-pressure shallow depths. Correct answer.
  2. Flux melting induced by heavy addition of water and sediment from subducting oceanic trenches.
  3. Thermal shock caused by cold seawater penetrating deep into the lower liquid outer core.
  4. Frictional heating generated by rapid strike-slip sliding along major transform faults.

Explanation

At divergent mid-ocean ridges, lithospheric extension pulls plates apart, allowing hot solid mantle peridotite to ascend adiabatically. As mantle rock rises without losing heat, the decreasing pressure drops below the peridotite solidus, inducing decompression melting and generating basaltic magma that forms new oceanic crust.

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

What causes the P-wave shadow zone observed between 104° and 140° from an earthquake epicenter?

  1. Refraction of P-waves as they pass through the low-velocity liquid outer core, bending waves inward. Correct answer.
  2. Complete absorption of P-waves by rigid continental crustal roots beneath major mountain ranges.
  3. Destructive interference between P-waves and surface Rayleigh waves across oceanic basins.
  4. Conversion of all P-waves into S-waves at the Mohorovičić discontinuity.

Explanation

When P-waves reach the mantle-core boundary (Gutenberg discontinuity), the abrupt decrease in wave velocity within the liquid outer core causes strong inward refraction (bending). This sharp deflection creates a zone on Earth's surface between 104° and 140° from the epicenter where direct P-waves fail to arrive.

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

Which geological domain of the Iberian Massif in Galicia represents exotic oceanic and island-arc terranes accreted during the Variscan collision?

  1. The Allochthonous Complexes (such as Cabo Ortegal and Órdenes). Correct answer.
  2. The Central Iberian Zone.
  3. The Asturian-Leonese Zone.
  4. The Cantabrian Zone.

Explanation

The Galicia-Trás-os-Montes Zone contains Allochthonous Complexes (including the Cabo Ortegal, Órdenes, and Morais complexes). These represent exotic terranes of oceanic lithosphere (ophiolites) and continental margin fragments that were obducted and thrust onto the Iberian passive margin during the Variscan collision in the Paleozoic.

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

How does the lithosphere differ from the asthenosphere in terms of mechanical behavior?

  1. The lithosphere is rigid and brittle, comprising the crust and uppermost mantle, while the asthenosphere is ductile and solid but capable of slow flow. Correct answer.
  2. The lithosphere is entirely liquid iron, while the asthenosphere consists of rigid granite and basalt.
  3. The lithosphere exists only under oceans, while the asthenosphere forms the continental crust.
  4. The lithosphere moves by rapid turbulent fluid flow, while the asthenosphere is completely immobile.

Explanation

Mechanical layering divides Earth's upper interior into the lithosphere and asthenosphere based on rheology. The lithosphere is cool, rigid, and brittle (breaking along faults to form tectonic plates), whereas the underlying asthenosphere is hotter, mechanically weak, ductile, and capable of solid-state plastic creep over geological timescales.

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

Which type of seismic wave causes the most severe surface structural damage during shallow earthquakes due to complex elliptical ground displacement?

  1. Rayleigh waves. Correct answer.
  2. P-waves (Primary waves).
  3. S-waves (Secondary waves).
  4. Infrasonic body waves.

Explanation

Rayleigh waves are surface seismic waves that travel along Earth's exterior, producing an elliptical, retrograde rolling motion in a vertical plane parallel to wave propagation. Because surface waves decay slower with distance than body waves and generate large ground displacements, Rayleigh and Love waves cause the severe shaking responsible for structural collapse.

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

What petrological feature distinguishes an oceanic ophiolite complex, such as those exposed in the Cabo Ortegal complex in Galicia?

  1. A sequence representing ancient oceanic crust and upper mantle, including serpentinized peridotites, layered gabbros, sheeted dikes, and pillow basalts. Correct answer.
  2. Thick sequences of subaerial rhyolitic ash-fall tuffs interbedded with continental lacustrine coal seams.
  3. Granitic batholiths containing abundant pink potassium feldspar phenocrysts and quartz veins.
  4. Evaporite deposits composed of halite and gypsum formed in desiccated endorheic basins.

Explanation

An ophiolite is a section of Earth's oceanic crust and underlying upper mantle uplifted and exposed above sea level via tectonic obduction. A complete ophiolite pseudo-stratigraphy includes tectonized ultramafic mantle peridotites (often serpentinized), cumulate gabbros, sheeted basaltic dike complexes, pillow lavas, and overlying deep-sea pelagic sediments.

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

What geological phenomenon occurs at Wadati-Benioff zones?

  1. Planar zones of dipping earthquake foci that trace the descent of a cold subducting lithospheric slab into the mantle. Correct answer.
  2. Horizontal rift zones where continental crust splits to form new ocean basins.
  3. Circular regions of intense intraplate volcanism caused by stationary mantle plumes.
  4. Aseismic transform faults connecting offset mid-ocean ridge segments.

Explanation

A Wadati-Benioff zone is a dipping planar zone of seismicity produced by the interaction of a subducting oceanic plate with the overriding plate. Earthquakes occur along this interface and within the interior of the cold, brittle subducting slab down to depths of up to 670 km, mapping the subduction slab path into the mantle.

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

Which mineral group constitutes over 90% of Earth's crust, characterized by silicon-oxygen tetrahedra ([SiO4]4-) as fundamental building blocks?

  1. Silicates. Correct answer.
  2. Carbonates.
  3. Sulfides.
  4. Halides.

Explanation

Silicates are the most abundant mineral class in Earth's crust and mantle. Their basic structural unit is the silicon-oxygen tetrahedron ([SiO4]4-), which links in various geometric arrangements (isolated, chains, sheets, frameworks) to form quartz, feldspars, micas, pyroxenes, amphiboles, and olivine.

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

What is the predominant petrological characteristic of Hercynian two-mica granites widespread across Galicia?

  1. Peraluminous plutonic rocks containing both biotite and muscovite, formed by partial melting of crustal metasediments (S-type granites). Correct answer.
  2. Extrusive basaltic rocks composed of olivine and calcic plagioclase formed at ocean ridges.
  3. Ultramafic intrusive rocks consisting entirely of coarse-grained pyroxene and olivine.
  4. Alkaline volcanic rocks enriched in leucite and nepheline formed in continental rifts.

Explanation

Galician Hercynian granites are predominantly peraluminous (aluminum-saturated) S-type granites containing both biotite (dark mica) and muscovite (light mica), along with quartz and K-feldspar/plagioclase. They were generated during the Variscan orogeny by widespread partial melting (anatexis) of continental metasedimentary rocks.

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

In regional metamorphism of pelitic (shale) rocks, which sequence of index minerals indicates increasing metamorphic grade (from lowest to highest grade)?

  1. Chlorite → Biotite → Garnet → Staurolite → Kyanite → Sillimanite. Correct answer.
  2. Sillimanite → Kyanite → Staurolite → Garnet → Biotite → Chlorite.
  3. Quartz → Calcite → Olivine → Pyroxene → Plagioclase.
  4. Gypsum → Halite → Anhydrite → Fluorite → Barite.

Explanation

Barrow's metamorphic zones define increasing pressure and temperature conditions in pelitic rocks. The classic index mineral sequence representing lowest to highest grade is: Chlorite (low-grade/greenschist fac.) → Biotite → Garnet → Staurolite → Kyanite (medium-grade/amphibolite fac.) → Sillimanite (high-grade/granulite fac.).

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

Which stratigraphic principle states that in an undeformed sequence of sedimentary rocks, older layers lie beneath younger layers?

  1. Principle of Superposition. Correct answer.
  2. Principle of Cross-Cutting Relationships.
  3. Principle of Original Horizontality.
  4. Principle of Lateral Continuity.

Explanation

Formulated by Nicolas Steno, the Principle of Superposition states that in an undisturbed sequence of sedimentary strata or extrusive igneous rocks, each layer is younger than the one below it and older than the one above it, establishing relative age relationships.

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

Why are the high-pressure metamorphic rocks of the Cabo Ortegal Complex in Galicia (such as eclogites and granulites) scientifically significant?

  1. They record deep subduction (>50 km) of oceanic and continental crust during the closure of Paleozoic oceans prior to the Variscan collision. Correct answer.
  2. They represent recent Quaternary volcanic activity associated with active rifting along the Atlantic margin.
  3. They formed by shallow meteorite impact melting during the Cretaceous-Paleogene extinction event.
  4. They are undeformed Precambrian evaporite beds formed in hypersaline shallow lagoons.

Explanation

The Cabo Ortegal Complex in NW Galicia exposes rare high-pressure and ultra-high-pressure metamorphic rocks, including eclogites (omphacite + pyrope-rich garnet) and high-pressure granulites. These rocks record the deep subduction of oceanic lithosphere (Rheic/Iapetus oceans) and continental margin fragments to mantle depths (>50–90 km) followed by rapid tectonic exhumation during the Variscan orogeny.

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

How does the diagenetic process of lithification transform loose sediment into solid sedimentary rock?

  1. Through compaction driven by overburden pressure and cementation by precipitating minerals (e.g., calcite, silica) in pore spaces. Correct answer.
  2. Through complete melting at high temperatures followed by rapid subaerial volcanic quenching.
  3. Through intense recrystallization driven by directed differential stress at mantle subduction depths.
  4. Through biological digestion of quartz grains by anaerobic soil bacteria.

Explanation

Lithification is the set of diagenetic processes that convert loose sediment into cohesive sedimentary rock. As sediment accumulates, overburden weight causes compaction, squeezing out pore water and reducing volume. Concurrently, dissolved ions in groundwaters precipitate secondary mineral cements (such as calcite, quartz, or iron oxides) in remaining pore spaces, binding grains together.

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

What defines an ideal index fossil (guide fossil) used in biostratigraphic correlation?

  1. Wide geographic distribution, rapid evolutionary rate (short vertical stratigraphic range), high abundance, and easy identification. Correct answer.
  2. Narrow geographic distribution restricted to a single alpine lake, long evolutionary persistence across 500 million years.
  3. Composition made exclusively of pure elemental gold, residing only in deep metallic veins.
  4. Large physical size exceeding 10 meters in length, restricted to deep abyssal oceanic trenches.

Explanation

Index fossils (guide fossils) allow precise relative age dating and correlation of sedimentary rock strata across distant geographical regions. An effective index fossil species must have had a broad geographic distribution, lived for a brief geological duration (rapid evolutionary change), been abundant, and possess distinctive, easily recognizable hard skeletal features (e.g., ammonites, graptolites, trilobites).

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

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

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

Explanation

An angular unconformity is a structural boundary where older underlying sedimentary rocks were deformed (tilted, folded) and eroded prior to the deposition of younger, flatter horizontal strata. It represents a significant temporal gap involving tectonism, uplift, erosion, and renewed sedimentation.

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

If a mineral sample originally contained 100% of a radioactive parent isotope and now contains 25% parent and 75% stable daughter product, how many half-lives have elapsed?

  1. 2 half-lives. Correct answer.
  2. 1 half-life.
  3. 3 half-lives.
  4. 4 half-lives.

Explanation

Radioactive decay follows exponential decay kinetics. After 1 half-life, 50% of the original parent remains (50% daughter). After 2 half-lives, half of that remaining parent decays, leaving 25% parent and 75% daughter product (100% -> 50% -> 25%).

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

Which igneous texture indicates a two-stage cooling history where large crystals (phenocrysts) are embedded in a fine-grained groundmass?

  1. Porphyritic texture. Correct answer.
  2. Phaneritic texture.
  3. Pegmatitic texture.
  4. Vesicular texture.

Explanation

A porphyritic texture features large, well-formed crystals (phenocrysts) surrounded by a finer-grained matrix or groundmass. This texture forms when magma begins cooling slowly at depth (allowing phenocrysts to grow), followed by rapid ascension and quenching of the remaining liquid at or near Earth's surface.

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

What physical property of minerals describes their tendency to break along weak atomic crystallographic planes, producing smooth flat surfaces?

  1. Cleavage. Correct answer.
  2. Fracture.
  3. Streak.
  4. Hardness.

Explanation

Cleavage is the tendency of a mineral to split along specific crystallographic planes where chemical bonds are weakest. Minerals can display one, two, three, or more distinct directions of cleavage (e.g., basal cleavage in micas, rhombohedral cleavage in calcite).

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

What is the 'Ollo de Sapo' in Galician geology?

  1. A famous Neoproterozoic-Ordovician glandular gneiss unit characterized by large eye-shaped microcline feldspar and quartz augen. Correct answer.
  2. A Quaternary glacial cirque lake located in the Serra do Gêres mountain range.
  3. A deep subterranean karst cavity system formed in coastal limestones near Lugo.
  4. An active active hydrothermal geyser field along the Miño River fault zone.

Explanation

The 'Ollo de Sapo' (Galician for 'Toad's Eye') is an iconic regional megastructure and stratigraphic marker unit in the Iberian Massif of NW Spain (Galicia and Zamora). It consists of Ordovician metavolcanic rocks and glandular gneisses featuring large, pale eye-shaped porphyroclasts ('augen') of microcline feldspar and quartz set in a fine-grained schistose matrix.

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

Which rock type is classified as a chemical sedimentary rock formed by the inorganic precipitation of calcium carbonate from marine or lacustrine water?

  1. Travertine / Oolitic limestone. Correct answer.
  2. Quartz sandstone.
  3. Conglomerate.
  4. Bituminous coal.

Explanation

Chemical sedimentary rocks form when mineral constituents in solution become supersaturated and precipitate inorganically. Travertine (precipitated at hot springs/caves) and oolitic limestone (precipitated in warm shallow marine currents) are classic calcium carbonate (CaCO3) chemical sedimentary rocks.

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

What type of metamorphism occurs adjacent to igneous intrusions, driven primarily by high temperature under low differential stress?

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

Explanation

Contact (thermal) metamorphism occurs when host rocks are baked by heat emanating from an intruding hot magma body. It takes place at high temperatures but low hydrostatic pressures, producing non-foliated metamorphic rocks such as hornfels within a localized metamorphic aureole surrounding the pluton.

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

Which extinct group of Paleozoic marine arthropods with calcified exoskeletons serves as key index fossils for the Cambrian and Ordovician periods?

  1. Trilobites. Correct answer.
  2. Ammonites.
  3. Belemnites.
  4. Rudists.

Explanation

Trilobites were highly diverse Paleozoic marine arthropods with hard chitinous/calcitic exoskeletons divided into three longitudinal lobes. Their rapid evolution and distinct morphological variations make them premier index fossils for biostratigraphic dating of Cambrian through Devonian strata.

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

According to Bowen's Reaction Series, which silicate mineral crystallizes first at the highest temperatures from a cooling basaltic magma?

  1. Olivine. Correct answer.
  2. Quartz.
  3. Muscovite.
  4. Biotite.

Explanation

Bowen's Reaction Series describes the order of silicate mineral crystallization from a cooling magma. Olivine (an isolated nesosilicate) crystallizes first at the highest temperatures (~1200°C) along the discontinuous branch, whereas quartz crystallizes last at the lowest temperatures (~700°C).

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

What do cross-bedding structures in sandstone strata reveal about the ancient depositional environment?

  1. The direction of ancient fluid flow (paleocurrent direction) from wind or water currents. Correct answer.
  2. The absolute age of the rock in millions of years.
  3. The exact depth of subduction during metamorphic alteration.
  4. The concentration of dissolved oxygen in deep atmospheric layers.

Explanation

Cross-bedding forms when inclined layers of sediment are deposited by moving wind or water currents on the lee side of dunes or ripples. The tilt direction of the cross-beds provides unambiguous evidence of the paleocurrent direction and helps determine whether deposition occurred in desert dunes, rivers, or deltaic channels.

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

What is a lithostratigraphic unit defined purely on observable rock characteristics regardless of age?

  1. Formation. Correct answer.
  2. Biozone.
  3. System.
  4. Era.

Explanation

A Formation is the primary formal unit of lithostratigraphy. It consists of a body of rock identified and mapped based on distinct, observable lithological characteristics (rock type, bedding, composition) that distinguish it from adjacent rock bodies, independent of geological age.

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

What metamorphic texture is characterized by parallel alignment of platy mineral grains (like micas) under directed pressure?

  1. Foliation. Correct answer.
  2. Granoblastic texture.
  3. Porphyroblastic texture.
  4. Cataclastic texture.

Explanation

Foliation refers to any repetitive planar arrangement of mineral grains or structural features within a metamorphic rock. Under directed differential stress during regional metamorphism, platy or elongated minerals (micas, amphiboles) rotate and grow perpendicular to the principal stress direction, creating slaty cleavage, schistosity, or gneissic banding.

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

Which mineral ranks as number 7 on Mohs scale of mineral hardness and can scratch glass?

  1. Quartz. Correct answer.
  2. Calcite.
  3. Fluorite.
  4. Orthoclase feldspar.

Explanation

Mohs hardness scale ranks minerals from 1 (softest) to 10 (hardest). Quartz is assigned hardness 7. It easily scratches window glass (hardness ~5.5) and steel pocketknives (hardness ~5.5–6.0).

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

How does frost wedging (gelifraction) mechanically shatter rocks in temperate and high-altitude environments?

  1. Water expands by approximately 9% upon freezing in rock fractures, exerting immense hydrostatic pressure against fissure walls. Correct answer.
  2. Freezing water reacts chemically with quartz to dissolve silicate bonds into soluble silica gel.
  3. Ice crystals shrink rapidly upon freezing, pulling rock surfaces inward until they implode.
  4. Freezing ice generates magnetic forces that repel iron atoms within surrounding granite.

Explanation

Frost wedging (gelifraction) is a mechanical weathering process. When liquid water enters micro-fractures in rock and freezes, it expands in volume by about 9%. This volumetric expansion generates immense outward pressures (up to 200 MPa) against crack walls, progressively widening fissures and wedging rock blocks apart.

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

Which chemical weathering process breaks down potassium feldspar in Galician granites into kaolinite clay and soluble ions?

  1. Hydrolysis. Correct answer.
  2. Oxidation.
  3. Carbonation of calcite.
  4. Dissolution of halite.

Explanation

Hydrolysis is the chemical reaction between silicate minerals and acidic water (H+ and OH- ions). In Galicia's humid climate, rainwater containing dissolved carbonic acid reacts with K-feldspar in granites, leaching potassium and silica while altering the mineral structure into kaolinite clay (kaolin deposits in Galicia).

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

What distinctive granitic weathering landforms, common in Galicia, consist of isolated rounded boulders resting atop bedrock pillars?

  1. Tors (penedos / bolaus). Correct answer.
  2. Dolines.
  3. Drumlins.
  4. Yardangs.

Explanation

In Galician granitic landscapes, subsurface chemical weathering (hydrolysis along orthogonal joint systems) rounds subsurface rock blocks into corestones ('bolaus'). Subsequent erosion strips away the surrounding weathered mantle ('xabre' or grus), exposing isolated, stacked rounded granite blocks known as tors ('penedos').

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

What is the primary origin of the Galician Rías (Rías Baixas and Rías Altas)?

  1. Fluvial valleys formed along tectonic fault lines that were submerged by sea-level rise during post-glacial marine transgression. Correct answer.
  2. Deep glacial fjords carved by massive continental ice sheets sliding directly into the Atlantic Ocean during the Last Glacial Maximum.
  3. Volcanic collapse calderas flooded by ocean water following explosive explosive eruptions.
  4. Impact craters formed by a meteorite cluster striking the Iberian coast in the Holocene.

Explanation

Galician rías are flooded coastal river valleys (incised estuaries). During the Cenozoic, rivers eroded valleys along pre-existing Variscan fracture and fault systems. Following the last glacial period, the Holocene (Flandrian) marine transgression flooded these lower river valleys, creating the characteristic arm-like inlets of the Rías Baixas and Rías Altas.

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

At which position in a meandering river bend does active sediment deposition occur, forming a point bar?

  1. Along the inner bank (inside of the bend), where water velocity is slowest. Correct answer.
  2. Along the outer bank (cut bank), where hydraulic action and erosion are strongest.
  3. Directly in the center of the thalweg, where turbulence reaches its maximum.
  4. At the river source in high mountain headwaters.

Explanation

In a meandering channel, helical flow causes water velocity to drop along the inside of a meander bend. This reduction in kinetic energy and shear stress decreases sediment transport capacity, causing bedload sediment (sand and gravel) to deposit along the inner bank, building a point bar.

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

How does longshore drift transport sediment parallel to a coastline?

  1. Waves approaching the shore at an angle carry sand up the beach face via swash, while backwash pulls sand straight down the slope under gravity. Correct answer.
  2. Tidal currents drag sand in circular underwater vortexes without net directional displacement.
  3. Wind blows sand dunes in zig-zag patterns across deep underwater submarine canyons.
  4. Oceanic upwelling currents lift heavy boulders and carry them along cliff tops.

Explanation

Longshore drift occurs when waves approach a beach at an oblique angle dictated by prevailing winds. Wave swash pushes sand grains up the beach slope diagonally. When the wave recedes, backwash pulls sediment straight down the slope under gravity. Repeated swash-backwash cycles move sediment laterally in a zig-zag path along the coast.

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

Which slow mass wasting process is evidenced by tilted fence posts, bent tree trunks (J-shaped trunks), and displaced soil on gentle grassy slopes?

  1. Soil creep (solifluction). Correct answer.
  2. Rock avalanche.
  3. Debris flow.
  4. Lahari avalanche.

Explanation

Soil creep is the extremely slow (millimeters to centimeters per year) downhill movement of soil and regolith under gravity. It is driven by freeze-thaw or wet-dry expansion and contraction cycles. The slow movement curves tree trunks into J-shapes, tilts retaining walls and utility poles, and ripples soil surfaces.

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

What process creates karst geomorphology, such as sinkholes, caves, and speleothems?

  1. Dissolution of carbonate rocks (limestone/dolomite) by acidic groundwater containing dissolved carbon dioxide. Correct answer.
  2. Mechanical abrasion of granite cliffs by wind-blown quartz sand grains.
  3. Thermal expansion and contraction of volcanic basalt under desert temperature swings.
  4. Glacial scouring of deep granite basins beneath moving ice sheets.

Explanation

Karst landscapes form through the chemical dissolution of soluble rocks (primarily limestone, CaCO3) by weak carbonic acid (H2CO3) in rainwater and groundwater. The dissolution widens fractures into underground drainage networks, caves, and surface depressions like dolines and poljes.

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

What coastal landform is a narrow ridge of sand or gravel connected to land at one end and extending out into open water or across a bay mouth?

  1. Spit (or baymouth bar). Correct answer.
  2. Stack.
  3. Marine terrace.
  4. Fjord.

Explanation

A spit is an elongated ridge of sand or gravel deposited by longshore drift that projects from the mainland into open water across a bay or river mouth. If longshore drift extends the spit completely across the bay mouth, it forms a baymouth bar, enclosing a lagoon.

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

What distinctive U-shaped cross-profile distinguishes valleys eroded by alpine glaciers from V-shaped valleys carved by rivers?

  1. Glaciers erode both valley floors and steep sidewalls uniformly across their entire contact width, forming steep-sided, flat-bottomed U-shaped valleys. Correct answer.
  2. Glaciers cut razor-thin vertical slot canyons with perfectly smooth glass-like walls.
  3. Glaciers deposit horizontal layers of river gravel that build wide triangular deltas.
  4. Glaciers generate hot thermal fluids that dissolve surrounding limestone into circular sinkholes.

Explanation

Rivers concentrate kinetic energy at their channel base, cutting narrow V-shaped valleys with interlocking spurs. In contrast, massive valley glaciers fill entire valley cross-sections, plucking and abrading both the floor and lower sidewalls simultaneously to transform V-shaped river valleys into steep-sided, flat-bottomed U-shaped glacial troughs.