100+ Free Galicia PAU Geology and Environmental Sciences Practice Questions

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.