Earth's Geological Cycles Flashcards

During subduction, oceanic crust descends into the mantle and undergoes metamorphism. At approximately what depth does the subducting slab release water into the mantle wedge, triggering flux melting and arc volcanism?

Answer: 80–120 km

Water is released from hydrated minerals in the subducting slab (primarily serpentinite and amphibolite) at roughly 80–120 km depth, where pressure and temperature conditions cause dehydration reactions. This water lowers the melting point of the overlying mantle wedge, generating magma that rises to form volcanic arcs. Shallower depths (10–20 km) are too cool for these reactions, and deeper levels (200–500 km) are below the zone where most arc magmas originate.

The Wilson Cycle describes the opening and closing of ocean basins. Which sequence correctly represents the stages in order from initial continental rifting to final suturing?

Answer: Embryonic → Juvenile → Mature → Declining → Terminal → Suture

The Wilson Cycle progresses as: Embryonic (continental rifting, e.g., East African Rift) → Juvenile (narrow ocean forms, e.g., Red Sea) → Mature (wide ocean with mid-ocean ridge, e.g., Atlantic) → Declining (subduction dominates, e.g., Pacific) → Terminal (closing ocean, remnant sea, e.g., Mediterranean) → Suture (continents collide, ocean gone, e.g., Himalayas). The other sequences scramble these stages.

Carbon stored in marine sediments is returned to the atmosphere over geological timescales primarily through which process?

Answer: Metamorphic decarbonation reactions at convergent plate boundaries

When carbonate-rich marine sediments are subducted at convergent boundaries, high pressure and temperature drive decarbonation reactions (e.g., CaCO₃ + SiO₂ → CaSiO₃ + CO₂), releasing CO₂ that outgasses through arc volcanoes back into the atmosphere. This closes the long-term carbon cycle (silicate-carbonate cycle). Phytoplankton respiration is short-term; hydrothermal vents at ridges primarily dissolve, not oxidize, carbonates; glacial erosion affects surface rocks but is not the primary geological-timescale return pathway.

Isostatic rebound following deglaciation causes measurable uplift of Earth's crust. Which of the following best explains why Scandinavia is still rising today, thousands of years after the last ice sheet melted?

Answer: The viscous mantle flows back slowly, and the response time of the asthenosphere lags behind ice removal by thousands of years

Isostatic rebound is governed by the viscosity of the asthenosphere. Because the mantle behaves as an extremely viscous fluid, it flows very slowly (on timescales of thousands to tens of thousands of years). Even though the Fennoscandian ice sheet melted roughly 10,000 years ago, mantle material is still slowly flowing back beneath the region, producing ongoing uplift of up to ~8 mm/year. Ridge push and mantle temperature are not the driving mechanisms here, and elastic rebound (instantaneous) is only a small fraction of the total adjustment.

During the Milankovitch cycles, which orbital parameter has the greatest influence on the ~100,000-year glacial-interglacial cycles observed in ice core records?

Answer: Eccentricity of Earth's orbit, which cycles on ~100,000- and ~413,000-year periods

The dominant ~100,000-year cycle in Pleistocene glacial records corresponds most closely to variations in orbital eccentricity (how elliptical Earth's orbit around the Sun is), which modulates the total annual solar energy received and amplifies the effects of other orbital parameters. Obliquity (~41,000-year cycle) is important for high-latitude insolation, and precession (~23,000-year cycle) controls seasonal timing, but neither matches the dominant 100,000-year glacial periodicity. Earth's rotational speed is not a recognized Milankovitch parameter.

In the rock cycle, granulite-facies metamorphic rocks form under extreme conditions deep in continental crust. Which combination of conditions is required for granulite facies metamorphism?

Answer: High temperature (>700°C) and moderate-to-high pressure (>0.8 GPa), with very low water activity

Granulite facies requires temperatures above ~700°C and pressures typically above 0.8 GPa (representing depths of ~25–45 km), combined with very low water activity (dehydrated conditions). These dry, high-grade conditions are diagnostic — the near-absence of hydrous minerals like amphibole and the presence of orthopyroxene characterize the facies. Blueschist facies (high pressure, low temperature) and ultra-high pressure facies (>2.5–3 GPa, e.g., coesite/diamond-bearing eclogites) are distinct regimes. Low-pressure contact metamorphism produces hornfels, not granulites.