Earth's Geological Cycles Flashcards

During subduction, oceanic crust descends into the mantle and partially melts. Which component of the subducted slab is primarily responsible for lowering the melting point of the overlying mantle wedge, triggering arc volcanism?

Answer: Water released from hydrated minerals in the subducting slab

Water released from hydrated minerals (such as serpentinite and amphibole) in the subducting slab lowers the solidus temperature of the overlying mantle wedge through flux melting. This water-induced melting — not simple heat transfer — is the dominant mechanism generating magmas in subduction-zone volcanic arcs.

The concept of 'residence time' in geochemical cycles refers to the average time an atom spends in a particular reservoir. If the ocean contains approximately 3.8 × 10¹⁹ kg of water and rivers deliver roughly 3.7 × 10¹⁶ kg per year, what is the approximate residence time of water in the ocean?

Answer: ~1,000 years

Residence time = reservoir size ÷ flux rate = (3.8 × 10¹⁹ kg) ÷ (3.7 × 10¹⁶ kg/yr) ≈ 1,027 years, roughly ~1,000 years. This calculation (reservoir/input rate) is the standard definition of residence time in geochemical cycling.

Metamorphic rocks formed at high pressure but relatively low temperature — such as blueschist — are most diagnostic of which tectonic setting and why?

Answer: Subduction zones, because cold oceanic crust descends faster than it can equilibrate with the surrounding mantle temperature

Blueschist-facies metamorphism requires an anomalously low geothermal gradient (high pressure, low temperature). This occurs in subduction zones where cold, dense oceanic lithosphere plunges into the mantle at rates fast enough that it remains cooler than the surrounding mantle. The glaucophane (blue amphibole) that gives blueschists their color is stable only under these unusual P-T conditions.

In the long-term carbon cycle, silicate weathering is considered a key climate-stabilizing (negative feedback) mechanism. Which reaction sequence correctly describes how atmospheric CO₂ is ultimately transferred to marine sediments through this process?

Answer: CO₂ dissolves in rainwater → carbonic acid weathers silicate rocks → bicarbonate ions transported to ocean → organisms build CaCO₃ shells → shells buried as limestone

The Urey reaction describes silicate weathering feedback: atmospheric CO₂ combines with rainwater to form carbonic acid (H₂CO₃), which dissolves calcium silicate minerals. The resulting Ca²⁺ and HCO₃⁻ ions are carried by rivers to the ocean, where marine organisms use them to build calcium carbonate (CaCO₃) shells. When these organisms die, the shells accumulate as limestone on the seafloor, effectively sequestering the carbon for millions of years.

Isostatic rebound (glacial isostasy) occurs when ice sheets melt and the underlying lithosphere rises. Which statement most accurately describes a subtle but geologically important consequence of rapid deglaciation on volcanic activity?

Answer: Decreased surface load reduces pressure on the underlying mantle, lowering the melting point and promoting decompression melting

Rapid removal of ice (reduced overburden pressure) causes decompression melting in the underlying asthenosphere — the same process that drives mid-ocean ridge volcanism. Studies in Iceland and other glaciated volcanic regions show statistically elevated eruption rates during and shortly after periods of rapid deglaciation. Lowering the pressure reduces the solidus temperature, allowing partially molten mantle rock to melt more completely without any change in temperature.

The Wilson Cycle describes the episodic opening and closing of ocean basins. Which sequence correctly orders the stages of a complete Wilson Cycle from initiation to completion?

Answer: Continental rifting → proto-ocean with passive margins → mature ocean → subduction initiation → ocean closure → continental collision

The Wilson Cycle begins with (1) continental rifting as a mantle plume or lithospheric extension splits a continent, (2) formation of a proto-ocean (like the modern Red Sea) with young passive margins, (3) a mature ocean basin with full seafloor spreading (like the Atlantic), (4) subduction initiation as old, dense oceanic crust begins to sink, (5) progressive ocean closure as subduction consumes the basin, and finally (6) continental collision and suturing when the opposing continents meet (producing mountain belts like the Himalayas). This cycle typically takes hundreds of millions of years to complete.