Earth's Geological Cycles Flashcards

During a Wilson Cycle, which sequence correctly describes the transition from continental rifting to ocean basin closure?

Answer: Rifting → passive margin → mid-ocean ridge spreading → subduction initiation → continental collision

The Wilson Cycle begins with continental rifting (e.g., East African Rift today), which widens into a passive margin as seafloor spreading begins at a mid-ocean ridge. As the ocean ages and cools, it becomes denser and eventually subduction initiates, consuming the oceanic lithosphere and ultimately driving continental collision — closing the cycle.

Ophiolite sequences found in mountain belts are significant because they represent:

Answer: Slices of oceanic crust and upper mantle obducted onto continental crust during ocean closure

Ophiolites are fragments of oceanic lithosphere — including pillow basalts, sheeted dike complexes, gabbros, and peridotite — that have been thrust (obducted) onto continental margins during the closure of an ocean basin. They serve as critical evidence for past subduction and plate tectonic reconstructions, preserving a record of ancient ocean floors now found atop mountain ranges.

In the carbon cycle, which process represents the PRIMARY long-term sink that removes carbon dioxide from the atmosphere on geological timescales (millions of years)?

Answer: Weathering of silicate rocks followed by carbonate burial in ocean sediments

On geological timescales, the dominant long-term carbon sink is the silicate weathering-carbonate burial cycle (Urey reaction). Silicate minerals react with CO₂ and water to produce bicarbonate ions, which rivers carry to the ocean where marine organisms use them to build calcium carbonate shells. When these organisms die, CaCO₃ is buried in sediments, effectively removing carbon from the atmosphere for millions of years. Photosynthesis and ocean dissolution are important but cycle carbon on much shorter timescales.

Isostatic rebound following the melting of a continental ice sheet results in which measurable geological consequence?

Answer: Crustal uplift that can temporarily outpace eustatic sea-level rise, causing relative sea-level fall in formerly glaciated regions

When massive ice sheets melt, the removal of their enormous weight allows the depressed lithosphere to rebound upward through isostasy. In regions like Scandinavia and Hudson Bay, this post-glacial rebound can occur faster than the eustatic (global) sea-level rise caused by meltwater, producing a net relative sea-level fall locally. This is why raised beaches and ancient shorelines are found well above current sea level in formerly glaciated areas.

Which pairing of rock type and the specific stage of the rock cycle it most directly represents is INCORRECT?

Answer: Mylonite — contact metamorphism adjacent to an igneous intrusion producing fine-grained texture

Mylonites are formed by dynamic metamorphism (cataclastic flow) along ductile shear zones and fault systems — not by contact metamorphism from igneous intrusions. Contact metamorphism near intrusions typically produces hornfels, not mylonite. Mylonite's characteristic fine-grained, foliated texture results from extreme mechanical deformation at depth under high stress. The other pairings are all correct: eclogite forms during subduction, migmatites form at the metamorphic-magmatic transition, and turbidites are classic deep-sea graded beds.

The Milankovitch cycles influence glaciation primarily through changes in insolation distribution. Which combination of orbital parameters produces the greatest amplification of Northern Hemisphere glaciation?

Answer: High orbital eccentricity + axial tilt at minimum obliquity (22.1°) + aphelion occurring in Northern Hemisphere summer

Glaciation is favored when Northern Hemisphere summers are as COOL as possible (preventing winter snow from melting) and winters remain cold enough to accumulate snow. This occurs when: eccentricity is high (amplifying seasonal differences), obliquity is at its minimum (less axial tilt means weaker summer insolation at high latitudes), and aphelion (Earth farthest from Sun) coincides with Northern Hemisphere summer (further reducing summer solar input). Perihelion in Northern Hemisphere summer would actually WARM summers and inhibit glaciation — the opposite of what's needed.