- Basic Math and Science Earth's Geological Cycles Questions and Answers Flashcards

During the rock cycle, which process directly converts sedimentary rock into metamorphic rock without first melting it?

Answer: Recrystallization driven by intense heat and pressure deep within the crust

Metamorphic rock forms when existing rock is subjected to intense heat and pressure deep within Earth's crust, causing minerals to recrystallize in a solid state — without melting. This distinguishes metamorphism from igneous processes, which involve melting. Lithification produces sedimentary rock, weathering breaks rock down, and surface cooling forms extrusive igneous rock.

A geologist observes varved sediments — alternating thin layers of fine and coarse particles deposited annually in a glacial lake. If she counts 1,200 couplets (coarse + fine pairs), approximately how many years of deposition do those sediments represent?

Answer: 1,200 years

Each varve couplet — one coarse layer deposited in summer meltwater and one fine layer deposited in winter — represents exactly one year of deposition. Therefore 1,200 couplets correspond directly to 1,200 years. This is a foundational principle of varve chronology used to date glacial and postglacial sediments.

Which of the following best explains why subduction zones are associated with explosive, silica-rich (felsic) volcanoes rather than the quieter, basaltic eruptions typical of mid-ocean ridges?

Answer: Water released from the subducting slab lowers the melting point of mantle rock, generating magma that assimilates silica-rich continental crust, increasing viscosity and gas content

As an oceanic plate subducts, water and other volatiles are driven off and enter the overlying mantle wedge, lowering the melting point and generating magma. As this magma rises through thick continental crust, it assimilates silica-rich material, becoming more viscous and volatile-rich. High viscosity traps gases, building pressure that leads to explosive eruptions — contrasting with the low-viscosity basaltic flows at mid-ocean ridges where magma rises more directly from the mantle.

Carbon fixed by photosynthesis on land returns to the atmosphere via several pathways. Which pathway represents the SLOWEST carbon return over geological timescales?

Answer: Burial and lithification of organic carbon into fossil fuels followed by eventual weathering or extraction

Respiration by decomposers, combustion by wildfire, and animal respiration all return carbon to the atmosphere on timescales of days to centuries. In contrast, when organic matter is buried, compressed, and converted to coal, oil, or natural gas, carbon is locked away for millions to hundreds of millions of years — only returning through slow tectonic uplift, weathering, or human extraction. This long-term geological storage is the slowest pathway in the carbon cycle.

During a complete water cycle, a water molecule evaporates from the ocean surface. Rank the following subsequent stages in the correct temporal order as the molecule eventually returns to the ocean: (1) Condensation into cloud droplets, (2) Infiltration into groundwater, (3) Precipitation as rain, (4) Surface runoff into a river.

Answer: 1 → 3 → 2 → 4

After evaporation, water vapor rises and cools, first undergoing condensation (1) to form cloud droplets. Clouds then release water as precipitation (3). Once at the surface, water can infiltrate into groundwater (2), which moves slowly and may eventually discharge into rivers or the ocean, or it may flow directly as surface runoff (4). In this pathway the molecule infiltrates before contributing to surface runoff, making the order 1 → 3 → 2 → 4 correct for the groundwater pathway.

Earth's nitrogen cycle includes a step called denitrification. Which of the following accurately describes what occurs during denitrification and why it is critical to the long-term balance of the nitrogen cycle?

Answer: Denitrification converts nitrates (NO₃⁻) back into N₂ gas, returning nitrogen to the atmosphere and preventing indefinite accumulation of nitrates in soil and water

Denitrification is carried out by anaerobic bacteria that reduce nitrates (NO₃⁻) — and sometimes nitrites — to N₂ (or N₂O) gas, which escapes to the atmosphere. This completes the nitrogen cycle by counterbalancing nitrogen fixation: without denitrification, reactive nitrogen compounds would accumulate indefinitely in soils and aquatic systems, causing eutrophication and disrupting ecosystem chemistry. The other options describe nitrogen fixation, nitrification, and marine nitrogen assimilation respectively.