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

The Milankovitch cycles describe periodic variations in Earth's orbital parameters. Which combination of orbital changes is primarily responsible for triggering the onset of ice ages over 100,000-year cycles?

Answer: Changes in Earth's orbital eccentricity modulating the precession signal

The dominant ~100,000-year glacial cycle is driven by Earth's orbital eccentricity variations, which modulate the precessional signal. Eccentricity itself causes only modest direct insolation changes, but it amplifies or dampens the seasonal contrast created by precession. When eccentricity is high, precession can cause large seasonal insolation differences; when low, precession has less effect. The 41,000-year obliquity cycle dominated before ~1 million years ago, but the 100,000-year eccentricity-modulated precession cycle has dominated since. Solar luminosity changes are not part of Milankovitch theory.

During the rock cycle, a granite batholith is exposed at the surface through erosion. The eroded sediment is transported to a marine basin, compacted, and lithified into sandstone. Later, tectonic collision buries the sandstone to 25 km depth where temperatures reach 650°C. What sequence of rock types and processes correctly describes this progression?

Answer: Igneous → sedimentary → metamorphic; processes: weathering/erosion, lithification, regional metamorphism

Granite is a plutonic (intrusive) igneous rock. Erosion and weathering break it down; transport and deposition form clastic sediment; lithification (compaction and cementation) produces sandstone (a clastic sedimentary rock). Burial to 25 km at 650°C under tectonic collision produces regional metamorphism — the large-scale, pressure-and-temperature-driven metamorphism associated with mountain-building events (not contact metamorphism, which is localized heat from magma intrusion). The full sequence is igneous → sedimentary → metamorphic via weathering/erosion, lithification, and regional metamorphism.

The carbon cycle includes both fast (biological) and slow (geological) subcycles. Which process MOST directly couples the fast and slow carbon cycles over million-year timescales and acts as a long-term thermostat for Earth's climate?

Answer: Silicate weathering consuming atmospheric CO₂ and delivering bicarbonate to oceans where it precipitates as carbonate rock

The silicate weathering-carbonate precipitation feedback (the Urey reaction) is the primary long-term carbon cycle thermostat. When CO₂ is high and temperatures rise, silicate rock weathering accelerates, drawing down CO₂ via: CaSiO₃ + CO₂ → CaCO₃ + SiO₂. Bicarbonate is delivered to oceans and locked into carbonate sediments. When CO₂ is low and temperatures drop, weathering slows, allowing volcanic outgassing to rebuild CO₂. This negative feedback operates over millions of years. Photosynthesis/respiration balance the fast cycle but don't provide long-term net storage. Volcanic outgassing is a source, not a sink. Ocean upwelling is a regional flux, not a geological-scale thermostat.

A geologist discovers an ash layer in a sedimentary sequence with a zircon U-Pb radiometric age of 300 Ma. The layer sits between limestone containing Carboniferous marine fossils below and red-bed sandstone with wind-blown structures above. Using the principle of cross-cutting relationships and superposition, what is the most geologically valid interpretation?

Answer: The ash layer provides a maximum age for the overlying sandstone and a minimum age for the underlying limestone

By the principle of superposition, layers lower in an undeformed sequence are older. The ash layer at 300 Ma is younger than the limestone below it (the limestone must predate 300 Ma — so 300 Ma is a minimum age for the limestone's upper boundary) and older than the sandstone above (the sandstone must postdate 300 Ma — so 300 Ma is a maximum age for the sandstone). Therefore, the ash provides a MAXIMUM age constraint for the overlying sandstone (it can't be older than 300 Ma) and a MINIMUM age constraint for the underlying limestone (the limestone is at least as old as 300 Ma, likely older). Option D reverses these relationships incorrectly.

In the nitrogen cycle, which process converts N₂ gas directly into a biologically available form, and which enzyme is uniquely responsible for catalyzing this reaction in prokaryotes?

Answer: Nitrogen fixation; nitrogenase converts N₂ to NH₃, and the enzyme is irreversibly inactivated by oxygen

Nitrogen fixation is the process by which dinitrogen gas (N₂) is converted to ammonia (NH₃), making nitrogen biologically available. The enzyme nitrogenase catalyzes this reaction. A critical advanced detail: nitrogenase is extremely oxygen-sensitive — O₂ irreversibly inactivates the enzyme. This explains why nitrogen-fixing organisms have evolved elaborate adaptations to exclude O₂ (e.g., heterocysts in cyanobacteria, the oxygen-consuming leghaemoglobin in root nodules). Nitrification is the conversion of NH₃ to NO₃⁻ (performed by bacteria like Nitrosomonas/Nitrobacter). Denitrification returns NO₃⁻ to N₂. Ammonification releases NH₄⁺ from organic matter.

Earth's water cycle transports heat as well as mass. A parcel of air rises orographically over a mountain range, causing precipitation on the windward side. Which thermodynamic principle explains why the leeward (lee) side experiences higher temperatures than the windward side at the same elevation, even though the air parcel has returned to its original pressure?

Answer: Latent heat released during condensation on the windward side is retained in the air parcel, so it descends the leeward side at the dry adiabatic lapse rate rather than the moist rate

This describes the foehn (or chinook) effect. On the windward side, rising air first cools at the dry adiabatic lapse rate (~10°C/km) until it reaches the dew point, then cools more slowly at the moist adiabatic lapse rate (~6°C/km) as condensation releases latent heat. Precipitation removes water vapor, effectively 'exporting' latent heat from the system. On the leeward side, the now-drier air descends and warms at the full dry adiabatic lapse rate the entire way down. Since it cools slowly (moist rate) going up but warms quickly (dry rate) going down, it arrives on the leeward side warmer than when it started on the windward side at the same elevation. The Coriolis effect acts on large-scale flows, not individual orographic parcels. Solar radiation differences are secondary. Bernoulli's principle does not increase temperature on descent.