- 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 the onset of glacial periods over the past 800,000 years?

Answer: Decreased summer insolation at high northern latitudes driven by precession and low obliquity acting together

Glacial inception is most strongly triggered when Northern Hemisphere summers receive reduced insolation — primarily when precession places perihelion during NH summer (reducing summer solar intensity) and obliquity is near its minimum (~22.1°), both effects lowering summer temperatures enough to allow snow and ice to persist year-round and accumulate. This is the core of Milankovitch theory as validated by deep-sea sediment records.

During metamorphism, a rock undergoes isochemical transformation. A schist with the mineral assemblage quartz + muscovite + garnet + staurolite is heated further. According to Barrovian metamorphic zones, which reaction best describes the next prograde transformation?

Answer: Staurolite + quartz → kyanite + garnet + H₂O

In Barrovian sequences, the staurolite zone is succeeded by the kyanite zone at higher grades. The defining reaction is staurolite + quartz breaking down to kyanite + garnet + H₂O (a dehydration reaction). This reaction occurs at roughly 550–600°C and moderate pressures, producing the kyanite index mineral diagnostic of that zone.

A geologist measures the Sr⁸⁷/Sr⁸⁶ ratio in a suite of cogenetic igneous rocks and plots a Rb-Sr isochron. The slope of the isochron gives a date of 1.2 Ga. If the initial Sr⁸⁷/Sr⁸⁶ ratio (the y-intercept) is 0.7050, what does an unexpectedly HIGH initial ratio (e.g., 0.720) instead suggest about the magma source?

Answer: The magma incorporated or was derived from old continental crust with a long history of elevated Rb/Sr ratios

The initial ⁸⁷Sr/⁸⁶Sr ratio reflects the isotopic composition of the source at the time of melting. A high initial ratio (~0.720) indicates the source had a long prior history of high Rb/Sr — characteristic of old continental crust, which has been enriching in radiogenic ⁸⁷Sr for hundreds of millions to billions of years. Depleted mantle has very low Rb/Sr and correspondingly low initial ratios (~0.702–0.704).

The carbon cycle includes both fast (biological/atmospheric) and slow (geological) reservoirs. Which process represents the PRIMARY long-term sink that removes CO₂ from the atmosphere on timescales of 100,000–1,000,000 years, stabilizing Earth's climate over geologic time?

Answer: Silicate weathering followed by carbonate precipitation and subduction-driven metamorphic degassing recycling

The silicate-carbonate cycle (also called the Urey reaction: CaSiO₃ + CO₂ → CaCO₃ + SiO₂) is Earth's primary long-term CO₂ thermostat. Silicate weathering consumes atmospheric CO₂, ions are carried to the ocean where carbonates precipitate and are eventually subducted, releasing CO₂ back via metamorphism and volcanism. This negative feedback operates over millions of years and is why Earth has not experienced runaway greenhouse conditions like Venus. Photosynthesis and ocean dissolution are fast-cycle processes that don't permanently sequester carbon on these timescales.

A volume of basaltic magma rises through the lithosphere. As it ascends, which sequence of physical and chemical changes correctly describes decompression melting in the asthenosphere beneath a mid-ocean ridge?

Answer: Pressure decreases → solidus temperature rises → previously solid rock now exceeds its melting point → partial melting occurs without added heat

Decompression melting at mid-ocean ridges occurs because as asthenospheric mantle upwells, pressure decreases and the solidus temperature (the temperature at which rock begins to melt) drops faster than the rock's actual temperature decreases adiabatically. The rock's temperature therefore crosses above the solidus without any external heat source being required — it melts purely due to pressure reduction. This is distinct from flux melting (subduction zones, driven by water) or heat-induced melting.

In the nitrogen cycle, which two-step microbial process is responsible for converting ammonium (NH₄⁺) to nitrate (NO₃⁻) in well-oxygenated soils, and what is the correct intermediate compound produced between the two steps?

Answer: Nitrification in two stages: NH₄⁺ → NO₂⁻ (by Nitrosomonas) then NO₂⁻ → NO₃⁻ (by Nitrobacter); intermediate is nitrite (NO₂⁻)

Nitrification is a two-step aerobic oxidation process. First, ammonia-oxidizing bacteria (classically Nitrosomonas) oxidize ammonium (NH₄⁺) to nitrite (NO₂⁻). Second, nitrite-oxidizing bacteria (classically Nitrobacter) oxidize nitrite to nitrate (NO₃⁻). The intermediate, nitrite, is toxic to plants at elevated concentrations but typically doesn't accumulate because step two proceeds quickly under normal aerobic soil conditions. Both steps require oxygen, making this process oxygen-dependent.