Questions: Weathering and Soil Chemistry

Question 1

The silicate weathering feedback is considered Earth's long-term thermostat. How does this feedback stabilize climate?

• A. Silicate weathering produces greenhouse gases that warm the climate
• B. Higher temperatures and more rainfall increase silicate weathering rates, which consume more CO2, reducing the greenhouse effect and cooling the planet; lower temperatures reduce weathering, allowing volcanic CO2 to accumulate and warm the planet -- a negative feedback that stabilizes climate over million-year timescales
• C. Silicate weathering reflects sunlight back to space
• D. The feedback only operates during ice ages

Answer: (B)

This is the Walker feedback: CO2 is continuously added to the atmosphere by volcanism and removed by silicate weathering. Because weathering rates increase with temperature (through kinetics) and precipitation (through water availability and runoff), any warming increases CO2 drawdown, eventually cooling the planet. Any cooling reduces weathering, allowing volcanic CO2 to build up and warm the planet. This negative feedback has maintained habitable surface temperatures for >4 Gyr despite a 30% increase in solar luminosity.

Question 2

A tropical soil profile shows gibbsite (Al(OH)3) as the dominant secondary mineral, with virtually no primary minerals remaining. This indicates extreme weathering conditions.

• T. True
• F. False

Answer: True

Gibbsite is the end-product of silicate weathering -- all silica, alkalis, and alkaline earths have been completely leached, leaving only the most insoluble component (aluminum hydroxide). This extreme weathering (lateritization) occurs under prolonged tropical conditions with high rainfall, warm temperatures, and good drainage. The weathering sequence from fresh rock is: feldspar -> smectite -> kaolinite -> gibbsite, with each step removing more silica and cations. Gibbsite dominance indicates that weathering has progressed to its maximum extent.

Question 3

Explain why chemical weathering rates are highest in warm, wet tropical environments but the thickest soil profiles can also develop on stable, low-relief surfaces in these regions.

Model answer: Chemical reaction rates increase with temperature (Arrhenius relationship), and water is both a reactant and transport medium that removes dissolved products (preventing saturation that would slow reactions). Tropical conditions maximize both kinetic rates and water throughput. Thick soils develop on stable surfaces because: (1) high weathering rates transform bedrock to soil faster than erosion removes it, (2) low topographic relief reduces physical erosion, allowing residual soil to accumulate, and (3) biological activity (root penetration, organic acid production, bioturbation) accelerates weathering at depth. The combination of intense chemical weathering and minimal physical erosion produces deeply weathered profiles (laterites) tens of meters thick that represent millions of years of cumulative weathering.

Thick tropical soils require two conditions: high weathering rates (climate) and low erosion rates (geomorphology). Where both are satisfied, the weathering front advances into bedrock faster than the surface is lowered.