Granite and limestone are both common surface rocks. Which statement best explains why limestone dissolves far faster than granite in a humid climate, even though calcite (the main mineral in limestone) is softer than quartz?
ALimestone is more porous than granite, so water penetrates deeper and removes material faster.
BGranite contains iron oxides that form a protective coating, shielding it from rainwater.
CCalcite is soluble in weakly acidic water, while quartz resists dissolution — solubility, not hardness, governs chemical weathering rate.
DFrost wedging is more effective on limestone because its crystals are smaller.
Hardness (resistance to scratching) measures the strength of atomic bonds against mechanical disruption — it has little bearing on how readily a mineral dissolves in water. Calcite (CaCO₃) reacts with carbonic acid (H₂CO₃, formed when CO₂ dissolves in rainwater) to produce soluble calcium bicarbonate that washes away. Quartz (SiO₂) is nearly insoluble in neutral to weakly acidic water and weathers almost entirely by mechanical means. This is why caves and karst landscapes form in limestone regions but not in granite terrain.
Question 2 True / False
Weathering and erosion are two names for the same geological process: the physical breakdown and removal of rock material from Earth's surface.
TTrue
FFalse
Answer: False
Weathering and erosion are distinct processes that often operate together but are conceptually separate. Weathering is the *in-place* breakdown of rock and minerals — the rock disintegrates or changes composition where it sits. Erosion is the *transport* of weathered material (or even unweathered bedrock) by an agent such as water, wind, ice, or gravity. Erosion can happen without prior weathering (e.g., a river scouring bedrock), and weathered material can remain in place for long periods before being eroded. Conflating them hides the distinction between chemistry and mechanics.
Question 3 Short Answer
Why does frost wedging (a form of mechanical weathering) operate most effectively in climates with frequent freeze-thaw cycles rather than in climates that remain frozen year-round?
Think about your answer, then reveal below.
Model answer: Frost wedging requires liquid water to infiltrate rock fractures and then freeze. In a climate that stays perpetually frozen, water in cracks remains ice year-round and does not exploit new fractures. Frequent cycling between above- and below-freezing temperatures repeatedly drives liquid water into cracks, where it expands roughly 9% upon freezing, wedging the crack open with each cycle. Repeated cycles progressively widen fractures and eventually split the rock.
This question tests whether students understand the mechanism rather than just the name. The key insight is that it is the *transition* — liquid water entering, then freezing — that does the work. Constant freezing provides no new intrusion events; constant thawing provides no expansion force. The most efficient frost-wedging environments are alpine and periglacial zones where daily temperature swings cross 0°C repeatedly throughout the year.