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Examines the physical and chemical breakdown of rocks and the factors influencing weathering rates.
Could a soft rain shower eventually topple a granite skyscraper? Over geological time, the answer is a resounding yes through the silent, relentless power of denudation.
Mechanical weathering is the physical disintegration of rock into smaller fragments without changing its chemical composition. The most potent form is frost wedging. When water enters rock fissures and freezes, its volume increases by approximately . This expansion exerts a massive outward pressure—up to —which is often greater than the tensile strength of the rock. Other processes include thermal expansion, where repeated heating and cooling cause minerals to expand and contract at different rates, and exfoliation, where the removal of overlying pressure causes rock to crack in sheets like an onion.
Imagine a crack in a granite boulder filled with of liquid water. 1. Upon freezing, the volume of ice () increases by . 2. Calculate the new volume: . 3. This extra of volume forces the crack wider, eventually shattering the rock.
Quick Check
Why is frost wedging most effective in alpine environments compared to the poles?
Answer
Alpine environments experience frequent 'freeze-thaw cycles' (diurnal temperature changes), whereas polar regions remain frozen for most of the year, preventing the necessary melting and refreezing.
In regions like the Mammoth Cave system, carbonation works over millions of years: 1. Acidic groundwater seeps through joints in limestone. 2. The chemical reaction dissolves the rock. 3. This creates underground voids (caves) and surface depressions called sinkholes.
Quick Check
If a rock contains high amounts of Iron (Fe), which chemical weathering process is it most likely to undergo?
Answer
Oxidation, which occurs when iron reacts with oxygen to form iron oxide (rust).
The rate of denudation (the overall wearing away of the Earth's surface) is controlled by the Goldich Dissolution Series and climate. High temperatures and high moisture levels accelerate chemical reactions, making chemical weathering dominant in the tropics. Conversely, cold, dry climates favor mechanical weathering. Lithology also plays a role; for instance, sedimentary rocks with many bedding planes weather faster than massive igneous rocks. The surface area to volume ratio is key: as a rock breaks into smaller pieces, its total surface area increases, providing more 'attack points' for chemical agents.
Consider a cube of rock with sides of . 1. Initial Surface Area: . 2. If we split this cube into 8 smaller cubes (sides of ): 3. New Surface Area: . 4. By breaking the rock mechanically, we have doubled the area available for chemical acid to react with, exponentially increasing the denudation rate.
Which process is responsible for the 'onion-skin' peeling of granite domes?
According to the principles of denudation, where would you expect to find the deepest soil profiles?
Mechanical weathering increases the rate of chemical weathering by increasing the available surface area.
Review Tomorrow
In 24 hours, try to sketch the carbonation formula from memory and explain why a tombstone in a rainy city erodes faster than one in a desert.
Practice Activity
Go outside and find a rock with a crack. Look for 'biological weathering'—can you find moss, lichen, or a root physically or chemically altering the stone?