Lecture 3: Weathering Flashcards
The type and nature of weathering that occurs depends on what three characteristics of the rock material?
Structure, texture (Particle size = different SA;V), composition (chemical stability and proportion of different minerals present)
Name the three primary weathering agents.
Water, heat, dissolved reactants
How is physical weathering different to chemical weathering?
Physical weathering associated with some form of volume change - chemical is the breakdown of rock by chemical reactions that usually involve water.
Describe the process of thermal weathering.
Differential heating of rocks and differential expansion may cause exfoliation.
Salt weathering is a form of physical weathering but how do it operate?
Salt precipitates in cracks in the rocks, expanding crystals exert stress, and this repeated cycle of saturation and drying weathers rock over time.
What are the three general reactions in chemical weathering and how do they occur?
Dissolution - ions from the mineral dissolve in water and carried away
Hydration - water enters rock carrying ions that combine with the mineral.
Exchange of ions = hydrolysis.
Hydrolysis occurs when a free hydroxyl or hydrogen ion enters the minerals structure to create a new compound. the presence of what helps to speed this up? How?
Carbon dioxide - by reacting with water to form a weak carbonic acid, which can then dissociate to form hydrogen carbonate and the hydrogen ions needed for the chemical breakdown.
Weathering rate is fastest under what climatic conditions? under what vegetation conditions? And in parent rock of what properties?
Hot and heavy rain - Thick, abundant vegetation - highly soluble minerals and fractured or thinly bedded minerals.
Name three ways of estimating weathering rate.
Time-controlled (/mimicked) experiments. Observations of architecture of a known age (inc. gravestones). Radiometric ages of rock/sediments.
The feldspar and Fe/Mg material in basalt is weathered to produce clay minerals. What does the magnetite weather to and what ions are formed in the overall process?
Goethite. Feldspar produces Na+ and Ca2+ whilst Fe/Mg material produces Mg2+.
Whilst Fe/Mg material in granite weathers in the same way as in basalt, the weathering of feldspar is slightly different; how? What other mineral in granite is this like?
Feldspar produces na+ ions as before, but also K+ ions, not Ca2+. Mica in granite also weathers to produce K+ ions.
Which mineral in granite does not weather to form a new product?
Quartz
What different forms can weathering products take? What may happen to them?
Dissolved substances, clay materials, weathering rind, soil. Secondary enrichment through chemical weathering may occur, leaching away certain materials to concentrate metal elements.
How does weathering affect CO2 in the atmosphere?
Decreases atmospheric CO2 levels as converts it to HCO3-
