Weathering

Weathering - physics & chemistry in action

Physical weathering:

The simplest, of these processes is freeze-thaw weathering; its power is also easily demonstrated by freezing water in a glass bottle, sealed into a plastic bag. The bottle should burst as the water expands on freezing to form ice, with a 9% expansion. Note that plastic bottles rarely burst and broken glass bottles should be handled with extreme caution - the old-fashioned cast-iron tubes are still best, but expensive! Exfoliation weathering (expansion & contraction of mineral grains) can also be demonstrated using granite chips (see 'experiments & demostration'). However, the reality of this process is actually much more complex, as it usually involves microscopic-scale chemical weathering by dew-fall that also soaks into the rock surface; evaporation then leads to the crystallisation of salts; these crystals gradually prize rock grains apart as they grow, making the surface layers of rock expand and break away.

Chemical weathering:

To most of us, the idea of “chemistry in action” means stinks, bangs or, at the very least, colour changes that occur before our eyes. Weathering processes are very slow however, so only their results are observable for most processes. The exception to this rule is the dissolution of limestone by a weak solution of carbon dioxide, which may be readily shown in the lab, and is included in the 'Experiments and Demonstrations' section.

Hydrolysis is by far the most important (but also perhaps the most complex) weathering process, as it is by this means that most silicate minerals - apart from quartz (SiO₂) - are broken down to form clay minerals and soluble salts. Silicate minerals are the main constituents of igneous and metamorphic rocks, hence quartz and clay are by far the most common constituents of sedimentary rocks. That said, the chemistry itself is not pretty - here’s one of the simplest reactions (not for student consumption!):

2KAlSi₃O₈ + 2CO₂ + 3H₂0                                            Al₂Si₂O₅(OH)₄ + 2k⁺ + 2HCO3^- + 4SiO₂
Potassium Feldspar                                                      Kaolinite (clay)       soluble salts

Biological weathering:

Some scientists prefer not to use this term as it describes both physical and chemical processes that are simply brought about, or assisted, by the presence of living organisms. Thus, a tree root helping to prize a rock-joint apart is a physical process, whilst the action of acid secreted by a rock-boring piddock shell is a chemical process. For all that textbooks tend to quote such large-scale examples of “biological weathering”, it is actually the biochemical action of microbes (bacteria, algae etc.) that contribute most to the breakdown of rocks and are crucial in soil-forming processes.

Teaching resources:

Teachers are encouraged to collect interesting bits of weathered rock for class discussion as and when they may find such things - often, effects such as colour banding on joint surfaces, are hard to explain - it is the generation of ideas that matters. Local resources such as old stone walls or graveyards can be used very effectively to show weathering effects, either directly to students or via photographs. Graveyards are often especially useful, since they can demonstrate how different rock types respond to weathering processes over measured time periods, and even the effects of different aspect (exposure to sun and prevailing wind).