Weathering refers to the breakdown and dispersal of rocks and soil into smaller particles, while erosion refers to their movement into a different location.
Erosion can be caused by water, wind, ice and living organisms alike; for example, plant roots could wedge themselves into cracks in rocks over time, widening those cracks further with each passing year.
Physical Weathering
Physical weathering (also called mechanical weathering) breaks rocks apart into smaller pieces, leading to erosion (see Figure 8.5). Water, ice, salts, plants and even animals may act as agents of physical weathering.
Unloading is the primary form of physical weathering, occurring when confining pressure is released on a rock, leading it to fragment into thin sheets parallel to its surface topography and fracture.
Abrasion is another method of physical weathering that removes rough edges on rocks as water flows over them, wearing away any rough patches over time and leading to smoother rocks in riverbeds. Wind can further aid abrasion by blowing dirt particles against a rock face’s sides.
Massive rocks often crack along natural zones of weakness, such as bedding planes in sedimentary rocks, exfoliation in metamorphic rocks or joints in igneous rocks – this increases their susceptibility to chemical and physical weathering processes.
Chemical Weathering
Chemical weathering affects many rocks in addition to mechanical erosion. Chemical weathering changes the composition of rocks by altering their mineral constituents, leading to disintegration of rocks. Minerals more stable under Earth’s physical conditions – like quartz – tend to weather less than others such as olivine, pyroxene and feldspar (Figure 6.4). Chemical stability depends on pressure and temperature when crystallizing from magmas, as these factors determine their chemical stability (see below).
Chemical weathering includes processes like oxidation, hydrolysis and carbonation. Iron-containing minerals tend to turn to rust over time, weakening and fragmenting rock structures. Hydration and hydrolysis play an integral part in weathering along discontinuities like faults. Their actions result in formations such as tafoni and honeycombs in sedimentary rocks as well as frame weathering (dissolved ions travel along joints to soften rock surfaces) – processes which scientists now believe play an even greater role than physical erosion in landscape denudation.
Frost Wedging
Erosion is a physical process driven by water, gravity (see Chapter 10), wind and ice (Chapter 14) that transports rock and sediment away from sites of weathering over long time periods. Erosion usually happens slowly.
Frost wedging is a physical/mechanical weathering process in which water seeps into cracks in rocks, freezes, expands (ice has 9 percent more volume than water), widening cracks and possibly splitting rocks. Frost wedging occurs frequently in locations experiencing extreme daily temperature variations.
Minerals’ ability to dissolve other minerals is measured by their crystallization ranking in the Bowen reaction series. Minerals with higher crystallization rankings tend to weather more quickly than their counterparts with lower crystallization rankings; biological weathering occurs when plant roots penetrate cracks in rocks, prizing them apart. Bacteria, algae and lichens secrete chemicals which break down rocks they inhabit; this chemical could either be acidic or abrasive in nature.
Root Wedging
Roots from plants can penetrate small cracks in rocks and break it apart over time, geologists refer to this phenomenon as root wedging. Sometimes this can even cause rapid erosion – that’s why you see rocks falling apart on roads or sidewalks and also in ocean mudslides!
One way of measuring how fast erosion has taken place is through the “rounding” of sediment particles – well-rounded sediments have smooth corners while very angular particles retain sharp corners. We can also gauge how much energy was involved by looking at how sorted the sediments are.
Erosion can create incredible landforms such as canyons and cliffs or spectacular beaches. We can observe it slowly over time with rivers flowing through valleys depositing sediment into lakes below; or it can occur quickly such as with mudslides. Erosion is part of Earth’s natural landscape that we depend on for life to exist on this planet.