Introduction Erosion and weathering are part of the forces of gradation which do battle with tectonic forces Tectonic forces: strive to build up rock structures Gradational forces: strive to bring rock structures to a level or a uniform slope; this can be done in two ways--by tearing down (i.e. degradation or erosion) or filling in (i.e. aggradation or deposition)

Introduction Erosion can be divided into two processes: a) the breaking up of rock masses (i.e. weathering ) b) the carrying away of the weathered rock fragments (i.e. transportation ) Cycle of gradation: weathering, transportation, and deposition

Cycle of gradation weathering transportation deposition

Weathering vs Erosion if a particle is loosened, chemically or mechanically, but stays put, call it weathering. Once the particle starts moving, call it erosion.

Regolith The broken rock particles that are the result of weathering

Bedrock solid unweathered rock lying beneath surface deposits of soil

Physical Weathering Usually occurs in northern climates due to temperature fluctuation above and below freezing more rock is exposed because soil is not very deep Agents of Physical Weathering: rain, wind, heat cold

Physical Weathering Types: Frost Shatter/Freeze Thaw Thermal Expansion Exfoliation/Pressure release Wetting Drying Salt Crystallization

Frost Shatter Water enters cracks in rock and freezes As water expands it expands the crack Over time, it will break the rock apart

Salt Crystallization All water has salt When water evaporates it leaves salt crystals behind. This fill pores and cracks in rock and works in a similar fashion to frost shatter. It is normally associated with arid climates where strong heating causes strong evaporation and therefore salt crystallization. It is also common along coasts.

Thermal Expansion Usually an arid region that has hot days and cold nights (deserts) Usually impacting just the top few centimetres The constant expansion and contraction weakens the rock leading to it breaking apart

Exfoliation/Pressure release As a rock is exposed at the surface of the earth (due to erosion), pressure is released allowing layers of rock to ‘peel’ off Also known as ‘unloading’ and ‘sheeting’ Eventually, rock becomes what is called an exfoliation dome

Plants and Roots Tree roots, for example, growing down into cracks can exert powerful forces Even mosses and lichens can help to split rocks apart

Wetting/Drying All rocks absorb water, some more than others This causes the rock to swell During the drying out process, the outer surface of the rock shrinks- this weakens rock The weakened rock cracks and eventually breaks apart. Usually near the coast (tides) and tropical climates where rocks could dry out quickly

Chemical Weathering Usually happens in warm moist climates (equatorial regions) where there is a lot of rainfall and most rock is buried under deep soil. Greater chemical weathering takes place below the surface

Chemical Weathering Main Types: Solution Hydrolysis Oxidation Hydration

How water becomes acidic As water droplets fall through the atmosphere they absorb carbon dioxide creating carbonic acid Normal pH of water is 5.6- the lower the number the more acidic it is- ex. Acid rain is ~4.1 When water hits the earth and combines with decomposing plant and animal matter

Solution Some rocks are made of minerals that are completely soluble. By the time rain water reaches the earth, it is a mild carbonic acid. This acid works to break down the rock Ex. Clints and grikes

Hydrolysis “-lysis” means to split apart- the H and O2 molecules split and attach to other minerals in the rock Carbonic acid works on silicate minerals Water ions replace the silicate ions and they get washed away Clay minerals are not soluble so areas that have a high clay content in the soil are areas where hydrolysis is prominent Ex. Spheroidal weathering

Hydration Entire H2O molecule bonds with mineral in rock causing it to become soluble.

Oxidation Oxygen binds with iron to form iron oxide When this contacts water it can be dissolved and carried away Sign of oxidation in a rock is the rust colour of the iron oxide

Notice the difference between the upper oxidized layer and the grey unweathered bedrock

Biological/Biotic weathering

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