Weathering and Erosion Lets look at Gradational Processes. These are the forces that are trying tolevel the earth through weathering and erosion.
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… think “add” or deposition)
Erosion erosion can be divided into two processes: a) the breaking up of rock masses (ie weathering ) b) the carrying away of the weathered rock fragments (i.e. transportation ) - cycle of gradation: weathering, transportation, and deposition
Weathering Controls Chemical stability Rock structure (type of rock, joints, etc.) Climate Hot, wet favors chemical weathering Cold, dry favors physical weathering Presence of soil Soil acts as a lid, keeping water in Length of exposure
Weathering increases surface area = more weathering
exfoliation or spheroidal weathering or thermal expansion and contraction or stress release daily variations in temperature cause rocks to expand and contract various minerals in a rock expand and contract at different rates; as a result there is a gradual splitting apart of the rock in humid climates, running water tends to round off the surface features causing the "skin" of the rock to peel off this is also an important form of weathering in desert areas where the daily temperature range can be high Bedrock that was formerly buried can expand when overlying pressure is removed
Mechanical (or Physical) Weathering the breaking up of rocks into smaller fragments without any change in the chemical composition of its minerals
ExfoliationNotice the top layer of bedrock is cracking and separating from theunderlying bedrock
freeze-thaw or frost action or frost shattering when the water in the cracks and pore spaces of rocks freezes, the force created by its expansion is tremendous - rocks are literally split apart resulting in shattering An example of the powerful force of freeze thaw action on a large boulder near Vernon, BC.
The steep slopes of fractured rocks accumulating around BlackTusk are a result of freeze-thaw action
Biological Weathering plants and roots Animals tree roots, for burrowing animals can example, growing make the soil less down into cracks can stable exert powerful forces - even mosses and lichens can help to split rocks apart
Chemical Weathering the decay of rock through actual chemical change in the composition of its minerals most common in warm and humid climates where both water and heat speed up chemical reactions Can be expedited by human influences (ie. Acid rain caused by industries)
Notice how the engravings on the tombstone on the right are nolonger clearly visible.
carbonation or solution occurs with limestone (easiest to erode) groundwater absorbs carbon dioxide to form a slightly acidic solution destroys joints (joint: line of weakness in a rock) and bedding planes to form karst topography
When limestone interacts with underground water, the water dissolves thelimestone to form an amalgamation of caves, underground channels, and arough and bumpy ground surface.Named for the Kras plateau region of eastern Italy and western Slovenia (Krasis Karst in German for “barren land”)
hydrolysis occurs especially with granite Minerals like feldspar easily react with slightly acidic water to create clay and salts Clay is easily carried away in solution by water causes rock to whiten (erodes slowly)
oxidation creates rusty red rock occurs especially with rock containing iron nitrate erodes and forms soil
hydration occurs especially with rocks containing salt minerals water is absorbed into the internal structure of the rock causing swelling and making it vulnerable to breakdown due to pressure and potential chemical structure changes a physical-chemical process eg gypsum results from water being added to anhydrite (CaSO4)
General Rule: igneous and metamorphic rocks are more susceptible to chemical decomposition than mechanical disintegration the reverse is true for sedimentary rocks = more mechanical disintegration A notable exception to this is limestone, a sedimentary rock that is highly susceptible to chemical weathering.
Weathering vs Erosion Weathering: The general process by which rocks are broken (physically and chemically) at the Earth’s surface Erosion: The process by which the surface of the Earth is worn away (encompasses weathering and transport)