Weathering and ErosionWeathering and Erosion

Let's look at Gradational Let's look at Gradational Processes. These are the Processes. These are the forces that are trying to forces that are trying to level the earth through level the earth through weathering and erosion. weathering and erosion.

IntroductionIntroduction- erosion and weathering are part erosion and weathering are part of the forces of of the forces of gradationgradation which which do battle with tectonic forcesdo battle with tectonic forces- tectonictectonic forces: strive to forces: strive to build build upup rock structures rock structures

- gradationalgradational forces: strive to forces: strive to bring rock structures bring rock structures to a level to a level or a uniform slopeor a uniform slope; ; - this can be done in two ways:this can be done in two ways:

- by tearing downby tearing down (i.e. (i.e. degradation or erosion) or degradation or erosion) or

- filling infilling in (i.e. (i.e. aggradation… think “add” or aggradation… think “add” or deposition)deposition)

ErosionErosion

erosion can be divided into two erosion can be divided into two processes: processes: a) the a) the breaking upbreaking up of rock masses (ie of rock masses (ie weatheringweathering ) )b) the b) the carrying awaycarrying away of the weathered of the weathered rock fragments (i.e. rock fragments (i.e. transportationtransportation ) )

- - cycle of gradation:cycle of gradation: weathering, weathering, transportation, and depositiontransportation, and deposition

Weathering ControlsWeathering Controls

Chemical stabilityChemical stability Rock structure (type of rock, Rock structure (type of rock, joints, etc.)joints, etc.)

ClimateClimate Hot, wet favors chemical weatheringHot, wet favors chemical weathering Cold, dry favors physical weatheringCold, dry favors physical weathering

Presence of soilPresence of soil Soil acts as a lid, keeping water inSoil acts as a lid, keeping water in

Length of exposureLength of exposure

Factors Contributing to Factors Contributing to WeatheringWeathering

Weathering increases Weathering increases surface area = more surface area = more

weatheringweathering

exfoliation or spheroidal exfoliation or spheroidal weathering or thermal weathering or thermal

expansion and contraction or expansion and contraction or stress releasestress release

daily variations in temperature cause daily variations in temperature cause rocks to expand and contractrocks to expand and contract

various minerals in a rock expand and various minerals in a rock expand and contract at different rates; as a result contract at different rates; as a result there is a gradual splitting apart of the there is a gradual splitting apart of the rockrock

in humid climates, running water tends to in humid climates, running water tends to round off the surface features causing the round off the surface features causing the "skin" of the rock to peel off"skin" of the rock to peel off

this is also an important form of this is also an important form of weathering in desert areas where the daily weathering in desert areas where the daily temperature range can be high temperature range can be high

Bedrock that was formerly buried can Bedrock that was formerly buried can expand when overlying pressure is removedexpand when overlying pressure is removed

Mechanical (or Mechanical (or Physical) WeatheringPhysical) Weathering

the breaking up of rocks into the breaking up of rocks into smaller fragments without any smaller fragments without any change in the chemical change in the chemical composition of its minerals composition of its minerals

ExfoliationExfoliationNotice the top layer of bedrock is cracking and Notice the top layer of bedrock is cracking and separating from the underlying bedrockseparating from the underlying bedrock

Coquihalla Hwy, BCCoquihalla Hwy, BC

freeze-thaw or frost freeze-thaw or frost action or frost action or frost

shatteringshattering when the water in the when the water in the cracks and pore spaces cracks and pore spaces of rocks freezes, the of rocks freezes, the force created by its force created by its expansion is tremendousexpansion is tremendous- rocks are literally - rocks are literally split apart resulting in split apart resulting in shatteringshattering

An example of the An example of the powerful force of freeze powerful force of freeze thaw action on a large thaw action on a large boulder near Vernon, BC.boulder near Vernon, BC.

The steep slopes of fractured rocks accumulating The steep slopes of fractured rocks accumulating around Black Tusk are a result of freeze-thaw around Black Tusk are a result of freeze-thaw actionaction

Biological WeatheringBiological Weathering

plants and rootsplants and roots tree roots, for tree roots, for example, growing example, growing down into cracks down into cracks can exert can exert powerful forcespowerful forces- even mosses and - even mosses and lichens can help lichens can help to split rocks to split rocks apartapart

AnimalsAnimals burrowing animals burrowing animals can make the soil can make the soil less stableless stable

Biological WeatheringBiological Weathering

Chemical WeatheringChemical Weathering

the decay of rock through the decay of rock through actual actual chemical changechemical change in the in the composition of its mineralscomposition of its minerals

most common in most common in warm and humid warm and humid climatesclimates where both water and where both water and heat speed up chemical reactionsheat speed up chemical reactions

Can be expedited by human Can be expedited by human influences influences

(ie. Acid rain caused by industries)(ie. Acid rain caused by industries)

Notice how the engravings on the tombstone on the Notice how the engravings on the tombstone on the right are no longer clearly visible.right are no longer clearly visible.

carbonation or solution carbonation or solution

occurs with occurs with limestonelimestone (easiest (easiest to erode)to erode)

groundwatergroundwater absorbs carbon absorbs carbon dioxide to form a slightly dioxide to form a slightly acidicacidic solution solution

destroys jointsdestroys joints (joint: line of (joint: line of weakness in a rock) and bedding weakness in a rock) and bedding planes to form karst topographyplanes to form karst topography

When limestone interacts with underground water, the water dissolves the limestone to form an amalgamation of caves, underground channels, and a rough and bumpy ground surface. Named for the Kras plateau region of eastern Italy and western Slovenia (Kras is Karst in German for “barren land”)

hydrolysis hydrolysis occurs especially with occurs especially with granitegranite

Minerals like feldspar Minerals like feldspar easily react with easily react with slightly acidic water slightly acidic water to create clay and to create clay and saltssalts

Clay is easily carried Clay is easily carried away in solution by away in solution by waterwater

causes rock to whiten causes rock to whiten (erodes slowly)(erodes slowly)

oxidationoxidation

creates rusty red rockcreates rusty red rock occurs especially with rock occurs especially with rock containing iron nitratecontaining iron nitrate

erodes and forms soil erodes and forms soil

hydrationhydration occurs especially with rocks occurs especially with rocks containing salt mineralscontaining salt minerals

water is absorbed into the internal water is absorbed into the internal structure of the rock causing swelling structure of the rock causing swelling and making it vulnerable to breakdown and making it vulnerable to breakdown due to pressure and potential chemical due to pressure and potential chemical structure changesstructure changes

a physical-chemical processa physical-chemical processeg gypsum results from water being eg gypsum results from water being added to anhydrite (CaSO4) added to anhydrite (CaSO4)

General Rule:General Rule: igneous and metamorphic rocks are igneous and metamorphic rocks are more susceptible to chemical more susceptible to chemical decompositiondecompositionthan mechanical disintegrationthan mechanical disintegration

the reverse is true for sedimentary the reverse is true for sedimentary rocks = more mechanical rocks = more mechanical disintegrationdisintegration

A notable exception to this is A notable exception to this is limestone, a sedimentary rock that is limestone, a sedimentary rock that is highly susceptible to chemical highly susceptible to chemical weathering.weathering.

Weathering vs ErosionWeathering vs Erosion

Weathering: The general process by Weathering: The general process by which rocks are broken (physically which rocks are broken (physically and chemically) at the Earth’s and chemically) at the Earth’s surface surface

Erosion: The process by which the Erosion: The process by which the surface of the Earth is worn away surface of the Earth is worn away (encompasses (encompasses weatheringweathering and and transport)transport)