2WeatheringWeathering is simply the breaking down of rocks and other materials at Earth’s surface.This may be done by mechanical OR chemical means.
3Types of Weathering Mechanical Weathering (physical) Takes place when rock is split or broken into smaller pieces of the same material without changing its chemical makeup.Example: Breaking of a rock cliff into boulders and pebbles
4Weathering Mechanical Common weathering processes:Ice wedgingTemperature changesRoot actionWindRunning Water
51. Ice Wedging (Frost Action) Water takes up about 10% more space when it freezes.This expansion puts great pressure on the walls of a container.Water held in the cracks of rocks wedges the rock apart when it freezes.Often occurs in places where temperatures vary from below the freezing point of water to above the freezing point.
7Where does Ice Wedging Occur? Occurs mostly in porous rocks and rocks with cracks in themBare mountaintops are especially subject to ice wedging.
8Where do I see ice wedging? Ice Wedging causes:Vast fields of large, sharp-cornered bouldersPotholes on streets and highways
92. Temperature Changes (Exfoliation) Heat makes things expand, or become larger.Cooling makes things contract, or become smaller.
10How does temperature change break rocks? During the day, heat can cause the outside of a rock to expandAt night, the outside of the rock cools and contracts This constant expanding and contracting can weaken the rock and cause it to break
11Repeated heating and cooling may cause pieces to flake or peel off.
123. Root ActionPlant roots in search of minerals and water can grow into cracks in rocks.The pressure of the growing root can make rocks break apart.Example: Cracks in sidewalks
201. HydrolysisThe chemical reaction of water with other substances is called hydrolysis.When heated or dried a little, minerals that have undergone hydrolysis may fall apartFor example:Feldspar + H20 Clay
212. OxidationThe chemical reaction of oxygen with other substances is called oxidation.Iron-bearing minerals are the ones most easily attacked by oxygen.Examples:MagnetitePyriteOxidation of these minerals results in rusting, or iron oxides Rocks weaken and crumble
233. Carbonation Carbon dioxide dissolves easily in water. It forms a weak acid called carbonic acidThis is the same compound that is in carbonated drinks.This is how acid rain is produced.
24What does Carbonation Affect? Has the greatest effect on calcite than any other mineral.This process changes the mineral calcite Limestone and marble are made of calcite so they are affected by carbonation
254. Other Acids Some green plants can produce weak acids MOSSES Decaying organisms also produce acidsThese acids can seep into cracks and break the rock apart.
271. ClimateThe amount of water in the air and the temperature of an area are part of the CLIMATE.The more water there is in the air, the faster weathering will occur.Weathering occurs fastest in hot, wet climates; slowest in hot, dry climates
28This is a monument called Cleopatra’s Needle This is a monument called Cleopatra’s Needle. It was carved in Egypt around 1450 B.C. The sides are carved with hieroglyphs, the writing of ancient Egypt. What is the climate like in Egypt?
29In 1800, the monument was moved to New York City In 1800, the monument was moved to New York City. Almost immediately, the hieroglyphs began to fade. In only a few years, the Egyptian writing became indistinct! What is the climate like in NYC?
30What happened to Cleopatra’s Needle? Cleopatra’s Needle was carved from granite, a hard tough, crystalline rock.Although it is tough, granite is changed by the atmosphere.Some of the minerals that make up granite change to clay.Chips and flakes of minerals break away from the granite surface.
312. Surface AreaMost weathering occurs on exposed surfaces of rocks and minerals.The more surface area a rock has, the more quickly it will weather.
333. Rock CompositionSome minerals that hold rocks together weather more quickly than others.Quartz resists weathering.Feldspar weathers easily.Think about chemical weathering:Rocks with calcite, such as limestone, will weather easilyRocks with iron will weather faster than those without.
364. Chemical ReactionCarbonic acid dissolves calcite, a mineral found in marble and limestone.Acid rain Pollution speeds up the process of weatheringFactories and cars release carbon dioxide and other gases, such as sulfur and nitrogen.These gases contribute to acid rain.