GEOL 101 1nd Edition Lecture 12 Outline of Last Lecture I Earth s External Processes II Mechanical Weathering Outline of Current Lecture I Chemical Weathering II Rate of Weathering III Soil IV Slope Stability Current Lecture Chemical Weathering Chemical weathering breaks down rock components and the internal structures of minerals In this process water is the leading cause Water is the responsible for dissolving and transporting ions and molecules Three major processes Dissolution Some minerals are easy to dissolve in acidic water o Rain water with CO2 in the air to form carbonic acid o Weak acids dissolve soluble ions out of the rock o As rock dissolves small voids are formed and eventually get larger to form caves best example would be Limestone and marble which are both made of calcite Oxidation rust any chemical reaction where a compound loses electrons to oxygen o When iron mixes with oxygen it makes iron oxides like hematite Fe2O3 and with this bond iron loses an electron to oxygen so it is oxidized o This is important in decomposing mafic minerals they have high iron o Pyrite is another example as it oxidizes one of the byproducts is sulfuric acid which is very corrosive and is a huge hazard Hydrolysis Reaction of any substances or rocks with water o Hydrogen ions attack and replaces positive ions o Common reaction is when granite is weathered Since it contains the mineral KFeldspar it breaks down into a clay mineral Kaolinite when reacted ti rain water These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Chemical reactions also make physical changes to the rocks For example the edges and surface areas are more exposed which makes is weathered faster Causes spheroidal weathering rocks get round Rates of Weathering Some factors that affect the rate where rocks are weathered Climate o Temperature and moisture are huge factors o Temperature is good for physical weathering o Moisture warm is good for chemical weathering Freeze thaw cycles frost wedging Warm moist more vegetation More organic matter more acidic soil Rock Characteristics o Rocks with calcite are easily dissolved in weak acid solutions Obvious with cave and statue formations o Silicates are more resistant to weathering quartz Mafic weather faster than Felsic Granite has quartz and feldspar are very resistant Differential weathering o Masses of rock do not weather regularly because of local factors Igneous and Metamorphic rocks are more resistant to weathering than sedimentary rocks Fractures called joints form during tectonic movements where weathering happens more often Soil After rock material is weathered it may be incorporated into the soil as regolith rock mineral fragments that come from weathering Soil is a combination of sediments organic matter water and air This combination supports plant life Factors controlling soil Parent Material the source of the weathered material from which soil forms o Residual underlying bedrock o Transported carried from somewhere else and deposited o Usually transported by wind or water Time always important o Affects amount of weathering o Affects the thickness longer it forms thicker it is Climate most influential control o Affects kind of weathering which affects the kind of soil that forms o Two key factors are temperature and precipitation Plants and Animals o Important to add nutrients to the soil when they decompose o Soil is mixed by insects that burrow down o Bacteria fungi or other microorganisms decompose remains to make humus organic material within soil Slope o Steep slopes have poorly developed soils As water runs off it affects the growth of vegetation o Flat or rolling land is more suitable The soil profile Soil forming processes happen from surface downward Different layers are called horizons and there are five O A E B C remember Oscar Always Eats Banana Cake From surface downward o O horizon loose organic matter o A horizon organic and mineral matter Highly biological activity Both O and A make topsoil Generally dark in colord o E horizon little organic matter Light colored with mostly mineral particles Eluviation fine particles are carried by water going downward Leaching soluble particles dissolved by water going downward o B horizon zone of accumulating of clay sized particles o C horizon partially weathered parent material Below C horizon is unweathered parent material O A E and B together are called true soil Soil is classified using a system known as Soil Taxonomy Based on physical and chemical properties of the soil profile Has hierarchical categories of classification ranging from broadest to most specific 12 soil orders Useful for agricultural and related land use purposes Soil Erosion is part of natural recycling of earth materials Natural rates of soil erosion depend on several factors Soil characteristics Climate Slope Vegetation Slope Stability Gravity is the driving force for mass wasting If a surface is sloped then gravity pulls the material downward parallel to slope Since steeper slopes have a greater slope parallel force the material is most likely to slide down Some slope materials are stronger than others Slopes made of unconsolidated material are less stable o Loose sediments or sand Slopes made of solid material are more stable o Bedrock Factors Water content o A little bit of water can make a slope more stable due to its cohesive properties imagine a sand castle But too much water can decrease the stability o Water increases weight of material increasing the gravity and making the potential for sliding a lot higher o Water can saturate slope material to increase pore pressure the weakens the stability Vegetation o Roots help stabilize the land by strengthening the unconsolidated soil sediment and other materials o Removing vegetation from slopes leaves material exposed during heavy rains that can lead to failure