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Transcript
Weathering and Soils Weathering and Soils Weathering and Soils #: 1 Weathering Weathering is the breakingbreaking-down of rock and sediments over time. -Most rocks, like granite, form under earth’s surface. -The rocks uplift and eventually make their way to earth’s surface. -Conditions on the surface are much different from inside earth. -Rocks are not used to these conditions, become unstable, and break down. There are two major types of weathering: Mechanical Weathering Chemical Weathering: Weathering and Soils #: 2 1 Mechanical Weathering Mechanical Weathering: - Physically breaks rock into smaller pieces. - Rock actually disintegrates, but does not change in composition. composition. - Example: like a jackhammer smashing up rock or concrete. The major types of mechanical weathering are: - Frost wedging - Exfoliation/Unloading - Organic Activity Weathering and Soils #: 3 Mechanical Weathering: Frost Wedging - Water gets into fractures in rock. - When the water freezes, it expands and breaks the rock apart. - Most often in regions of constant “freeze and thaw” thaw” cycles. Weathering and Soils #: 4 2 Mechanical Weathering: Exfoliation 1. Rock weathers and erodes away. 2. There is less weight and pressure pushing down on rock. 3. The rock expands upward under the reduced pressure and breaks into sheets or slabs. Pressure Weathering and Soils #: 5 Mechanical Weathering: Organic Activity 1. Plant roots grow into rocks and split it apart. 2. Burrowing animals loosen sediments. Weathering and Soils #: 6 3 Mechanical Weathering Area= L X W Surface Area: (Area) X (# of sides.) - Once a rock weathers, its surface area increases. - Increase in surface area exposed allows for more weathering. THUS: Weathering speeds up over time. Weathering and Soils #: 7 Chemical Weathering Chemical Weathering: - Breaking down (dissolving) of minerals in rocks. - Occurs when rock reacts to air, water, and other chemicals. The major types of chemical weathering are: - Oxidation - Acids Reactions Weathering and Soils #: 8 4 Chemical Weathering: Oxidation 1. When rocks containing iron are exposed to air and water. 2. The rocks oxidize or “rust” rust” producing a red color. Example: a rusty nail or a rusty bike chain Weathering and Soils #: 9 Chemical Weathering: Acid Reactions 1. When water reacts with carbon dioxide it forms carbonic acid. 2. The acid reacts with rocks that contain calcite (limestone). 3. Rocks like limestone will dissolve from the acid. Example: When we used Hydrochloric Acid in lab to test for calcite. Carbonic Acid Chemical Reaction with Limestone: H2O Water + CO2 Carbon Dioxide CaCO3 + H2CO3 Calcite Carbonic Acid H2CO3 Carbonic Acid Ca2+ + 2HCO3Calcium Ion Bicarbonate Ion Dissolved Limestone Weathering and Soils #: 10 5 Factors affecting weathering: Two major factors that affect weathering: 1. Mineral Composition of Rock 2. Climate of a region. Weathering and Soils #: 11 Factors affecting weathering: Mineral Composition of Rock - Some minerals are less stable (dissolve faster) than others. Quartz (sand) is one of the most stable minerals. Which is why it is so common on earth’s surface. Weathering and Soils #: 12 6 Factors affecting weathering: Climate of a Region. - The warmer and more wet a region is, the more weathering occurs. occurs. - Climate affects both mechanical and chemical weathering. Weathering and Soils #: 13 From weathering to soils: Soil Formation Once rock weathers, a layer of rock and mineral fragments is produced. produced. - This layer of sediment is called Regolith. - Regolith eventually turns into soil over time. HOW DOES REGOLITH TURN INTO SOIL? -The The rock and mineral fragments continue to break down. -Organic Organic matter from plants & animals is added to the regolith. -The further. ther. The decaying organic matter helps to break down the regolith fur -Eventually Eventually the rock and mineral fragments turn into fertile soils. soils. Weathering and Soils #: 14 7 Factors affecting Soil Formation There are 5 major factors that affect soil formation. - Climate Rate of weathering changes with climate: warmer and wet = more weathering - Slope of the Land Erosion removes soil-forming materials. Steep land has poor soils. - Types of rocks weathering Some rocks weather easier than others: granite vs. limestone. - Types of vegetation in the region Vegetation adds organic matter to soils which helps in soil formation. - Amount of time rock has been weathered. The more time that goes by, the more weathering = more rich fertile soils. Weathering and Soils #: 15 Weathering and Soils #: 16 Soil Composition Composition of Soils: A mixture of: 1. 2. 3. 4. Rock & mineral fragments Organic matter Air Water. 8 Soil Profile - A soil profile is a cutcut-away look at layers of soil built up over time. - Each layer in a profile is called a horizon. horizon. - There are 3 major soil horizons each with different characteristics. characteristics. •Layer known as “top soil.” •Littered with organic material. •Humus: decayed organic material. •Dark and fertile; nutrient rich. o A- H o B- H rizon rizon n orizo C- H •Less organic material. Less fertile •Accumulation of mineral matter that was leached from A horizon. •Contains the “parent rock.” •Rock that has produced above soil. •Little weathering has taken place Weathering and Soils #: 17 Soil Types • Soils rich in Al- and Fe-oxides and hydroxide • Generally with a thin A-Horizon over a thicker B-Horizon • Typical of soil in areas of moderate-to-high rainfall such as the eastern US and Canada as well as Europe •Generally good agricultural soils Weathering and Soils #: 18 9 Soil Types • Soils rich in Ca (typically as CaCO3) • Thin A- and B-horizons commonly with a crust of soil cemented by CaCO3 (caliche) •Typical of soils developed in warm, dry areas (e.g. southwest US) •Generally poorer soils (little organic matter) Weathering and Soils #: 19 Soil Types • Deep, red soils stripped of all silicates leaving mostly Fe- and Al-oxides and hydroxides • Very thin soil organic-rich humus layer due to constant recycling to the surface plants • Typical of equatorial rain forests (e.g. Brazil) • Results in poor soils subject to rapid damage Weathering and Soils #: 20 10