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CHAPTER 10: WEATHERING AND SOIL FORMATION: Weathering is the breakdown of rocks into smaller pieces called sediment. Two types of weathering: 1. Mechanical— physical breakdown of a rock into smaller fragments, each with the same properties as the original. WHAT CAUSES MECHANICAL WEATHERING 1. Ice wedging: water squeezes into cracks in rocks and expands making the crack wider and thus wedging the rock apart. 2. Wind: nature’s method of sand blasting. 3. Water: has abrasive powers. Abrasion is where rocks rub against each other. This occurs mostly in streams and rivers where fragments bounce off each other and become rounded. Rounded Rocks Used to be Underwater 4. Plants/animals: roots can penetrate cracks and physically pry rocks apart as the root grows in size. Animals burrowing in soil expose fresh surfaces that will continue weathering. Root Wedging 2nd Type of Weathering 2. Chemical—chemical reactions dissolve minerals in rocks or change them into different minerals or into a new substance. Look for words like dissolved, rusting, mixing, oxidation and decaying RUSTING DECAYING HOW FAST DOES WEATHERING HAPPEN? It depends on several things: 1. Different rock types weather at different rates 2. The environment: cold, warm, wet, dry, humid, etc. WHAT HAPPENS WHEN ROCKS ARE COMPLETELY BROKEN DOWN? •You have soil!!! •Soil is a mixture of pieces of broken down rocks and organic materials When soil forms, it ends up in a series of layers called horizons. This monument in Egypt has weathered little in 3000 years. The same kind of monument in New York City has weathered significantly in only 100 years. Soil profiles are when the soil is looked at in cross sections. When looking at soil profiles, you can tell the difference between mature soil formation and immature soil formation. Mature soils have had enough time to develop distinct horizons. Immature soils do not have distinct horizons. A Immature MATURE SOIL An SoilPROFILE Profile …no horizons Horizon Profile: “A” horizon—top soil layer Also called topsoil Is very dark in color Contains the most humus (organic soil where plants grow) and less rock particles “B” horizon—middle soil layer Also called subsoil Much lighter than A Contains less humus than A Minerals travel from A to B horizon in a process called leaching “C” horizon — bottom soil layer •Contains weathered or broken rock—the parent material of the soil •Has very little organic matter “D” horizon — bedrock which is solid rock…no dirt AMAZING EARTH SCIENCE FACTS Weathering •__________________ is the process that rocks are broken down by water, air and organisms. Mechanical •___________________ weathering occurs in cold climatesice wedging warm,humid climates •Chemical weathering occurs in ________________ •Soil evolution starts with the weathering of _____________ bedrock Organic material must be present in order to have soil •___________ •Soil profile consists of 3 horizons Top soil A-_______________ (most evolved) leaching from A B-Less humus, ____________ Weathered rock C-______________ D- Known as solid rock or _______________ bedrock aaaaaaaaaaaaaaaa …and this glorious cliff-top property has recently been reduced by 50%