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Transcript
Weathering The breaking down of rock into smaller and smaller pieces. Mechanical Weathering • Mechanical weathering is the breakdown of rock into smaller pieces by physical means. Weathering Mechanical Weathering Chemical Weathering Ice Wedging Water Abrasion – Wind, Water and Gravity Acid Precipitation Plants Acid in Groundwater / Living Things Animals Oxidation - Air Ice Wedging Ice wedging is a process by which water flows into cracks in rock and expands as it freezes, enlarging the cracks. Ice Wedging Animation Root Wedging The roots of plants can grow into cracks in rocks, and the roots can enlarge the cracks as they grow. Abrasion Gravity, water, and wind are agents of abrasion in mechanical weathering. Sphinx Cleopatra’s Needle Plants and Animals The activities of plants and animals can mechanically weather rock. Burrowing animals for example Chemical Weathering Chemical weathering is the breakdown of rock into smaller pieces by chemical means. Water can dissolve some rocks and minerals. Acid Precipitation Sulfuric and nitric acids from pollution can cause chemical weathering. Rain and snow are naturally acidic and contain carbonic acid. Sulfuric Gases from Volcanoes also add to acid precipitation. Acid in Groundwater Natural acids found in air and water are produced by plants can cause chemical weathering. Limestone contains calcite which is reactive with acid. Caves form this way. Acids in Living Things Lichens produce organic acid. Oxidation can cause chemical weathering when oxygen combines with iron and other metallic elements. (Rust) Differential Weathering The process by which softer less weather resistant rocks weather and wear away leaving harder more weather resistant rocks behind. Rates of Weathering – Section 2 The rate at which weathering occurs depends partly on the composition of the rock being weathered. If the minerals have higher hardness values then they will weather slower than softer minerals. Rocks with a lot of quartz (7) will last longer than rocks with a lot of talc (1). Differential Weathering The elevation changes around here are due to DIFFERENTIAL WEATHERING. Rocks weather at different rates due to the different hardness of the minerals. Sandstone Shale Limestone Erosion carries sediment away after the rocks is weathered. Surface Area The greater the surface area of a rock is, the faster the rate of weathering. The only part of the cubes available to be weathered are the “faces”. 1. Has 6 faces to weather. 2. Has 48 faces to weather. 3. Has 384 faces to weather. Which one will weather the fastest? The one with more surface area (3.) The Shape of Weathering Page 253. Which pile of rocks will weather quicker? They may be the same volume of material but one will definitely weather quicker. Why? Weathering and Climate Different climates promote different rates of weathering. What climates are worse for weathering? Hot and dry climates (New Mexico) OR Wet and humid climates (Florida) Why? Oxidation (rusting) – occurs quicker in moister environments. Weathering and Elevation Factors that affect weathering: Water, ice, ice wedging. Plants, animals Oxidation Look at the factors and think about how each would relate to elevation. How does elevation affect weathering? Weathering usually occurs at a faster rate at higher elevations. From Bedrock to Soil Soil – a loose mixture of small mineral fragments and organic material. Everything above the “bedrock” Sources of Soil Soils are made from weathered rock fragments. The rock that creates the soil is called the “parent rock”. A soil derived from granite will be different than a soil derived from limestone. If the soil stays over top of the parent rock it is known as “residual soil”. Residual soils rest on top of their parent rock, and transported soils collect in areas far from their parent rock. Sources of Soil LIMESTONE GRANITE Living things also add to soil….. Soil also contains organic material. Organic – material derived from living things. Humus – very small particles of decayed plant and animal material. (HYU muhs) or (Hugh mus) not (hum mus) 20% - 30% humus is considered very good soil for growing plants. LEACHING Soil and Climate Tropical Climates – humid and a lot of rain Soils are very good for growing plants Lot of humus (20%-30%) However, lots of rain leaches the material downward and create a very thin soil. Any removal of vegetation will cause topsoil to erode away and be lost. Soil and Climate Desert Climates – very dry Low rate of chemical weathering. Water moving through the ground will evaporate as it reaches the surface leaving “salts” behind. Too much salt is toxic to plants. Salt Flats Soil and Climates Temperate Climates – “us” – both chemical and mechanical weathering occur. Not enough rain to leach out nutrients so you have thick fertile soils. Breadbasket – able to grow wide variety of plants. Soil and Climates Arctic Climate – much like desert climates, arctic areas can have very little precipitation. “Cold Deserts” Less chemical weathering and slow soil development. Very few plants can survive in this environment. Soil Conservation Soil Conservation – the various methods used to take care of the soil. Importance of soil Plant Health – better soil = better plants Housing – burrowing creatures live there. Storage – soil holds water Preventing Soil Erosion Erosion – the process by which wind and water transport soil and sediment from one location to another. Soil Conservation Soils need to be protected from nutrient depletion and erosion through the use of soil conservation methods. Cover Crops Farmers can plant crops between harvests to protect their soil. Crop Rotation Growing the same crop will deplete the soil quicker. Alternating crops allows a farmer to prolong the life of the soil. Contour Plowing Contour plowing follows the lay of the land like a topographic contour on a topo map. This prevents soil from eroding during heavy rains. Terracing Creating small flat areas out of steep hillsides for planting crops.