Weathering and Soil Formation Weathering… • Weathering – the breaking down of rock and other materials on the Earth’s surface. A slow, continuous process. The effects are not always easily observed and others are obvious. Example: paint peeling off a house. Weathering cont. • Rocks are broken down by two types of weathering. • Mechanical – when the forces of weathering break rocks into smaller pieces but do not change the chemical makeup of the rocks. • Chemical – the chemical makeup of the rock is changed. Mechanical Type 1 • Temperature – heat causes the outside of rock to expand, cooling causes the rock to contract. This continuous cycle causes the rock to go through exfoliation (curved sheets or slabs). Mechanical Type 2 • Ice Wedging– the repeated freezing and melting of water that has seeped into the cracks of the rocks. Example: road ways, drive ways…) Mechanical Type 3 • Organic Activity – a plant growing in a crack, roots of plants loosen rock material; or any activity caused by living things. Mechanical Type 4 • Gravity – pulls loosened rocks off of cliffs or mountain sides. • Example: Landslide – large movement of loose rocks and soil. • Example: Avalanche – large movement of loose rocks and snow. Landslide Story Mechanical Type 5 • Abrasion – windblown sand causes weathering of rocks. Carried by wind, water, or other forces. Causes riverbed rocks to be rounded and smooth. Chemical Type 1 • Water – (universal solvent) can dissolve most of the minerals that hold rocks together. Can also form an acid when mixed with certain gases which speeds up decomposition. Chemical Type 2 • Oxidation – oxygen chemically combines with another substance. The result is an entirely different substance. Iron in rocks turns to rust. Chemical Type 3 • Carbonation – carbon dioxide dissolves in rain water, forming a weak acid called carbonic acid. Dissolves certain rocks (feldspar and limestone). • Acid Precipitation – air polluted with sulfur oxides (from burning coal) dissolve in rain. Causing acid rain to fall which corrodes rocks, metals, and other materials quickly. • Plant Acids – produce weak acids that dissolve certain minerals in rocks (example: mosses and lichens) Formation of a Cave Is this weathering your brain? Differential Weathering • The composition of rock greatly affects the rate at which rock weathers • Softer rock weathers away leaving the harder more resistant rock behind • Limestone and other sedimentary rocks that contain calcite weather rapidly while other sedimentary rock weather faster depending on what cements them together • Some are more resistant than igneous Rate of Weathering… • Amount of exposure can determine the rate, the amount of rock that is exposed, and the amount of time • More surface area means more weathering • Fractured and jointed rock weathers faster • Climate that has extreme seasons allows the greatest rate of weathering. Hot dry climates allow the least amount of weathering • Higher elevations and steeper topography allow for faster weathering • Plant, animal, and human activities can accelerate weathering. Soil Formation • Soil- formed when rocks are continuously broken down by weathering and is extremely important for most living organisms.\ • Plants - food (minerals/water) • Animals – plants/animals to eat. Types of Soils • Residual soil – remains on top of its parent rock and has similar chemical composition. • Transported soil – moved from its origin by wind, water, glaciers, waves…and can be completely different than the rock is lays upon • Humus – decayed material (plants/animals). Important for the growth of plants because it helps to speed up the breakdown of rocks into soil. Moles, earthworms, ants, and beetles help break down big pieces of soil. Filled with nutrients. Soil Composition • Pieces of weathered rock and organic material (humus) are the two main ingredients of soil. • Rock particles are more than 80% and air and water are present in the pore spaces. • Clay and quartz are the most abundant because they are very stable. • Nitrates (potassium, phosphorous, and nitrogen) are vital to plant growth and found in fertilizers. • Why are pore spaces important for plants? Worm Farm Soil Texture • Type of weathering affects the composition of the soil and the texture of the soil (size of grains). • Gravel – between 2 and 64 mm in diameter. • Sand – less than 2mm • Silt – 1/16th of a mm • Clay – less than 1/256th of a mm Soil Horizons • Soil Horizons – soil layers. • Each horizon is different. A cross section is called a soil profile. • Soil that has 3 layers is mature (takes many thousands of years and the right conditions. • Factors: surface features, time, climate and type • Soil that has 2 layers is immature. Horizons… • Horizon O-part of the A layer where all the organic material is produced. • Horizon A – uppermost layer, dark colored, much activity by living occurs, topsoil (humus); most fertile. • Horizon B – leached out minerals, clay and some humus, called subsoil, formed very slowly. • Horizon C – partly weathered rocks, extends down to top of un-weathered parent rock; composition similar to parent rock. Soil and Climate • Tropical soils- soils develop quickly, top layer called laterites that contain Fe and AL • Temperate soils- A, B, and C are thick. Areas that receive 65 cm of rain have a top layer called pedalfur that contains clay, quartz, and Fe. • Areas that receive less rain have a top layer called pedocal that contains CaCO3 • Desert and Arctic soils- thin with little humus Try and Get excited about dirt… Soil Erosion • Weathered material are transported by gravity, wind, glaciers, and water Gullying and Sheet Erosion • Soil that is washed away with each rainfall can furrow and eventually become gullies. • Sheet erosion - when the top layer of soil is washed away Soil Conservation • Contour Plowing Strip Cropping Terracing A true soil lover!!!!