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Weathering – the chemical and physical breakdown of rocks at or near Earth’s surface. Two types of weathering: 1) Chemical weathering – breakdown of rock through a change in mineral or chemical composition. ex: when iron combines with oxygen to make rust (oxidation) ex: the effect of water on minerals (water is a universal solvent) 2) Physical weathering – breakdown of rock into smaller pieces without a chemical change. ex: frost action – water going through temperature changes, breaks the rock apart ex: abrasion – when rocks grind against each other Factors that Affect the Rate and Type of Weathering Exposure – generally the closer a rock is to the surface, the faster it will weather Particle size – when rock particles are smaller, the total surface area per unit volume exposed to weathering is greater. Factors that Affect the Rate and Type of Weathering Cont. Mineral composition – different minerals have different physical and chemical properties Climate – chemical weathering mostly occurs in warm, moist climates. In cold climates, frost action is most common (physical weathering). Soil Formation Physical and chemical weathering processes are important in the formation of soil. Soil is the mixture of rock particles and organic matter on Earth’s surface that supports rooted plants. Soils develop horizontal layers with distinctive profiles Soils can be transported from other areas by wind, moving water, or glaciers. Erosion weathering of rock particles that are transported as sediments Erosion, shapes and lowers Earth’s surface Involves a transporting system which includes: 1) an agent of erosion, such as a stream, glacier, wave, current, wind or human activity 2) the sediments being moved 3) a driving force (gravity) Energy from the Sun plays an important role: Sun’s energy drives the water cycle, which produces precipitation and running water Gravity Erosion Gravity is the driving force behind erosion Gravity can pull rocks and sediments downhill, this is called mass movements Running Water Erosion and Streams Running water is the most common agent of erosion Running water erosion that is confined to a channel is called streams A smaller stream that flows into a larger stream is called a tributary Streams carry sediments in different ways: 1) dissolved minerals are carried in solution 2) solid sediments are carried in suspension 3) larger sediments are carried by rolling, sliding or bouncing on the stream bottom (results in rounding of sediments by stream abrasion) Running Water Erosion and Streams Continued Over time, streams carve deeper channels and a V-shaped valley forms The area of land drained by any one stream is called its watershed or drainage area. Stream Velocity Three factors that determine average velocity: 1) gradient (slope of the stream) 2) discharge (volume of water in the stream) 3) stream channel shape (the shape of the bedrock or loose materials that confine the stream) • • • • If the stream has a wide, flat stream channel, then there is a large surface in contact with moving water (more friction) If the stream has a semicircular stream channel, the velocity will be greater Velocity changes depending on the path of the stream: -if the stream is straight, the greatest velocity exists in the center of the stream -if the stream curves, the greatest velocity exists on the outside of the curves A stream with greater velocity can carry larger sediment particles Stream Velocity Velocity changes depending on the Shape of the Stream Sediment Load changes depending the Stream’s velocity Evolution of a Stream Streams slowly change their characteristics over time. In its uppermost part, the stream, starts as a finger lake. In early stages there are many abrupt changes resulting in waterfalls and rapids Farther down the stream, more water is added to the stream (from tributaries and ground water) to increase velocity As the stream grows larger, it begins to shift and meander (bend or curve) If there is flooding, a lake may form from a meander and/or floodplain Near the end or mouth of a stream there is often a fan-shaped delta. Wind Erosion * Two main points – 1) deflation 2) sandblasting (abrasion) * Deflation takes place in areas with small, loose sediments exposed to the atmosphere. Winds blow away the loose sediments, lowering the land surface. * Sandblasting occurs when winds blow sand or silt grains and other objects. The pelting by the grains can erode, or abrade rocks. Glacial Erosion A glacier is a naturally formed mass of ice and snow that moves downhill on land under the influence of gravity Mountain glaciers are formed in high mountain valleys Ice-sheet or continental glaciers cover huge landmass regions The glacier acts like a fluid, it moves fastest in the center than on the sides If more snow and ice accumulates than wastes away, the glacier advances If more snow and ice wastes away then accumulates, the glacier retreats. -When a glacier moves over land, rocks and other materials beneath in freeze into the ice and are dragged along -These sediments that are dragged along, scratch the surface beneath them -This action creates glacial grooves and glacial parallel scratches -The direction of exposed glacial grooves and scratches shows the direction of former glacial movement - The wide, thick ice of a glacier erodes its valley walls to form a U-shaped valley Wave and Current Erosion Waves and currents act as agents of erosion When waves enter shallow water near the shore, they drag against the bottom. The waves then become unstable and water rushes toward the shore as breaking waves or surf. These waves continue to pound against the shore line usually at an angle. Due to this angle, the water flows in one direction along the shore. This is called a longshore current. Over time, parts of the shore erodes away In the zone of breaking waves, deposited sediments can create a series of mounds called sandbars. Wind Erosion Glacial Grooves Striations Glacial Flow Longshore current