Physical and Chemical Weathering - Q3 SA#1 PDF

PHYSICAL WEATHERING What is physical (mechanical weathering)? -​ Breaking down of rocks into smaller particles (sediments) without changing the composition of the rock Frost Action/Ice Wedging: -​ Repeated freezing and thawing of water in the cracks of rocks -​ Ex. Pothol...

PHYSICAL WEATHERING What is physical (mechanical weathering)? -​ Breaking down of rocks into smaller particles (sediments) without changing the composition of the rock Frost Action/Ice Wedging: -​ Repeated freezing and thawing of water in the cracks of rocks -​ Ex. Potholes on the streets, broken rocks in the mountains Abrasion: -​ Bumping and rubbing of rocks as they collide -​ Rocks become smaller, smoother, and rounder -​ Sediments carried by a river or a stream -​ Ex. Rounded pebbles found near water and in deserts (wind) CHEMICAL WEATHERING What is chemical weathering? -​ A change in the rock that forms a new compound -​ Mostly caused by water reacting with the mineral grains in rocks to form new minerals -​ These reactions occur particularly when the water is slightly acidic from acid rain -​ Takes long and signs aren’t obvious -​ Look for bubbles released, substances dissolved, change in color -​ Composition of rock determines the effects of chemical weathering -​ Ex. Calcite decomposes quickly in acidic water -​ Limestone and marble are made from calcite  -​ Agents include water, oxygen, carbon dioxide, and acid precipitation Oxidation: -​ When iron-bearing rocks are exposed to air and water, the iron atoms combine with oxygen to form iron oxide (rust) -​ Ex. Rusty nails, planet Mars -​ Common mineral that contains oxidized form of iron is hematite Carbon Dioxide (CO2): -​ Contributes to chemical weathering -​ Combines with water and forms the acid carbonic acid and falls as precipitation -​ Can dissolve rocks such as limestone -​ Decaying matter and respiration can produce high levels of carbon dioxide -​ Acidic water from precipitation combines with this and forms stronger agent -​ Carbonic acid reacts with calcite to dissolve rocks Carbonation: -​ Acid rain dissolves rocks containing the mineral calcite -​ Eg. caves and sinkholes in limestone Hydrolysis: -​ Takes place when water reacts with some of the minerals in a rock (ex. Feldspar in granite) to produce clay and salts -​ Ex. statues made of granite, mountain Chemical reaction of ions: -​ The formation of secondary minerals in soils generally results from the recombination and addition of ions and molecules from the soil solution to the solid phase -​ Process is slow but happens -​ Ex. The weathering of feldspar to kaolinite  -​ Next soil solution Al3+ will react with soluble silica to form kaolinite: RATE OF WEATHERING Factors that affect the rate of weathering: 1.​ Climate -​ The weathering of rocks is influenced by temperature and rainfall intensity -​ Strong chemical weathering occurs in warm and very humid climates (ex. rainforest) -​ Strong physical weathering (frost action) occurs in cold and humid climates (ex. Mountains and higher latitudes) -​ Very slight weathering occurs in arid (dry) climate with low amounts of precipitation -​ The amount of chemical weathering will increase if air temperature increases and precipitation increases How does climate affect landscape development? Humid Climate Arid Climate Amount of precipitation = high Amount of precipitation = low Areas with strong chemical weathering Areas with very slight chemical weathering Landscape with smooth, rounded surface Landscape with sharp, angular surface features features 2.​ Exposed Surface Area -​ The more surface area exposed while a particle is weathering, the faster it will weather (break)  3.​ Mineral Composition -​ Rocks weather at different rates depending upon mineral composition -​ Rocks composed of harder, more resistant minerals will weather away more slowly -​ Rocks composed of softer, less resistant minerals will weather away more rapidly SOILS How does soil form? -​ Soil: the mixture of weathered rocks and biological activity over long periods of time -​ Soil profile: -​ The top layer (horizon) contains the highest amount of organic matter and weathered material -​ The bottom layer of soil contains the most amount of weathered material EROSION AND DEPOSITION Vocabulary: Weathering Weathering causes the rocks to break into fragments (sediments) Erosion Erosion moves the sediments downhill to another place (transportation) Deposition The sediments get ultimately deposited into the ocean or a lake (to settle/drop) Agents of Erosion: What are sediments? -​ Described in 3 ways: size, rounding (well rounded/angular), and sorting (well sorted/poorly sorted) based on size How are sediments deposited? Sorted sediments Unsorted sediments When larger, denser, rounder particles settle out When sediments drop out in no particular order (get deposited) first  * Organized by size * All mixed up Occurs in streams, beach, desert Occurs in landslide, glacier Erosion-Deposition by waves: How do ocean waves erode and deposit sediments? -​ Shoreline erosion is an ongoing process caused by the force of ocean waves -​ Waves are formed by the wind blowing over the water -​ As waves crash onto the shore, they carry sand and pebbles that act like natural sandpaper -​ Gradually wearing down the coastline overtime = abrasion -​ Waves also transport eroded materials (ex. sand) along the shore -​ Coastal erosion: -​ Affected by urbanization (building moves sediments around) Erosion-Deposition by wind: How does wind erode and deposit sediments?  -​ The wind picks up small particles such as sand, silt, and slay, and carries them to different locations Erosional features formed by wind: -​ Arches National Park -​ Monument Valley Erosion-Deposition by gravity (mass movement): How does gravity affect erosion and deposition? -​ Mass movement: downhill movement of rock or sediment due to gravity Soil Creep Debris Flow Mud Flow Rock Fall Gradual downhill Rapid downslope flow Downward flow of fine Rapid falling of pieces movement of soil of debris particles (mud) and of rock from a cliff or * Bent trees are the large amounts of water steep slope most obvious indicators -​ Sediments eroded by gravity are ANGULAR and UNSORTED