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Download weathering - Valhalla High School
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Which comes first Erosion or weathering? Weathering must happen before erosion can take place! WEATHERING Definition: The physical and chemical processes that break down rock on Earth’s surface. 2 Types of Weathering PHYSICAL/ MECHANICAL WEATHERING – rocks are broken apart increasing the surface area. ROCK BEFORE WEATHERING = ROCK AFTER WEATHERING (just smaller) CHEMICAL WEATHERING – Chemical reactions CHANGE minerals. ROCK BEFORE WEATHERING ≠ ROCK AFTER WEATHERING PHYSICAL WEATHERING INCREASES SURFACE AREA MORE CHEMICAL WEATHERING CAN OCCUR Surface area vs Size and Shape There are four main sources of power for mechanical weathering. Gravity, Water, Wind, & Waves Of these, water appears to be the leader in changing the surface. PHYSICAL WEATHERING MOST IMPORTANT IN OUR CLIMATE FROST ACTION – FROST WEDGING ALTERNATE FREEZE AND THAW Water expands when frozen ROCKS ARE CRACKED BY WATER FREEZING EXAMPLES: POT HOLES AND FROST HEAVES Spheroidal Weathering Fluids enter rock along joints or fractures and the edges become rounded. PHYSICAL WEATHERING – ROOT ACTION PLANT ROOTS UPLIFT AND FRACTURE ROCK Root Penetration Powerful plant roots grow into rock cracks and cause fractures. PHYSICAL WEATHERING ABRASION ROCK PARTICLES GRIND AGAINST ONE ANOTHER WATER WIND Abrasion Both wind and water can cause abrasion as rock fragments bounce off each other. Abrasion This “moon looking” picture of an Antarctic rock, shows weathering from the wind blown sands. PHYSICAL WEATHERING EXFOLIATION – PEELING AWAY OF ROCK – due to reduced pressure at earth’s surface rocks will expand but they will crack because they are brittle Unloading Fluctuating Temperatures will cause rocks to contract and expand causing cracks Examples: deserts and mountains PHYSICAL WEATHERING EXFOLIATION Unloading and Exfoliation of Igneous Rocks Dome Exfoliation Exfoliation domes form as the slabs of rock slip or slide off the bedrock. E.g. Yosemite Glacial Weathering Glaciers and avalanches can cause weathering as ice and rock interact. Other Agents of Weathering Things like flash floods, mud slides, landslides, and other forms of mass wasting can cause weathering. Chemical Weathering EXAMPLES OF CHEMICAL WEATHERING OXIDATION OCCURS when free oxygen combines chemically with metallic elements (usually iron) AKA RUST Chemical Weathering Oxygen dissolved in water will oxidize some materials like nails and rocks! (Rust) Carbon dioxide dissolved in water forms a weak acid called carbonic acid, which helps dissolve rock material. Chemical Weathering of Statues, Bath, UK CARBONATION Water containing carbonic acid dissolves minerals (all rain water is slightly acidic) Most strongly affected are calcite minerals: limestone and marble CARBONATION Stalagmites and stalactites CARBONATION – SINK HOLES Karst Topography forms caves, caverns, and sinkholes CAVES AND SINK HOLES How Granite Weathers Chemical weathering – the alteration of the internal structure of minerals by chemical reactions Potassium Feldspar + carbonic acid + water Clay mineral + Potassium bicarbonate + Silica CHEMICAL WEATHERING HYDRATION OCCURS WHEN Water combines with minerals – most often in granite (mica and feldspars) to form CLAY Chemical weathering CLIMATE CONTROLS WEATHERING PHYSICAL WEATHERING: COLD AND MOIST ALTERNATE FREEZE / THAW CHEMICAL WEATHERING: WARM AND MOIST IN BOTH CASES – WATER IS THE PRIMARY INGREDIENT THAT PROMOTES WEATHERING WHAT KIND OF CLIMATE SUPPORTS WEATHERING? HIGH PPT ↑ ↑ ↑ ↑ ↑ ↑ ↑ LOW PPT COLD →→→→→→→→→→ HOT Differential Weathering due to Climate and Composition • Masses of rock do not weather uniformly due to regional factors and composition. • Results in many unusual and spectacular rock formations and landforms Differential Weathering Rocks weather at different rates because of different mineral makeup, degree of jointing, and exposure to the elements. More resistant rock protrudes as ridges and pinnacles. Bryce National Park, Utah More resistant rock on top Rates of Weathering Factors influencing weathering: -Mineral makeup Granite versus Marble WHICH LAYER IS MORE RESISTANT TO WEATHERING? LEAST RESISTANT? A C B D YOU MIGHT ALSO SEE IT LIKE THIS. WHICH DIAGRAM IS AN ARID CLIMATE? HUMID CLIMATE? A Humid – more rounded B Arid – more angular Weathering Review Chemical Rocks are broken down through chemical changes Common in warm, humid climates Example: Limestone highly affected by acids Mechanical Rocks are broken down through physical changes Common in harsher climates Example: water freezing in cracks expands and breaks rock apart WEATHERING SUMMARY WEATHERING – INCREASES SURFACE AREA COLD/HUMID CLIMATE PHYSICAL WEATHERING – WARM/HUMID CLIMATE CHEMICAL IN BOTH CASES, WATER IS THE PRIMARY INGREDIENT THE END PRODUCTS OF WEATHERING CALLED SEDIMENTS OR SOIL SOIL: a combination of sediment, rock minerals, and humus (organic material from biologic activity)
