Lecture 11 Finish Volcanoes Weathering, Erosion and Soil FormationQuestionsExplosivity of volcanoesPowerPoint PresentationMount St. Helens, 1980Slide 6Slide 7Slide 8Slide 9Slide 10TsunamisKrakatau 1883 (Krakatoa)Slide 13“Supervolcanoes”Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20What is Weathering? Erosion?Why is weathering important?Why do minerals and rocks weather?Slide 24Slide 25Slide 26Why do minerals weatherA. Physical (mechanical) weathering1. Frost actionSlide 30Slide 31Slide 322. Pressure releaseSheet joints in graniteSlide 35Slide 363. Thermal Expansion and Contraction4. Biological contributions to physical weatheringSlide 39Results of weatheringSlide 41Slide 42Slide 43Slide 44Slide 45The role of water in chemical weathering1. DissolutionSlide 48Slide 49Slide 50Slide 512. HydrolysisHydrolysisSlide 54Many silicates from Bowen’s Reaction Series weather into a variety of clays3. Chemical Weathering - OxidationChemical Weathering - OxidationOxidationSlide 59Weathered Fe3+ is insolubleWeathering is differentiation process because different minerals weather at different ratesChemical weathering affects minerals differentiallySlide 63Slide 64Interaction of lithosphere, hydrosphere, atmosphere and biosphereSlide 66Lecture 11Finish VolcanoesWeathering, Erosion and Soil FormationQuestions•Why are Supervolcanoes (calderas) particularly hazardous? Where are some notable calderas of concern?•What is chemical weathering? Physical weathering? What are the types of each?•How is weathering process related to rocks?•What are secondary mineralsExplosivity of volcanoes•Controlled by:–Magma/lava volume & composition•Felsic materials are resistant to flow and can build up pressure before sudden release–Water•Water seeping into magma chamber or groundwater can be instantly turned into steam•Expansion of water to steam results in an increase of volume of about 630 X–Sudden release of confining pressure•Seismic tremors caused a landslide on Mount St. Helens that resulted in an explosive eruptionSteam explosionMount St. Helens, 1980Bulge develops on north side of volcanoHumans for scaleTsunamis•Large sea waves attaining heights of 100 meters or more•Caused by a displacement of water•Violent eruptions of volcanic Islands, or the rapid deposition of pyroclastics into oceans can instigate tsunamis–(earthquakes on the seafloor are also causes)Krakatau 1883 (Krakatoa)A violent series of eruptions destroyed2/3’s of the islandAbout 5000 people died from pyroclastic flowsAnother 31,000+ died from the resulting tsunami“Supervolcanoes”•Calderas with very large magma chambers that erupt on cycles of up to several hundred thousand years•Calderas may exceed 100 km in diameter–Long Valley, CA (30 x 50 km)•Caldera formed 730,000 years ago–Yellowstone, WY (50 x 80 km)•Last major eruption 630,000 years ago–Toba, Indonesia (30 x 100 km)•75,000 years ago•May have affected human evolutionLong Valley CalderaYellowstone CalderaWhite dots – extent of ash deposits from Yellowstone eruptionsBrown dots – extent of ash deposits from Long Valley eruption75,000 years agoVolume of material ejectedfrom supervolcanoes compared to other eventsTamboraMount St. HelensPinatuboRock CycleTemperatureAnd PressureWhat is Weathering? Erosion?•Weathering – the physical and/or chemical breakdown of rocks and minerals–Weathering occurs at the intersection of the lithosphere, hydrosphere, atmosphere and biosphere•Erosion – The removal of weathered material•Transport – the mode in which weathered sediments are eroded and moved–Wind, Water, GlaciersWhy is weathering important?•Creates unique landforms•Allows nutrients and critical trace elements to be available to plants and animals•Creates and maintains soil•As a differentiation process, weathering can sometimes enrich and concentrate resources•Secondary minerals – form after primary minerals chemically alterWhy do minerals and rocks weather?Why do minerals Weather•Most rocks and minerals are formed at high temperatures and pressures–They are in a state of equilibrium at the T and P of formation–At the Earth’s surface, rocks and minerals are subject to weathering–Secondary minerals form at the T and P common to the Earth’s surfaceSecondary mineralsWhy do minerals weather•Many processes at earth’s surface can affect exposed rock materials1) Physical (mechanical) weathering–The disintegration or disaggregation of rocks by physically breaking them apart2) Chemical weathering–The decomposition of rocks and minerals as chemical reactions alter them into new minerals stable at the Earth’s surface•Usually Physical and Chemical weathering work togetherA. Physical (mechanical) weathering•Frost Action–Freeze-thaw (Ice wedging)–Frost heaving•Pressure release–Exfoliation and Sheet joints•Thermal expansion and contraction•Biological–Root wedging, burrowing, etc1. Frost action•Ice wedging (Freeze thaw)–Expansion of water as it freezes “wedges” apart rocksWater seeps into cracks in rocks, then freezesTalus forms as piles of loose rubble at the bottom of exposed, weathered rocksFrost action•Frost heaving–Loose unconsolidated sediments can become saturated with water. As water freezes, the expansion can lift up rocks and material as well as “churn” the ground2. Pressure release•Rocks formed at great depths and high pressures are at equilibrium with those conditions•Unstable at lower pressures at surface, the rock physically “adjusts” to lower pressures by expanding–Sheet joints•Form roughly parallel to exposed rock surfaces–Exfoliation – the shedding of sheeted material•Sheeting process may produce rounded features called exfoliation domes, especially in weathered plutonsSheet joints in graniteExfoliationYosemite National ParkStone Mountain, GAExfoliation Domes3. Thermal Expansion and Contraction•Most materials expand when heated•Different materials have differential expansion rates–Ex) Quartz expands 3 X as much as feldspar, when heated•Dark colored materials absorb more heat than do light colored materials–Therefore, rocks of differing compositions behave differentially•Thermal expansion–During fires–Deserts–Other planets4. Biological contributions to physical weathering•Root wedging–Roots may exert enormous forces in growing–Root tips pressures may exceed 10,000 kg per square meter–Seeds gather in cracks