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Unformatted text preview: Tearing Down the Volcanoes: Weathering, Erosion, Weathering, and Mass Wasting (L 5, EM/E-1) What is Weathering? What The process by which rocks are The broken down by chemical alteration chemical and mechanical fracturing mechanical Is a result of exposure to the Is environment (atmosphere or ocean) environment Weathering processes do not Weathering transport material! transport I. PHYSICAL WEATHERING (or Disintegration) (or
Kinds of Physical Weathering: Frost & (salt) wedging— water in cracks expands by 9% wedging water when it freezes (salt expands as crystals grow) when Thermal expansion — differential thermal expansion of minerals creates stress in rocks (very slow) minerals Exfoliation - pressure release expansion (continents) Exfoliation pressure Organic activity — tree roots to micro-organisms Mechanical abrasion — things go bump Frost wedging Salt wedging Role of Organisms in Weathering P eter Kr esa m Job of Physical Weathering Job Level everything to an angular flat Level surface by breaking everything into small pieces small The look of weathering on Mars & the The Moon Moon A shallow surface process II. Chemical Weathering (or Decomposition) (or This This process occurs because minerals formed deep in the earth’s interior are not stable under surface conditions. not is generally the reverse of Bowen’s reaction series. Bowen’s principle agent of chemical weathering is water & acid. acid Stability Stability The The Kinds of Chemical Weathering Weathering Oxidation - adds oxygen Hydration/Dehydration - +/- water Ion Exchange - exchanges H+ for a cation Dissolution - dissolves rock material NOTE - ALWAYS WORKING IN PRESENCE NOTE OF AN ACID!!!! OF H2O + CO2 -> H2CO3 -> H+ + HCO3(WATER & AIR -> CARBONIC ACID) (WATER CARBONIC Weathering Oxides Provide Color Weathering to the Desert Landscape to B etty Cr owel l Ion Exchange: making coffee Ion making -------> analogy of weathering ------->
fresh grounds + water => coffee + residue (a solution) K-feldspar + water => K+ + kaolinite (a clay mineral) A nice place to study the chemical weathering of rocks... NEW ASSIGNEMENT: GROUP VISIT TO A CEMETERY Marble (calcite) Granite (silicate minerals) Spheroidal Spheroidal Weathering Weathering M i cha el Fol l o Spheroidal weathering weathering rind The Job of Chemical Weathering:
Round everything to a flat surface by altering/dissolving everything The look of weathering in Kailua A shallow surface process Mechanical Weathering Mechanical In Hawaii, rocks are broken apart by: Freezing and thawing of water at high Freezing altitudes altitudes Plant roots at low altitudes Expansion of rock from chemical alteration
• can cause spheroidal weathering can spheroidal Mechanical weathering aids chemical Mechanical weathering by increasing the surface area of rock exposed to the enviroment rock Chemical Weathering Chemical Warm, wet conditions in Hawai‘i favor chemical weathering Silicate (igneous) minerals (and thus rocks) formed at high Silicate temperature are not chemically stable at ambient surface conditions conditions The earlier a mineral forms in Bowen’s reaction series, the more The easily it is chemically weathered easily Common minerals in Hawaiian igneous (i.e., silicate) rocks are
• • • • Olivine (Mg and Fe) Pyroxene (Mg, Fe, Na, Ca, and Al) Plagioclase (Ca, Na, and Al) Magnetite (Fe) These minerals break down to make new minerals stable at ambient These surface conditions surface
• • • • Hematite and Limonite (rusted Fe) Bauxite (Al) Clay minerals (e.g., kaolinite) and opal (Si) Na, Ca, K, P carries away in water, taken into clay mineral or plant Palagonite Palagonite Alteration of basaltic ash Ash deposited in hydromagmatic Ash eruptions can be wet eruptions Alteration of ash to palagonite cements Alteration the deposit the Some Si, Al, Mg, Ca, Na, K removed from Some ash and precipitated as new minerals between palagonite grains between TAKE NOTE!!!!! TAKE THIS IS WHAT HAPPENS TO ALL THIS THOSE LIBERATED CATIONS (Mg+, Na+, K+, Ca+, etc.) Na+, Be taken up by a plant as “food” Be incorporated into the structure of a new Be clay mineral clay Be carried away in solution to the ocean by Be groundwater & streams groundwater FACTORS CONTROLLING RATES OF WEATHERING
ROCK TYPE ROCK STRUCTURE SURFACE AREA VEGETATION SLOPE CLIMATE BURROWING ANIMALS TIME (mechanical) Soils - Weathering’s Goal Soils If weathered rock material can support plant life, it is If called a soil (removed without blasting) soil residual soils--formed in place from rocks (most Hawaiian --formed soils) soils)
• parent rock lies beneath transported soils--brought in from somewhere else Differences in local climate (especially rainfall) and Differences drainage conditions in Hawaii result in different soil Hawaii types types Laterite--produced by high degree of weathering
• forms in wetter areas • can be iron rich and include kaolinite clay: “red dirt” can kaolinite • if Al-rich, called “bauxite,” an aluminum ore Expanding soils--made of montmorillonite clays --made montmorillonite
• produced by incomplete weathering, drier areas & dark color The effect of landscape on soil development. Soil Profile Soil Soils: 3 major zones
• A horizon (may include an upper “O” horizon is the leached zone; roots & humus) • B horizon • C horizon Soils: 3 major zones
• A horizon • B horizon
- only most resistant of original rock-forming minerals remain -iron oxide and clays are redeposited in the B horizon. • C horizon Soils: 3 major zones
• A horizon • B horizon • C horizon - subsoil grading into bedrock Weathered basalt and ash Weathered Weathered soil in Hawaii -- A and C horizons; B not well developed horizons; Important Clay minerals Important Kaolinite = most common type of clay in Hawai`i Hawai`i Reasonably stable Very susceptible to incorporating water Very The clay then swells Called an “expandable“ clay ! Montmorillonite is also very common Soil Problems at U.H. Business School November, 1979 What is Erosion? What Processes that remove and transport Processes soil and rock soil Transportation can be by a fluid: air or Transportation water water
• dunes • stream erosion Transportation without requiring a fluid Transportation medium is called mass wasting mass e.g., landslides, rockfalls MASS WASTING
(L 8) The downhill movement of masses of rock & soil under the influence of gravity The erosion & transportation that occurs after weathering Labeled according to Kind of movement Kind of material moved Velocity of movement Kind of Material
Rock - solid rock Soil/mud - fine grained Regolith/debris - poorly sorted, unconsolidated material; combination of rock & soil Deposited material called colluvium Influences on (controls of) mass movement Gravity Rock type & structure Angle of slope Amount of water Triggers- human activity/climate Slopes susceptible to mass movement. Angle of Repose Angle
Steepest angle at which loose material will lie without cascading down. Over steepened slopes tend to collapse to angle of repose Function of grain size and shape And water How water works! Surface Tension WATER WEIGHS A LOT -ABOUT 63 LB/FT3 (c) Unsaturated Soil Soil Saturated Saturated Soil Soil Mass Wasting Mass Types: soil creep
• slow movement of soil downhill as the result of expansion and slow contraction contraction soil avalance--soil cover pulls loose and slides downhill rockfalls--loose rocks and boulders moving downslope --loose boulders
• deposit is called talus deposit talus landslides--large sections breaking loose at once, break --large up as they move up mudflows--mud and boulders flow together behaving as a mud viscous liquid viscous ROCK FALL TALUS APRON TALUS Talus Deposit Talus SLUMP Slump motion no rotation rotation Slide breaking up Flow Large slump in central Washington Pali Highway - Kailua mudflow deposit Let’s Make A Great Slide!! Dissect slope - cut toe Add weight - build on it Add water - leaky swimming pools Remove vegetation - for more convenient construction!
Trigger - Here comes a hurricane! Preventing Landslides Preventing Build to adapt to landscape Drain or divert water Terrace steep slopes Build walls Rock bolt Giant Submarine Landslides Giant Sometimes very large sections of the Sometimes islands break off and slide out to sea islands Similar mechanisms to smaller Similar landslides on land, just scaled up landslides Never witnessed, but known from Never submarine deposits submarine May move very fast--some move up May and over the Hawaiian Arch and Moloka‘i THE END ...
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This note was uploaded on 04/20/2011 for the course GG 103 taught by Professor Herrero-bervera during the Summer '10 term at University of Hawaii, Manoa.
- Summer '10