L1 Chapter 7 – Weathering PDF

Chapter 7 Weathering Hoodoos Weathering at a loss of ~1m per 100 years… https://www.gojobs.gov.on.ca/  Chapter 7 Weathering Recap the Rock Cycle What are the two main types of weathering? What factors influence weathering?  Weathering The First Ste...

Chapter 7 Weathering Hoodoos Weathering at a loss of ~1m per 100 years… https://www.gojobs.gov.on.ca/  Chapter 7 Weathering Recap the Rock Cycle What are the two main types of weathering? What factors influence weathering?  Weathering The First Step In The Rock Cycle Rock cycle: the set of crustal processes that form new rock, modify it, break it down, transport it, and deposit it anew.  Weathering The First Step In The Rock Cycle the rock cycle is affected by the hydrologic cycle as well as the tectonic cycle all powered by energy from the sun  Heat stored in the earth drives plate tectonics which results in the creation of mountain belts. The high relief in these areas results in extensive erosion turning the igneous and metamorphic rocks into loose sediment which blankets the surrounding area.  Over 70% of the loose debris lying on the earth’s surface comes from the mountain belts where the high relief promotes erosion. Rivers slowly move this sediment to the ocean where it will eventually be incorporated in a mountain chain by either being scraped off against the edge of the continent or melted. Weathering: The chemical and physical breakdown of rock exposed to air, moisture, and living organisms. Erosion: Transport of particles of rock by gravity or by a fluid (e.g. air, water, ice)  Weathering – The First Step How Rock Disintegrates Mechanical weathering The breakdown of rock into solid fragments by physical processes Chemical composition of rock NOT Amazing Places (c) - Monadnocks altered Chemical weathering The decomposition of rocks and minerals by chemical and biochemical reactions Water, air and microorganisms penetrate down through cracks and react with the rock slowly transforming it into loose debris.  Loose debris overlying bedrock is called regolith, formed by the weathering of the rocks immediately below it. Soil is the uppermost layer of regolith, which can support rooted plants Joint formation jointing can occur from the rock mass being subject to great pressures from overlying or surrounding rock, or tectonic squeezing Why are some granite bodies extensively jointed? While others are joint free? How might the presence of joints affect the rate of weathering?  Physical (Mechanical) Weathering Physical weathering destroys the bedrock producing rock fragments. Causes: 1) root and frost wedging 2) glacial grinding 3) heating of the rock 4) pressure release 5) abrasion in streams Tree roots promote weathering… Freeze/Thaw jointing in rocks aka “frost weathering” Very familiar in Northern Ontario….  Glacial processes responsible for weathering Glaciers flowing over the land scour the rock surfaces ripping off and transporting loose material. Sheet joints formed by pressure release This type occurs mostly in massive intrusive igneous rocks such as granite Granite exfoliation explained Spheroidal weathering Heating and cooling can also cause thin layers of the rock to crack off. As the rock heats and cools it expands and contracts. The outside heats and cools more on a daily cycle causing cracks to develop and the surface to peel off. Moving water (rivers and waves) are also major forces causing erosion. they will be discussed in more detail later in the course. Chemical weathering this process occurs because minerals formed deep in the earth’s interior are not stable under surface conditions stability is the reverse of the Bowen’s reaction series the principal agent involved is water Quartz: very stable Feldspars: form clay minerals think Hawaii…lots of quartz sand Mafic minerals: alter to oxides beaches, not a lot of olivine beaches Mechanisms of chemical weathering essentially minerals are re-equilibrating with their new environment Dissolution: reaction between minerals and the acidic or alkaline water can alter the stability of the mineral rainwater is slightly acidic because of dissolved CO2 it interacts with the water to form carbonic acid CO2 + H2O --> H+ + HCO3- dissolves Calcite in limestone- making large cavities  Weathering – The First Step Chemical Weathering Dissolution Separation of materials into ions in Figure 7.4 (a) solution by a solvent, such as water or and (b) - Dissolution acid Weathering – The First Step Chemical Weathering Ion exchange Figure 7.4 (c) – Ion exchange Feldspar to clay the greatest analogy  Salt weathering salt accumulates in rock pores or cracks generally occurs by evaporation sources: weathering products in water, precipitation (near coasts); polluted urban air and rainfall; sedimentary rocks may retain salts from their original deposition threat to the durability and appearance of stone buildings Salt crystals weaken the rock: a) pressure and expansion of the crystals; b) with heat, the crystals will expand, creating pressure; and c) salt crystals absorb water into their structure and form hydrated crystals The attraction to the dipolar water molecules causes dissolution.  Oxidation When an iron-rich mineral such as pyroxene is exposed to water it begins to disintegrate because of hydrolysis releasing ferrous iron to solution where it is oxidized to ferric iron by the oxygen in the water. Hydrolysis is a chemical reaction during which molecules of water (H2O) are split into hydrogen cations (H+, conventionally referred to as protons) and hydroxide anions (OH−) in the process of a chemical mechanism. The strength of the bonds between the ferric iron and oxygen make the new iron oxide compound insoluble. thus, it forms solid ferric iron oxide (rust). 4FeSiO3 + O2 = 2(Fe2O3) + 4(SiO2) Small amounts of iron oxide produce a red stain in the weathered debris and on the rock.  Acid Mine Drainage Weathering of the mineral pyrite not only produces iron oxide but also puts sulfur in solution- which reacts with the water to form sulfuric acid. Many mineral deposits have abundant pyrite which produces acid mine drainage at the sites of many mining operations. quick and easy  Factors Affecting Weathering The most important factors: Tectonic setting Rock composition Rock structure Figure 7.5(a) - Tectonic setting Topography Amount of vegetation and biological activity Climate Factors Affecting Weathering Rock structure Distribution of joints influence rate of weathering Figure 7.5(b) - Rock structure Relatively close joints weather faster  Factors Affecting Weathering Topography Weathering occurs faster on steeper slopes Figure 7.5(c) - Topography Rockslides can expose new underlying rock to weathering Factors Affecting Weathering Biological activity Microorganisms contribute significantly to weathering (e.g., Figure 7.5(d) – Biological activity algal mats) Factors Affecting Weathering Rock composition Minerals weather at different rates: Calcite weathers quickly through Figure 7.5(e) – Composition dissolution. Quartz is very resistant to chemical and mechanical weathering.  Factors Affecting Weathering Vegetation Contributes to mechanical and chemical weathering. Figure 7.5(f) – Vegetation Promotes weathering due to increased water retention. Vegetation removal increases soil loss. Weathering – The First Step Factors Affecting Weathering Climate Climate affects weathering in two primary ways: Chemical weathering is more prevalent in warm, wet tropical climates Occurs quickly and extends to great depths Mechanical weathering is more prevalent in cold, dry regions. In hot and dry climates, both chemical and mechanical weathering occur very slowly Where would the equatorial zones fit here? How about a climate like Arizona? Nunavut? Factors Affecting Weathering Figure 7.6(a) and (b) – Climate and Weathering  Why is weathering important to us? rock hazard analysis- protection of our roadways and railways mitigation techniques for personal properties may be necessary weathering of building stone deterioration of aggregates, construction materials design and performance of structures built upon weathered rock assessed at foundation sites Image Credit: Martin etc. etc. engineering and rock weathering.. on-site investigations: from house foundations to massive projects effects: decrease in strength, loss of elasticity, decrease in density, increases in moisture content and porosity example: deep weathering zones around faults Credit: TNS via Getty Images  Chapter 7 Products of Weathering First lab is this week please remember to print off your labs!!!! First lab will focus on learning how Figure 7.7 - Kaolin to understand maps recap Weathering Products of Weathering Sediment Rock that has been fragmented, transported, and deposited Clay Figure 7.7 - Kaolin Tiny mineral particles of any kind that have physical properties like those of the clay minerals A family of hydrous alumino-silicate minerals Products of Weathering Soil Humus Figure 7.8 (a) and (b) – Partially decayed organic matter in Earth and lunar “soil” soil Sand A sediment made of relatively coarse mineral grains Soil Mixture of minerals with different grain sizes, along with some materials of biologic origin  Products of Weathering: SOIL If the loose debris produced by weathering (regolith) is capable of sustaining vegetation it is called soil. Soil is a mixture of minerals and organic matter (humus). Products of Weathering Soil Profiles and Horizons Soil horizons One of a succession of zones or layers within a soil profile. Each horizon has a distinct physical, chemical, and biologic characteristic. Soil profiles The sequence of soil horizons from the surface down to the underlying bedrock. Soil profiles vary, influenced by factors such as climate, topography, and rock type. Climate controls the type of soil formed  Soil Profiles and Horizons Figure 7.9 – Soil profile Soil profile containing soil horizons (zones or layers) Products of Weathering Soil Profiles and Horizons A typical soil consists of several horizons, Figure 7.10 – Climate and Soils distinguished by colour, texture, and chemistry  Zone of Leaching, Zone of Accumulation When soils develop from weathering and the transport of dissolved and particulate matter, this will result in the formation of different coloured layers In temperate climates the rainwater percolates downward through the soil removing material from the upper layers and concentrating it in the lower layers. The thickness of the soil profile depends on the climate, length of time during which the soil has been forming and the composition of the parent rock  Typical layers in the type of soil in Ontario are: O, A, B and C. These types of soils are called pedalfers (they are also called spodosols and mollisols). Soil Sampling in mineral exploration https://www.royaleijkelkamp.com/products/augers-samplers/soil-augers- samplers/ https://www.gsi.ie/en-ie/programmes-and-projects/minerals/activities/mineral-exploration/Pages/Soil-Sampling.aspx In the upper zone, only the most insoluble precipitated iron and similar oxides remain, plus some quartz All soluble minerals are leached The whole profile may be considered to be an A-horizon directly overlying a C-horizon  Extreme leaching by downward percolating rainwater occurs in the wet areas of the tropics. This forms soils called laterites. They consist mostly of the insoluble oxides of iron and aluminum with little nutrient value. Laterites are sources of aluminum and nickel ores Sample of Bauxite (Aluminum Ore) https://tvird.com.ph/agata-nickel-laterite-dso/  Where does the nickel in laterite deposits come from? A sample of Garnierite, a mixture of nickel A generalized cross section of nickel laterite deposits bearing minerals. https://www.geologyforinvestors.com/nickel-laterites/  The A-Horizon is leached The B-Horizon is enriched in calcium carbonate precipitated by evaporating soil waters In pedocals the calcium carbonate (CaCO3, calcite) rich layer is called caliche. It is whitish in colour. steppe = grasslands Soils are classified according to the materials which are found in their various layers In Canada we use different names for various types of soil than in the US. This makes it difficult matching soil types across the border. Soils can also be classified according to their average grainsize  Soil erosion On Surface particle displacement---”raindrop erosion” ponding water channelized flows of water gullies develop and enlarge rapidly Underground piping - hydraulic removal of soil particles by subsurface flow raindrops Bashir, Safdar & Javed, Atif & Bibi, Irshad & Ahmad, Niaz. (2017). Soil and Water Conservation.  piping below surface- swelling clay minerals and particles “piping” exposed clay expands when wet; shrinks when dry piping can lead to damage under roads and buildings- eventual surface subsidence Bernatek‐Jakiel, A., & Poesen, J. (2018). Subsurface erosion by soil piping: significance and research needs. Earth-Science Reviews. piping How does wind contribute to soil erosion? Credit: Peter Robey/DPE Cloudia Spinner, 2013; Image copyright Loskutnikov, 2013 erosional rates very rough estimates- large number of variables Universal soil loss equation (more for agricultural) Climate Soil Topography Landuse A= RKLSCP A= average annual soil loss (tons/acre) R=rainfall factor- obtained from the product of the kinetic energy of raindrops and the max 30-min intensity of the storm (values are in an index) K= soil erodibility factor (fxn of grain sizes- silt and fine sand are the most erodible) LS= slope length/steepness factor (aka topography) C= cropping factor (high would be barren land; low for forests) P= conservation factor (crop management techniques)  other methods measure amount of carried sediment downstream measure amount of trapped sediments (dams nearby etc) changes in land use are going to give DRASTIC changes in erosion urbanization erosion control agricultural: contour farming, strip cropping, terracing, conservation tillage Contour farming: the orientation of crop rows parallel to the elevation contours of the land (disrupts water flow) strip cropping alternation of strips of erosion inhibiting crops- ex. grasses with more erodible row crops Image copyright Studio 37, 2013 terracing construction of alternating embankments and terraces on the slope (rice..China) Image copyright beboy, 2013 conservation tilling variety of practices that reduce the amount of disturbance and exposure of soil during cropping Courtesy of Jason Johnson, USDA NRCS Iowa Credit: Aaron Daigh soil erosion video https://www.youtube.com/watch?v=PQmon7Rj6ns&t=183s