The Production of Sediment. Contents. Weathering. Chapters 1, 3

Transcription

1 The Production of Sediment Chapters 1, 3 Contents Weathering Physical, chemical, biogeochemical processes Rates Products Carbon cycle and global change Erosion/Soils Sediment Texture Weathering General process by which rocks are broken down at the earth s surface Produces clays, soils, dissolved substances in rivers Operates on igneous, sedimentary & metamorphic rocks 2 types: Chemical & Physical (usually work together)

2 Weathering Physical weathering Solid rock affected by physical processes that do not change its chemical composition Weathering Physical weathering Frost wedging Thermal effects (fires, daily/annual temperature) Salt/mineral crystallization in fractures (arid regions) Wetting and drying Release of overburden pressure/unloading (uplift of crust) Organisms (incl. humans) Grinding and smashing action of river and ocean water Abrasive action of wind-blown sand Plucking by glaciers

3 Weathering Chemical weathering Changes that alter both chemical and mineralogical composition of rocks Water (and dissolved gasses) plays a key role Chemical Weathering Boggs

4 Hydrolysis Oxidation Hey! It s Bowen s reaction series! Boggs

5 What happens when granite is weathered? Unweathered granite contains these minerals: Na plagioclase feldspar (white) K feldspar (pink, but may be white in other granites) Quartz (gray) Small amounts of biotite and/or amphibole (black) and sometimes muscovite (not shown) What happens when granite is weathered? Here is what will happen to each of the mineral constituents in a granite under warm, humid weathering conditions: The feldspars will undergo hydrolysis to form kaolinite (clay) and Na and K ions The sodium and potassium ions will be removed through leaching and will be carried in solution in running water The biotite and/or amphibole will undergo hydrolysis to form clay, and oxidation to form iron oxides. The quartz (and muscovite, if present) will remain as residual minerals because they are very resistant to weathering. Weathering Chemical and physical weathering work together

6 Weathering Biogeochemical weathering Boggs doesn t really emphasize the role of (micro)organisms in promoting weathering New field Barker and Bamfield 1998 Weathering Products Weathering generates three products: Source-rock residues: chemically resistant minerals and rock fragments Secondary minerals formed in situ Soluble constituents

7 Weathering Products Grain size of particles governed by: the original grain size of the parent rock and the completeness of the weathering process E.g., granites yield sand-sized quartz crystals whereas siltstones yield rock fragments Weathering Products Boggs 2001 Weathering Rates depend on a variety of factors

8 Carbon Cycle and Rock Cycle Carbon Cycle and Rock Cycle

9 Erosion The set of processes that loosen and move soil and rock downhill or downwind See next chapter for more detail Moves weathered material from earth s surface exposes fresh rock to weathering Erosion Agents of erosion (some): gravity (mass-wasting) flowing water wind ice (glaciers) waves along coasts Erosion is a natural process which can be accelerated by human activities Associated with, but different to weathering Weathering vs Erosion Weathering: General process by which rocks are broken down at the earth s surface Erosion: The set of processes that loosen and move soil and rock downhill, downwind or downcurrent

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11 Soil Solid weathering products that are not removed by erosion can form soil above bedrock Eroded material may accumulate elsewhere to form soils Soil Unconsolidated deposits that support plant life ( geologist s definition ) Composed of: Fragments of bedrock (boulders -> silt in size) Clay minerals (from weathering) Dead organic material (roots, leaf litter, organisms) In the absence of plant life, it is called regolith E.g., Moon, Mars

12 Soil Takes time to develop Factors: climate topography parent material organisms time and geologic history surface water and groundwater Sediment Texture Small-scale features that arise from the size, shape and orientation of individual sediment grains Sediment texture reflects nature of sediment transport and depositional processes Useful for interpreting depositional environment

13 Sediment Texture Primary interest: Grain size (average, sorting, etc.) Particle shape (form, roundness, surface texture) Fabric (grain orientation, inter-particle contacts) These variables help to determine bulk density, porosity and permeability Udden-Wentworth scale Sediment Texture Phi scale φ = -log 2 d Example 1: What is phi value of.125 mm? φ= -log φ= 3 Example 2: What is mm size of 1.5 φ? 1.5 = -log 2 d = d = 0.35mm

14 Sorting of consolidated deposits is typically evaluated visually (see above) Sorting of unconsolidated deposits is defined using statistical analyses of grainsize data Sediment Texture Other Factors Roundess (angular->well rounded) Sphericity/shape (disk, sphere, etc.) Surface texture (SEM images) Fabric (orientation, packing) Grain Fabric Imbrication OR Rapid deposition

15 Summary Weathering - General process by which rocks are broken down at the earth s surface Physical, chemical, biological aspects Products include solid particles and dissolved substances Rates variable Plays role in global carbon cycle Summary Erosion - The set of processes that loosen and move soil and rock downhill, downwind or downcurrent Variety of processes Soil unconsolidated materials Can accumulate in situ above weathered rocks or form from eroded/transported material Summary Sediment textural parameters are studied to help determine environment of deposition Also influence properties such as porosity, permeability Phi scale & Udden-Wentworth scale used to describe particle sizes