Climate and Weathering Flashcards
Climate
Refers to characteristics atmospheric conditions over a long period of time
- years or decades
Weather
Refers to atmospheric conditions over short periods of time
- days or weeks
Influences on weathering
- rock type and structure
- topography
- presence of organisms
- age
Climate imposed effects of weathering
- style
- rate
- volume
- products
- landscape
Weak leaching
smectite
Moderate leaching
kaolinite
Intense leaching
Gibbsite
Peltier (1950) scheme
Weathering rates related to temperature and rainfall
- intensity of chemical weathering
Weathering
breakdown of rick by;
- mechanical
- chemical
- biological
Functions of weathering
- give rock lower strength and greater permeability
- produces landforms
- releases minerals in solution
Mechanical weathering
- Progressive breakdown
- increases surface area of the rock for chemical weathering
- fatigue, repetition of stress
Unloading (Mechanical)
- Rocks exhumed from depth
- Confining pressure is eased (erosion)
- Allows mineral grain expansion (rockbursts)
- Small surface-parallel fractures and joints
- Exfoliation
- May result in rock falls
Frost action (Mechanical)
- physical disintegration (frost shattering
- cold environments, cyclical freezing
freezing = 9% increase in water volume
Heating and cooling (Mechanical)
Isolation weathering • Rocks = low thermal conductivity • Outer surface heats and expands quicker than rock core • Exfoliation • Desert environments
Wetting and drying (Mechanical)
• Often known as ‘Slaking’ • Water is attracted to fractured minerals and is aDsorbed • In a crack this leads to swelling pressure • Clay minerals encourage water adsorption (e.g. mudstones and shales) • Cracks form on drying
Salt crystal growth (Mechanical)
Haloclasty • Evaporation of saline solution • Salt crystal growth in rock interstices = granular disintegration • Important in coastal and arid regions • Evaporation of brine where watertable close to surface
Action of plants and animals (Mechanical)
- lichens and mosses grow on rocks
- they wedge their tiny
roots into spores and
crevices - when the roots grow, the
rock splits
- they wedge their tiny
- trees and shrubs may grow in the cracks of boulders
Chemical weathering
• Caused by rain water reacting with the mineral grains in rocks to form new minerals (clays) and soluble salts. • Reactions occur particularly when the water is slightly acidic • Mainly from the action of rainwater, oxygen, carbon dioxide, and acids of plant decay • Water vapor is present in the air everywhere means that chemical weathering occurs everywhere
Solution (Chemical)
- mineral salts become dissolved in water
- solubility of minerals depend on pH of the water
- readily reversible = precipitate
- important in limestone areas
Hydration (Chemical)
- transitional between chemical and mechanical
- no change to chemical composition
- absorption of water molecules into minerals lattices
- stress from expansion of minerals
- embeds water
Oxidation and reduction (Chemical)
- involves oxygen combing with a substance
- oxygen dissolved in water is important oxidising agent
- iron is highly susceptible
element loses an electron to dissolved oxygen - Gives altered earth material a characteristic
yellowish brown to red colour - Water table is boundary between oxidizing and
reducing environments
Carbonation (Chemical) - chain reaction
- is the formation of carbonates
- occurs when carbonic acid attacks minerals (calcareous rocks)
- limestone reacts with carbonic acid to produce calcium hydrogen carbonate
- calcium hydrogen carbonate = very soluble
Hydrolysis (Chemical)
- water splits into hydrogen irons and hydroxyl anions
- these react with minerals
- plagioclase to kaolinite produces soluble calcium carbonate
Chelation (Chemical)
- biochemical process
- organisms produce organic substances, known as chelates
- chelating agents are produced by alteration of humus in plant acids and excreted by lichens
- decompose minerals and rocks from removal of metallic cations
- forms soluble organic matter-metal complexes
Reduction (Chemical)
- Reverse process of oxidation
- excess water present
- oxygen is released by soil/rock
- waterlogged conditions
- ferric iron converted to ferrous forms
- responsible for changing soil colour to grey, blue, green colours (gleying)
- water table is high with impermeable layer
Biological weathering
root penetration = bio-mechanical
- lichen growth or guano = thermal weathering
- coastal organisms bore and graze
- bacterial chemical secretions (alter minerals)
animal burrowing etc.
Tor formation
- Sedimentary cover rocks eroded quickly
- Unloading to expansion and uplift of granite to horizontal joint formation
- Granite weathering (kaolinisation) begins
- Deep chemical weathering
- Humid, wet climate
- Acidic waters
Weathering landforms
Tors, Dartmoor (280Ma) • Granite batholith cooled • Composition – Quartz, feldspar, biotite mica • Cooling to initial vertical cracks • Faulting accentuated jointing
Continued weathering of profile
Block-fields - mountain summits - periglacial activity Talus Cones - accumulations of material loosened by weathering Duricrust - soluble materials precipitate - more resistant layers formed (crust, nodules)
Human activities (Biological)
- quarries, dams, roads
- climate change;
- increased rainfall
- warmer temps
- chemical emissions
- land clearance;
- forestry and grazing land
Final stage of tor formation
• Quaternary glaciation • Too cold for chemical weathering • Mechanical processes take over (Freeze thaw/Solifluction in permafrost) • Removes weathered debris • Soil and boulders
Soil?
Controversial!
- Geologists and Engineers see soils as = regolith
- Pedologists see soils as the portion of the regolith supporting plant life
and where soil forming processes operate
