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• Weathering: the physical breakdown (disintegration) and chemical alteration (decomposition) of minerals and rocks at of near the Earth’s surface
• Erosion: removal of weathered material (by water, wind, or ice)
• 3 Types of Weathering:• 1. Physical/Mechanical Weathering:
– Physical forces break rocks into smaller pieces without changing chemistry
– 4 kindsa) Frost action/ice wedging: water
seeps into cracks, freezes, expands (9%) and pushes rock apart. Most effective in areas where temperature fluctuates above & below 0’C
animation link
3 Types of Weathering3 Types of Weathering
1. Physical 1. Physical (Mechanical) (Mechanical) Weathering:Weathering:– a) Ice Frost a) Ice Frost
WedgingWedging
• b) pressure release: many minerals/rocks are formed under immense pressure. When they are uplifted and exposed through erosion, the pressure is greatly reduced. The rock expands forming layer of fractures (joints)
Sheet joints
• Exfoliation: slabs of rock break along sheet joints and fall off
• Popping/rock bursts: sudden pressure relief in deep mines rock explodes like a grenade injuring/killing miners
beforeafter
Exfoliation dome
• c) Thermal expansion & contraction: rock is a poor heat conductor; in deserts where the temperatures rise and fall 30’C/day, the outside of the rock heats-expands and cools-contracts animation
day night
+ dark minerals get hotter than white minerals
• d) Salt crystal growth: salts in rock crevices grow. Expand prying the rock apart like ice wedging.
• Road salt in roads & bridges, inside of concrete foundation.
• 2. Biological Weathering = activities of organisms
• a) animals: burrow, tramp, bring materials to surface for more weathering, help transmit gases and water to depths, etc. Animation
• b) plants: roots wedge rocks apart• Animation
• 3. Chemical Weathering: chemical alteration of rock/parent material
• 3 kinds
• a) Solution: a solid dissolves (halite, calcite) and is usually helped by CO2 in water which forms carbonic acid (also in plant roots)
• Results in hard water, caves & karst topography (sink holes).
• b) oxidation: reacts with oxygen to form oxides (rust).
• Ferromagnesians (olivine, pyroxene amphibole & biotite) combine with O2 for form hematite or limonite.
• Hi sulfur/sulfide coal acid rain/runoff• Iron sulfides (pyrite FeS2) sulphuric acid + iron oxide
• c) Hydrolysis: cations (metals) in minerals are replaced by H+ from water
• Ex: Potassium (K) Fs + water clay + K + silica
• Ex: Plagioclase Fs + water clay + (Na, Ca) + silica
• Terminology:
• Differential Weathering: rocks that weather at different rates produce uneven surfaces
Resistant cap rock
plateau
Mesa/butteHodo/
pinnacle rock
• Spheroidal Weathering: reactangular rocks weather into roundish rocks because corners are attacked on 3 sides, edges on 2 sides, but flat planes on 1 side
• Result: rounded (& smaller) rocks
Higher temperature minerals weather most easily (olivine, calcium-rich plagioclase)
Lower temperature minerals weather least (quartz)