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Geology 12. Presents. Sedimentary Rocks. Weathering and Erosion. 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).
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Geology 12 Presents
Sedimentary Rocks Weathering and Erosion
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 kinds • 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
Ice wedging cliff Joints = rock fractures Talus = debris accumulation
3 Types of Weathering • 1. Physical (Mechanical) Weathering: • a) Ice Frost Wedging
Sheet joints • 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)
Exfoliation dome before after • 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
day night • 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 + 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
2. Biological Weathering • A) Animals:Moles
2. Biological Weathering • A) Animals:ants
2. Biological Weathering • A) Animals:clams
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).
3. Chemical Weathering • A) Solution
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 Mesa/butte Hodo/ pinnacle rock plateau
Differential Weathering Hard Caprock
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
Resistance to Weathering poor good