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Unformatted text preview: T&L-4-1 Weathering and Mass Wasting Chapter 4 Weathering and Mass Wasting
Weathering — The breakdown of rock into smaller pieces and diﬀerent minerals. The result is regolith (loose rock fragments) and eventually soil. This regolith can be moved by water, wind or ice (erosion). It can also be moved downwards by the action of gravity alone (mass wasting). (Later chapters will deal with the reverse of this process — uplift and mountain building.) Weathering — near the surface Metamorphism — far below Both weathering and metamorphism can be divided into either mechanical or chemical processes. T&L-4-2 Weathering and Mass Wasting Mechanical Weathering — the breaking of rocks into smaller pieces of the same minerals by mechanical forces. Examples Typical Processes 1. Breaking due to ice formation (frost wedging). 2. Unloading, caused by the release of internal pressure in rock formed far below the surface. This causes sheeting, the peeling away of layers, often resulting in exfoliation domes. 3. Abrasion (grinding) by ﬂowing water, air or ice. 4. Biological activity — roots and burrowing. 5. Breaking by rapid temperature change (mostly in forest ﬁres — daily and seasonal variations are not enough to cause signiﬁcant eﬀects.). Mechanical weathering increases the surface area of the rock and thus speeds up chemical weathering. Joints are large cracks in the bedrock. These are places where weathering can attack far into the rock. T&L-4-3 Weathering and Mass Wasting Chemical Weathering — Rocks react chemically with substances in their environments to form new minerals. These substances include 1. Oxygen from the air 2. Acids in water CO2 from the air or decaying vegetation produces carbonic (H2 CO3 ) acid. 3. Water slowly reacts with some rocks. Typical products are 1. Soluble (e.g. Calcium Carbonate) 2. Soft 3. Water Absorbing 4. Oxides of Iron and Aluminum Usually the last mineral to react is Quartz (white sand, silica) The products of chemical weathering result in soil. T&L-4-4 Weathering and Mass Wasting The order of weathering of silicate rock is often the same as their order of crystallization. Deep buried minerals tend to be the least stable under surface conditions. Those that form near the surface tend to be the most stable. Chemical weathering is fastest with 1. Suﬃcient moisture 2. Warm temperatures T&L-4-5 Common Chemical Weathering Reactions Weathering and Mass Wasting Carbonic acid produced by the solution of carbon dioxide H2 O + CO2 H2 CO3 H2 CO3 H+ + HCO3 − Hydrolysis of potassium feldspar 4K Al Si3 O8 + 4 H+ + 2 H2 O ⇒ 4 K+ + Al 4 Si4 O10 (OH)8 + 8 Si O2 Dehydration of geothite to form hematite 2 Fe O ·OH ⇒ Fe2 O3 + H2 O Dissolution of calcium carbonate by carbonic acid ⇒ Ca++ + 2 (HCO3 )− Ca CO3 + H2 CO3
Example T&L-4-6 Weathering and Mass Wasting Erosion and Mass Wasting
These processes move material loosened by weathering. Erosion — The movement of regolith by water, ice or air. Deal with this in more detail in the discussion of water, deserts and glaciers. Mass Wasting — The downhill motion of regolith by the action of gravity alone. Example Very Fast Landslide Rock Fall Avalanche Intermediate Slump EarthFlow Mudﬂow Very Slow Soil Creep Soliﬂuction T&L-4-7 Weathering and Mass Wasting Slump Downward slipping of material moving as a unit along a curved surface Picture Earthﬂow Turbulent ﬂow, often with water-saturated clay beneath. Mudﬂow Mud ﬂows like a river.
Picture Soil Creep Gradual downhill motion due to repeated expansion and contraction on cycle of wet/dry or freeze/melt.
Picture Soliﬂuction Caused by melt-water accumulation at surface when lower soil is still frozen.
Example T&L-4-8 Weathering and Mass Wasting End of Chapter 4 T&L-4-9
Back Weathering and Mass Wasting T&L-4-10
Back Weathering and Mass Wasting T&L-4-11
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Back Weathering and Mass Wasting T&L-4-18
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This note was uploaded on 01/06/2009 for the course PSC 103 taught by Professor Nichols during the Fall '08 term at Point Loma.
- Fall '08