Lecture notes 4 Weathering & Soils

[Pages:6]Lecture notes - Bill Engstrom: Instructor

Weathering & Soils GLG 101 ? Physical Geology

In this section we will focus on the Earth's External (solar heat) engine: The Hydrologic Cycle and the "weathering" that occurs as a result of the cycle, and the formation of soils.

Weathering

? Weathering = The mechanical and/or chemical breakdown of rocks and minerals.

? This differs from Erosion = The physical removal of weathered materials (picking-up).

? Transportation = The movement of eroded materials.

Mechanical weathering: Breaking the rock

Sheet Joints & Exfoliation

? Formed by pressure release. The outer layers of rock expand more than the inner layers and thin slabs of rock are formed.

? This is common when rocks (igneous and metamorphic) are formed at depth and are then exposed at the surface of the Earth).

Ice Wedging/Frost Action

? Common in environments with freezing/thawing cycles. Talus slopes are often formed at the edges of cliffs/mountains as a result.

? There is a 9% expansion that occurs in cracks/fractures when water turns to ice and this acts to mechanically break the rock.

Mechanical Weathering- Ice Wedging/Talus Formation

Thermal Expansion

? Common in desert environments with high daily temp variations

Plant Roots

? Plant and Animal activity. Plant roots can break up rocks and animals can burrow through.

Abrasion

? "Sand blasting" is another term that is sometimes used. Abrasion can be done by wind or water.

Mechanical and chemical weathering work together. Mechanical Weathering increases the exposed surface area and promotes chemical weathering.

Chemical Weathering

Dissolution and Hydrolysis (aka dissolving) ? Calcite, halite (salt), gypsum (and silica or quartz- under extreme conditions) dissolve in water and go into ions in solution. Carbon dioxide in the atmosphere increases the acidity of rainwater. This forms a weak carbonic acid that can easily dissolve calcite (limestone). Calcite/limestone dissolves into calcium and bicarbonate (soluble products). Example : Dissolution of Salt - Water (polar molecule) disrupts the ionic forces in salt and the chemical bonds are broken. ? In hydrolysis, hydrogen ions replace other positive ions in a crystal lattice. This acts to promote chemical weathering along with the weak carbonic acids from the atmosphere.

Oxidation (rust) ? Iron and Magnesium "rust" easily (e.g. Iron and oxygen react to form iron oxides)

Reactions (Changing the chemistry or phase) ? Example: Feldspars chemical weather into clay minerals

When the amount of CO2 in atmosphere is much higher than normal, the result is "acid rain". This can greatly increase the speed at which mechanical weathering occurs. The increase in "greenhouse" gases, including carbon dioxide has been tied to global warming and the increase in acid rain. Global warming occurs when the earth heats up (the temperature rises). It happens when greenhouse gases (carbon dioxide, water vapor, nitrous oxide, and methane) trap heat and light from the sun in the earth's atmosphere, which increases the temperature.

Bowen's Reaction Minerals and Weathering

Effect of Bowen's reaction series and mineral stability on chemical weathering ? Minerals highest on series are most out of equilibrium, therefore most susceptible to weathering. ? Minerals lowest on series- like Quartz, least out of equilibrium and least likely to weather

Chemical Weathering & Rock Type Chemical Weathering- Structures Climate and Chemical Weathering

? Abundance of water = more chemical weathering. ? Humid climates = more chemical weathering.

? Higher temperatures = more chemical activity - favors chemical weathering. ? Hot climates favor chemical reactions. And, by contrast, chemical weathering is not effective in polar regions.

Water = the dominant factor ? Hot dry climates (e.g. Arizona Desert) hampers chemical reactions. ? Hot moist climates like the tropics have the most chemical weathering.

Products of weathering = detrital sediments, organic matter and ions ? Individual grains - from break-up of materials ? Rock fragments ? Clay - from chemical reactions with ferromagnesian minerals and feldspars ? Ions in solution- calcium, sodium, potassium, and silica ? Fossils and organic matter.

Again....... Physical and Chemical Weathering work together ? Physical weathering increases surface area which promotes chemical weathering ? Chemical weathering weakens the bonds between grains loosening them to fall out physically

Features produced by weathering and erosion ? Spheroidal Weathering: With this type of weathering (common in granites), the corners on fractures decompose more readily. There is a gradual rounding of the rock, and it becomes more spherical, as the fractures are "attacked" by chemical weathering more readily ? Differential Weathering: Because certain rock types weather and erode more easily than others, exposed rocks weather "differentially". "Softer" rocks like clays/shales (sedimentary) may form slopes, while harder rocks like sandstones or limestones (sedimentary) may form cliffs. If you have ever been to the Grand Canyon you have observed these rocks that form slopes and cliffs.

Weathering is also critical in the formation of certain types of ore deposits. Diamonds can be concentrated after weathering of the host rock (diamond pipes). Important point

? weathering produces sediments or solutions, the materials which can be deposited or precipitated to become sedimentary rocks.

Soils

Some common terms.....

? Soil - a layer of weathered unconsolidated material overlying bedrock ? Bedrock = the rock beneath the soil ? Loam - mixture of sand, silt, clay, organic matter ~= topsoil ? Humus ? decayed plant and animal matter (organic) Factors that influence the formation of a soil ? Parent rock - the type of parent rock will affect the composition of a soil. ( e.g. basalt will tend

to produce soil higher in Fe and Mg than Granite). However, some soils may be transported from high to low areas thus combining effects of different bedrock.- residual (remaining in- place) versus transported soils ? Topography - Steep topography favors thin soils; flat topography favors thick soil development ? Vegetation - different kinds of plants produce different acids in soil which affect the development ? Time - it takes hundreds or thousands of years for a soil profile to develop ? Climate - probably the biggest single factor in the development of soils. Affects vegetation, leaching, evaporation etc. Soil Horizons (primary soil layers) ? Horizon A - organic rich layer usually fairly thin - zone of leaching ? Horizon B - zone of precipitation, few organics ? Horizon C - broken up pieces of underlying bedrock

Effects of Climate on Soil Profile ? KNOW these general soil profiles for different climates (Pedocals, Pedalfers and Laterites) Pedocals - developed in very dry climates ? like Arizona

? A-horizon thin humus and partially leached soil ? B-horizon- precipitated calcite (CAL)- caliche or hardpan ? C-horizon as always-broken pieces of bedrock Soil Horizons- Pedocal Pedalfers - developed in moderately humid and moderate temperature climates ? A-horizon thick humus and partially leached soil ? B-horizon- precipitated iron and aluminum oxides (AL + FER) ? C-horizon as always Laterites - developed in extremely humid and very hot tropical climates. Agriculturally unproductive soil even though formed in the rain forest. ? A-horizon - thin humus and totally leached soil- remnants are the most insoluble oxides of

iron, and aluminum. Bauxite, an ore of aluminum forms here ? B-horizon- virtually absent- anything that would dissolve and re-precipitate is gone ? C-horizon as always Economic/Consumer aspects of soils ? Shrink-swell potential of soil - Water is a polarized molecule - sticks to flat surfaces of biotite-

like clay minerals and separates them- expanding them. Drying allows clays to shrink again. ? Varies with clay content ? Can cause destruction to roads (US89), runways (DIA), houses etc. (e.g. cracks in foundations,

roads, and floors) ? Can be remedied but must know about it in advance. Soil maps are often available that can be used to determine if you live in an area where there is a high clay content in soils.

Please see the diagram (summary of soil types) below............

8/2011

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