EPS 101 Spring 2003– Adrian Brearley - Lectures 12-14

Colorado River and The Grand Canyon - Product of Erosion

Weathering and Erosion

Keywords and concepts

General Terminology

Weathering –general process of the breakdown of rocks on the Earth's surface into smaller fragments and/or new minerals.

Chemical weathering - weathering process involving the dissolution and or chemical breakdown of minerals into new minerals that are stable at the Earth's surface.

Physical weathering (mechanical weathering) - fragmentation of sold rocks by purely physical processes, without any change in chemical composition.

Erosion - combination of processes that result in the wearing away of mountains. Involves the chemical and physical breakdown of solid rock into smaller fragments by action of rainfall, wind, ice, snow, followed by their downhill transport

Chemical weathering

Soil – surface layer produced by weathering of rocks consisting of fragments of bedrock clay minerals and organic matter produced by organisms and decay of plant and other organic material.

Mineral solubilities – degree to which different minerals dissolve in water.

Hydration – reactions in which an mineral that contains no water (anhydrous) undergoes a reaction with water to produce a mineral that contains water (hydrous) e.g. a clay mineral.

Chemical stability – measure of tendency for a chemical substance to remain in a given chemical form, rather than react to become a different chemical compound(s).

Kaolinite (Al₂Si₂O₅(OH)₄) – clay mineral (hydrated sheet silicate) formed by alteration of potassium feldspar.

Bauxite – ore composed mainly of aluminum hydroxide. Produced by the extreme weathering of clay minerals resulting in the removal of all the silica in solution. Formed in topical areas with high rainfall where weathering is intense.

Carbonic acid (H₂CO₃) – mild acid produced by dissolving CO₂ in water (e.g. atmospheric CO₂ in rainwater)

Bicarbonate ion (HCO₃^-) – complex anion produced by the dissociation of carbonic acid.

Physical Weathering

Joints – regular fractures in rocks, one to several meters apart produced during uplift and expansion due to release of pressure.

Frost wedging – Expansion of freezing water as it turns to ice, that wedges open rocks and causes them to split open.

Thermal expansion – alternating cycle of heat and cold from day to night that can cause a rock to weaken and eventually break open.

Exfoliation – weathering process where large flat or curved sheets of rock become fractured and broken away from an outcrop.

Spheroidal weathering – weathering form in which cracking and splitting off of curved layers occurs, typically from a spherical boulder. Occurs on smaller scale than exfoliation.

Soil

Regolith – layer of loose hterogeneous weathered material (rock fragments etc) that occurs overlying bedrock.

Humus – organic component of soil. Mixture of organic materials, the remains and waste products of plants, animals and bacteria that inhabit the soil. Upper most layer of the regolith.

Soil profile – trench cut through soil to reveal its vertical structure.

A-horizon – topmost layer of soil typically consisting of organic material, clay and insoluble minerals (e.g. quartz).

B-horizon – layer below A-horizon that is low in organic matter and contains soluble minerals and iron oxides in small pods, lenses and coatings.

C-horizon – lowest horizon above bedrock consisting of broken and decayed rock fragments and clay minerals from chemical weathering.

Laterite – soil produced in warm humid climates in which rapid weathering of feldspar has occurred. Deep red soil consisting mainly of aluminum and iron oxides.

Pedocals – dominant soil type in arid regions were weathering is slow. Rich in calcium (from calcium carbonate) and other soluble minerals – low in organic matter.

Pedalfers – soil in regions with moderate rainfall and temperatures. Soil depends on climate, parent rock and length of time to develop. Upper layers of pedalfers contain insoluble minerals such as quartz, clay and iron alteration products. Soluble minerals (carbonates) are absent. Make good agricultural soils.

Paleosols – ancient soils, preserved in geological record. Provide record of climate change millions and sometimes billions of years ago in geological history.