Weathering

ï The general process by which rocks at the Earthís surface are broken down into:

ñ Sediments

ñ Clays

ñ Soils

ñ Dissolved substances

Two Types of Weathering

ï Chemical Weathering

ñ Chemical reactions between minerals and external agents like air or water

ï Physical Weathering

ñ Rocks broken down by mechanical processes such as fracturing, freezing-thawing, and breakage during transport

Chemical Weathering

Chemical weathering results from chemical reactions between rocks and external agents like air or water

Examples of Chemical Weathering

ï Conversion of silicates to clays

ï Dissolving of minerals

ï Oxidation

Conversion of Silicates to Clays

silicates + water = clays + dissolved substances

Silicates = feldspar, mica and amphibole

Dissolved substances = SiO₂ & dissolved cations (K⁺, Na⁺, Ca²⁺, etc.)

Conversion of Silicates to Clay

The type of clay formed depends on the composition of the original silicate:

feldspar + water = Al-rich clay (kaolinite) + dissolved substances

(Fe, Mg)silicate + water = (Fe, Mg)clay + dissolved substances

Physical Weathering Accelerates Chemical Weathering by Increasing Surface Area

Bauxite

ï In tropical regions, clays can further react with water to form bauxite (Al-hydroxide)

silicates + water = clays + more water = bauxite

ï Bauxite is a major source of aluminum

Dissolving of Minerals

ï Slightly acidic rainwater can also completely dissolve some non-silicates

ï Carbonic acid completely dissolves calcite

ÝÝÝÝÝÝÝÝÝÝÝ CaCO₃ + H₂CO₃ = Ca²⁺ + 2HCO₃^-

ï Halite dissolves directly into water

ÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ NaCl = Na⁺ + Cl^-

Acidic Rain Slowly Dissolves Limestone

Oxidation

ï Oxidation involves reactions with oxygen

ÝÝÝÝÝÝÝÝÝÝ 4Fe + 3O₂ = 2Fe₂O₃ (hematite)

ï Iron can also dissolve in water as cations

ñ Dissolved Fe can exist in two oxidation states, Fe²⁺ & Fe³⁺

ï Iron oxide (hematite) is a common product of oxidation

Physical Weathering

Rocks are broken apart by mechanical processes such as fracturing, freezing-thawing, wedging, or breakage during transport by rivers or glaciers

Types of Physical Weathering

ï Natural zones of weakness in rock

ï Activity of organisms (e.g. tree roots)

ï Frost wedging

ï Crystallization of minerals in fractures

ï Alternating heat and cold

ï Exfoliation

ï Spheroidal weathering

Zones of Weakness Along Natural Fractures

Bedding Planes Are Often Zones of Weakness

Mineral Crystallization

Minerals (calcite, gypsum, etc.) crystallize from solutions in rock fractures, forcing the fractures to expand

Alternating Heat and Cold

ï Repeated expansion and contraction of a rock during heating and cooling

ï Common in desert regions that experience hot days and cold nights

Exfoliation Common for Granitic Batholiths

Spheroidal Weathering: cracking and splitting of curved layers from spherical boulders (like skin on an onion)

Soils

ï Residual Soil:

ñ Form in place after weathering

ñ Develop on plains and lowlands having moderate to gentle slopes

ñ Consist of loose, heterogeneous material

ï Transported soil:

ñ Derived from elsewhere

ñ Deposited in lowland, valley or basin

The Soil Profile

ÝA-horizon

ï Topmost layer of soil

ï Usually a meter or two thick

ï Upper portion often rich in organic matter (humus)

ï May also contain insoluble clays & quartz

ï May take thousands of years to develop depending on climate & organic activity

ï Supports crops and other vegetation

ÝB-horizon

ï Precipitated soluble minerals leached from the A-horizon

ñ calcite, quartz, gypsum, salts and/or iron oxides

ï Precipitated minerals accumulate in small pods, lenses and coatings

ï Organic matter is sparse

Laterite, Pedocal & Pedalfer Soils