Weathering and Erosion

Weathering – the disintegration and decomposition of material at the surface Erosion – the transportation of weathered

material by water, wind, or ice

Weathering Two kinds of weathering

• Mechanical weathering • Chemical weathering

Weathering along joints – Balanced Rock, Joshua Tree National Monument

Weathering along joints – Balanced Rock, Joshua Tree National Monument

Unloading and exfoliation

of igneous rocks

Exfoliation joints – Tioga Road, Yosemite National Park

Exfoliation joints – Yosemite National Park: Half Dome

Ice wedging – expansion of water due to freezing

Salt weathering – growth of salt crystals due to evaporation of water

Prying apart of rocks due to growth of tree roots along joints and fractures

Water transport

Wind transport

Ice transport

Weathering • Chemical weathering

• Dissolution of minerals • Chemical alteration of minerals by

removal/addition of elements

Acids: chemical solutions in which hydrogen ions (H+) are released

Formation of carbonic acid

CO2 + H2O water and carbon dioxide

Formation of carbonic acid

CO2 + H2O H2CO3

carbonic acid

water and carbon dioxide

Formation of carbonic acid

CO2 + H2O H2CO3 H+ + HCO3

-

carbonic acid

hydrogen ion + bicarbonate ion

water and carbon dioxide

Formation of carbonic acid

CO2 + H2O H2CO3 H+ + HCO3

-

2H+ + CO3

2-

carbonic acid

hydrogen ion + bicarbonate ion

2 hydrogen ions + carbonate ion

water and carbon dioxide

Carbonic acid in the ocean

Chemical weathering -- dissolution of calcite

Granite Limestone

Chemical weathering – dissolution of limestone or marble

Weathering Chemical weathering of silicate minerals

• Goldich’s stability series • Stability of silicate minerals increases with the

percentage of Si-O bonds:

• Quartz – all Si-O bonds, most stable • Feldspars • Micas • Hornblende • Augite • Olivine – fewest Si-O bonds, least stable

Weathering Chemical weathering of silicate minerals

• Weathering of mafic minerals produces insoluble iron oxides and clay minerals

Weathering Chemical weathering of silicate minerals

• Weathering of mafic minerals produces insoluble iron oxides and clay minerals

• Weathering of feldspars produces clay minerals and silica in solution

Weathering Chemical weathering of silicate minerals

• Weathering of mafic minerals produces insoluble iron oxides and clay minerals

• Weathering of feldspars produces clay minerals and silica in solution

• Quartz remains substantially unaltered

Formation of Kaolinite, a clay mineral, from the weathering of Orthoclase

2KAlSi3O8

orthoclase

2KAlSi3O8 + 2H2O orthoclase water

Formation of Kaolinite, a clay mineral, from the weathering of Orthoclase

2KAlSi3O8 + 2H2O + 2H+ orthoclase water hydrogen ions

Formation of Kaolinite, a clay mineral, from the weathering of Orthoclase

2KAlSi3O8 + 2H2O + 2H+

2K+ potassium ions

orthoclase water hydrogen ions

Formation of Kaolinite, a clay mineral, from the weathering of Orthoclase

2KAlSi3O8 + 2H2O + 2H+

Al2Si2O5(OH) + 2K+ kaolinite potassium ions

orthoclase water hydrogen ions

Formation of Kaolinite, a clay mineral, from the weathering of Orthoclase

2KAlSi3O8 + 2H2O + 2H+

Al2Si2O5(OH) + 2K++ 2SiO2

0 kaolinite potassium ions silica in solution

orthoclase water hydrogen ions

Formation of Kaolinite, a clay mineral, from the weathering of Orthoclase

Clay crystals have a charge separation, with positive charges sandwiched between layers of negative

charge, making them attractive to water molecules

10 microns = 0.01 mm

Spheroidal weathering – chemical weathering along surfaces and joint planes

Sandcastles collapse when sand dries (pores contain only air) or when sand is saturated

(pores contain only water)

Sand grains stick together while damp, due to surface tension -- when pores contain water and air, water molecules stick to each other and to sand grains

When dry, clay holds about 20% water -- sculptures thus hold their shape through the firing process

Rates of weathering

Advanced mechanical weathering aids chemical weathering by increasing the surface area Important factors

• Rock characteristics • Mineral composition and solubility • Physical features such as joints

Rates of weathering

Important factors • Climate

• Temperature and moisture are the most crucial factors

• Chemical weathering is most effective in areas of warm temperatures and abundant moisture