Physical Geology 01C

J Bret Bennington

Weathering

Updated 10/99

Weathering does several very important things: [slide]

Breaks down rock to make sediments

Forms soils

Forms economic mineral deposits

Two types:

Mechanical weathering: the physical breakup of rock into smaller pieces [slide]

Mechanical weathering begins with the process of Jointing. Jointing is the spontaneous fracturing of rock as it adjusts to the removal of overlying pressures. The closely spaced cracks that result are called joints. Joints can be either horizontal or vertical.

Sheeting - the layered appearance of horizontal joints. [slide - jointing and sheeting]

Slide: Columnar jointing, top view

Slide: Columnar jointing in igneous rock, Mexico

Slide: Sheeting in Granite, Yosemite

Slide: 3 joint sets, Lysefiord, Norway

Wedging [slide]

Once cracks form in rock, these cracks can be widened and extended by several processes.

Crystal wedging: Groundwater moving through the fractures carry dissolved minerals, including salts. These salts and minerals can crystallize within the cracks, and the growing crystals exert a force on the walls of the crack. This process is most important in desert areas where extremes of wet and dry alternate.

Frost or Ice wedging: Ice is a particularly good wedging agent because it expands up to 9% in volume from liquid water when it forms. Freeze thaw cycles are most effective as agents of mechanical weathering at temperatures between 23° F and 5° F.

Plant roots: Plants are incredibly effective agents of mechanical weathering. Roots can penetrate cracks in rocks to depths of several meters. As the roots grow they place a tremendous amount of hydrostatic pressure on the walls of the cracks.

Heat spalling [slide]: Heat from forest fires and brush fires will cause the outer surface layers of rock to expand quickly and break away in spalls. Natural fires, although an infrequent occurence in human experience, occur on a scale from yearly to hundred yearly, and are thus very frequent events over geological time scales.

Chemical weathering [slide] exploits the fracturing in rock produced by mechanical weathering. The principle agents of chemical weathering are water solutions that act as weak acids.

The principle weak acid responsible for chemical weathering is Carbonic Acid.

Carbonic acid forms naturally in rainwater. As rainwater falls through the atmosphere it dissolves small amounts of carbon dioxide. Additional carbon dioxide is picked-up in the ground from decaying vegetation.

H2O + CO2 = H2CO3 = H+1 (carbonic acid)+ HCO3-1 (bicarbonate ion) [slide]

The hydrogen ion in solution (H+1) is very reactive. For example, it can attack Feldspar and cause the reaction of feldspar with water (hydrolysis or hydration): [slide]

K-Feldspar + hydrogen ion + water = K ions + Kaolinite (clay) + silica (solution)

4KAlSi₃O₈ + 4H^- + 2H₂O = 4K+ + Al₄Si₄O₁₀(OH)₈ + 8SiO₂

Acidic rainwater is also very effective at breaking down calcium carbonate, a principle sedimentary rock-forming mineral that makes-up limestones. [slide]

Calcium carbonate + carbonic acid = calcium ions + bicarbonate ions

CaCO3 + H2CO3 = Ca⁺⁺ + 2HCO3-1

The bicarbonate ions react with hydrogen ions to produce water and CO2.

HCO3-1 + H⁺ = H2O + CO2

Other chemical weathering reactions:

Oxidation (attack by oxygen): [slide]

2Fe₂SiO₄ (olivine) + H2O + O2 = FeO.OH (goethite) + dissolved silica

Goethite dehydrates to form hematite, a very stable iron oxide. Goethite has a yellowish color, while hematite is brick red. Thus, rocks rich in iron oxides tend to form very red soils.

Leaching: simple dissolution of minerals in water solutions.

Succeptibility of minerals to weathering

In general, many igneous and metamorphic rocks contain minerals that formed under conditions of high temperature and pressure. These minerals are not really stable at STP and tend to chemically breakdown when exposed at the surface. [slide]

Minerals prone to chemical attack: Feldspars, pyroxenes, amphiboles, micas, iron oxides, calcium carbonate.

Minerals resistent to chemical attack: Quartz, clay minerals (e.g. Kaolinite).

Some general weathering processes:

Exfoliation: the loss of outer layers of rock as it weathers and detaches from the main mass.

Slide: Fire exfoliation, Yellowstone

Spheroidal weathering: the tendency of initially angular rock fragments to weather into spherical shapes due to the increased vulnerability of edges and corners to attack. [slide]

Slide: Spheroidal weathering

Effect of climate:

Generally, the most rapid weathering occurs in hot, wet climates where chemical weathering is intense and mechanical weathering due to vegetation is ubiquitous. Cold, moderately dry climates experience intense mechanical weathering due to frost wedging.

Soils: Soils form from a mixture of regolith (the outer layer of weathered rock fragments on the earth’s surface) and organic matter from plants. Different climates produce different types of soils.

Mineral deposits:

Because weathering tends to remove unstable minerals, either through dissolution or through disintegration and transport, the remaining stable minerals are concentrated. If a sufficient quantity of these stable minerals are valuable, then economic deposits can form.

Laterite: A wet climate soil, common in the tropics, formed from the extensive weathering and leaching of the underlying rock. In a laterite, only the most stable minerals remain. Most laterites are rich in iron hydroxide that forms a red crust or ironstone. However, in some tropical regions laterites form that are rich in aluminum hydroxide. These laterites are mined as bauxite the principle ore of aluminium metal.

Slide: Lateritic soil, North Carolina

Slide: Bauxite deposit, Queensland Australia

Placer deposits: Gold and other dense, native minerals can weather out of rock and become concentrated in stream beds.

Slide: 58 gram gold nugget, Australia