Physical Geology

Lecture 7 Weathering

Weathering does several very important things:

Breaks down rock to make sediments

Forms soils

Forms economic mineral deposits

Two types:

Mechanical weathering: the physical breakup of rock into smaller pieces

Chemical weathering: the chemical reaction of minerals with air, water, etc to form new minerals with different properties.

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.

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

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. Frost wedging is effective as a mechanical weathering agent in regions where alternating freezing and thawing occur such as mountaintops and mid-latitudes with pronounced seasons.

Heat spalling: 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.

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 pressure on the walls of the cracks.

Chemical weathering 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.

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

Other chemical weathering reactions:

Oxidation (attack by oxygen):

Iron + Oxygen = Iron Oxide (rust)

Leaching: simple dissolution of minerals in water solutions.

Some general weathering processes:

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

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.

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.