Geol 33_Environmental Geomorphology Weathering

Geol 33 Environmental Geomorphology

J Bret Bennington

Weathering

As should be obvious by now, weathering is a very important process in the formation of geomorphic landforms. Weathering is also an important subject to study for other reasons:

Soil formation - weathering forms the soils that support terrestrial ecosystems
Behavior of geologic materials - all surface rock and geologic material used in construction is subject to weathering, which alters the rock's fundamental properties of hardness, durability, and stability over time.
Formation of economic mineral deposits - gold, diamonds, bauxite

Weathering can be defined as the physical and chemical changes that take place in a geologic material exposed at or near the surface of the Earth. Erosion refers to the movement of weathered material and its removal from an area.

Weathering occurs because all rock forms at temperatures and or pressures that are greater than those found near the surface of the Earth. Uplift and exposure at the surface brings the rock and its component minerals into an environment with different conditions and the material changes to come into equilibrium with these new conditions.

There are two basic types of weathering

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 unloading and jointing. Jointing is the spontaneous fracturing of rock as it adjusts to the removal of overlying pressures (unloading). The closely spaced cracks that result are called joints. Joints can be either horizontal or vertical. Large horizontal joints are called sheets and typically form in massive igneous rock where the orientation of the sheets parallel to the weathering surface mimics sedimentary layering.

Rapid excavation and unloading of rock can lead to equally rapid (sometimes explosive) jointing. Joint surfaces also commonly form failure plains for rock slides.

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 very important in desert areas where extremes of wet and dry alternate and has its greatest effect on permeable rock such as sandstone and porous limestone.

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, and at high altitudes where exposed rock surfaces are heated by the sun during the day and cooled rapidly at night.

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.

Wetting and drying of clays: Some clay minerals are capable of absorbing water to form sheets of water molecules between layers of adjacent silica sheets. The addition of water to these clays causes them to expand, exerting an outward pressure. Different clays are capable of absorbing different amounts of water and different degrees of swelling. Certain forms of Na-montmarillonite clay expand up to 1000X their original volume. These swelling clays can cause structural problems if buildings are constructed on them.

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.

Some other 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.

Chemical weathering exploits the fracturing in rock produced by mechanical weathering. Chemical weathering reactions are exothermic and result in volume increases in minerals that contribute to the physical disruption of rock. The effectiveness of chemical weathering is directly related to the amount of surface area exposed, which itself is exponentially related to the density of fractures in a rock.

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)

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):

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

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

In extremely wet conditions where fresh water continually flushes the weathering products, all of the available cations from the mineral can be eventually removed, leaving only Al₃O₄ (bauxite).

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

Calcium carbonate + carbonic acid = calcium ions + bicarbonate ions

CaCO3 + H2CO3 = Ca⁺⁺ + 2HCO3-1

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

HCO3-1 + H⁺ = H2O + CO2

When groundwater saturated with the products of the above reaction comes into contact with the air, CO2 is lost, resulting in less hydrogen ion and driving the reactions backwards to precipitate CaCO3 as solid mineral. This is how limestone formations develop in caves and around springs in karst regions.

Other chemical weathering reactions:

Oxidation (attack by oxygen):

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.

FeS₂ + 8HCO₂^- + 7.5 O₂ = Fe₂O₃ + 4SO₄^-2 + 4CO₂ +4H₂O

Pyrite oxidizes rapidly to hematite in the presence of water and oxygen.

Leaching: simple dissolution of minerals in water solutions.

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.

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

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

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. Cold, dry climates have very slow rates of rock weathering.

The relations between climate and weathering can be seen in soils.

Tropical climates - deep, highly weathered red soils with abundant iron oxides and aluminum oxides.

Desert climates - thin soils enriched in carbonate minerals that precipitate from ephemeral soil water.

Temperate climates - soils with intermediate characteristics -low in carbonate, enriched in clays.

Weathering profile across different climatic zones related to latitude.

from Strakhov, 1967

The effect of weathering on climate

The chemical reactions involved in weathering may also be important in understanding long term changes in the global climate.

In general, weathering of silicate minerals by acid hydrolysis consumes carbon dioxide from the atmosphere. This occurs because bicarbonate ion is produced by the disassociation of CO2 in water to form carbonic acid.

2H2O + 2CO₂ = 2H₂CO₃ = 2H⁺¹ + 2HCO₃^-1

Bicarbonate ion then reacts with Ca2+ in seawater to produce carbonate mineral.

Ca₂+ + 2HCO₃- = CaCO₃ + H₂O + CO₂

For every two molecules of CO₂ consumed, one becomes sequestered in limestone, with a net loss to the atmosphere. Reduction in atmospheric CO₂ reduces the greenhouse warming of the surface and leads to global cooling.

This process is self-limiting however, because cooling leads to a decrease in the rate of chemical weathering, which in turn decreases the uptake of CO₂ by the weathering process.

Metamorphism reverses the process by causing to reaction of silica with carbonate to produce complex silicates, releasing CO₂ into the atmosphere via volcanoes and hot springs.

For example, some have suggested that the Pleistocene glaciations may have been triggered, in part, by atmospheric CO₂ reduction caused by silicate weathering associated with the Himalayan orogeny.