Weathering: the decay of rocks and the source of sediments in sedimentary rocks

Geology 200

Geology for Environmental Scientists

Physical Weathering

• Ice Wedging (Geo-wedgies!) - the most effective form of physical weathering. Ice can exert 1500 lbs/in². Most effective with multiple freeze-thaw cycles.
• Sheeting or exfoliation - expansion joints
• Thermal expansion/contraction
• Plant roots
• Animal burrows

Chemical Weathering

• Two variables control most aspects of chemical weathering.
• Water - the more precipitation, the greater the rate of weathering. Dry climates have a slow rate.
• Temperature - warm, wet climates have the greatest rate of chemical weathering. Very cold climates have a slow rate.

Chemical Weathering - processes

• Dissolution
• Hydration
• Oxidation

Dissolution

• Water will dissolve many minerals because it is a bipolar molecule. It acts to loosen the bonds of ions at the surface of minerals. Salt and gypsum are easily dissolved.
• Carbon dioxide dissolved in water forms carbonic acid, which dissolves calcite.

Dissolution

• Some silicates, such as pyroxene, will also dissolve in carbonic acid:

Hydration

• A mineral reacts with either the H⁺ or the OH^- (hydroxide) from water to produce a new mineral. Aluminum silicates do not dissolve in water. Feldspars weather this way to form clay; e.g. plagioclase to clay:
• (equation not balanced)

Oxidation

• The chemical combination of oxygen with a mineral. Important in weathering iron-rich silicates: olivine, pyroxene, amphibole, biotite. Final product is hematite or limonite.

Results of Chemical Weathering

• Table 10.1 and 10.2
• Na, K, Ca, and Mg are removed into solution.
• Al and Si are concentrated in clays.
• Fe is incorporated into oxides.
• Clays and oxides are in equilibrium with the earth’s surface.

Figure 10.8 - Susceptibility to weathering

• Least stable: halite, gypsum, pyrite, calcite, dolomite
• Olivine, Ca-plagioclase, pyroxene, amphibole, biotite, Na-plagioclase, K-feldspar, muscovite
• Most stable: quartz, clay, aluminum oxides (bauxite), iron oxides.

Weathering of Major Rock Types

• Granite - physically weathers by exfoliation to form domes; chemically weathers to quartz grains and clay. This is the source of quartz sand grains.
• Basalt - weathers totally to clay and iron oxides; forms red or brown soils.

Weathering of Major Rock Types

• Sandstone - quartz grains are highly resistant to weathering and are recycled into new sand deposits; cement type, calcite, iron oxide, or silica, controls erosion of sandstones.
• Limestone - weathers rapidly in moist climates often forming karst features such as caves and sinkholes; forms cliffs in arid regions.

Weathering of Major Rock Types

• Shale - weathers rapidly because it is fine-grained and soft. The clays are transported in water by suspension to form muds which can later become mudstones and shales.

Differential Weathering

• Various rocks weather at different rates. This creates topography where the hills, ridges, or mountains are capped by resistant rock types. Most of the ridges of the Appalachians are capped by sandstone.

Products of Weathering

• Rocks weathered into spherical shapes
• Regolith, including soil, produced
• Ions go into solution

Shape of Weathered Rocks

• Fractures and joints provide opportunities for weathering
• Bedding planes - rocks break into slabs or sheets
• Cleavage or foliation - metamorphic rocks break along these planes
• Shattering - in dense rocks like quartzite

Shape of Weathered Rocks

• Granular disintegration - sandstone or granites
• Spheroidal weathering - angular fragments become rounded as spheres have the least surface area per unit volume
• Exfoliation - breaking into concentric layers

Regolith

• A blanket of loose decayed rock debris derived from the bedrock beneath it. The thickness may range from a few cms. to 100s of meters depending on climate, type of rock, and length of time for weathering.

Soil

• The uppermost layer of regolith. Composed of weathered rock and clays, plus decomposed organic matter.
• Soil horizons:

Soil and Climate

• Subtropical soils are often highly weathered and are red because of iron oxides.
• Tropical soils are highly weathered and are called laterites. They can be up to 60 m thick! They often accumulate aluminum oxides forming the mineral bauxite.
• Deserts and permafrost - thin soil
• Temperate - best agricultural soils

Soils: Parent Rock

• Good soils form on limestone and mafic igneous rocks. Many plant nutrients are released by chemical weathering.
• Poor soils form on quartz-rich rocks like sandstone, quartzite, or quartz-rich granites. Relatively few nutrients released for plants.

Ions in Solution

• Ions dissolved in water are invisible products of chemical weathering. Each year the world’s rivers carry about 4 million metric tons of dissolved material to the ocean (Table 10.2).

Rates of Weathering

• Controlled by the rock type and climate.
• Note the different rates of weathering of gravestones. Choose granite over marble.
• Even the pyramids in a desert climate show substantial effects of erosion.
• Volcanic rocks in tropical environments weather at a very rapid rate, >50 cm per 1000 yrs.