Chapter 5

Exfoliation domes

Accompanying this unloading, the outer layers expand more than the rock below and thus separate from the rock. Continued weathering eventually causes the slabs to separate and spall off, creating exfoliation domes. Examples include Stone Mountain, Georgia, and Half Dome.

Rills/Gullies

After flowing as a thin, unconfined sheet for a relatively short distance, threads of current typically develop, and tiny channels called rills begin to form. Still deeper cuts int he soil are known as gullies, which are created as rills enlarge.

Climate

Climate is the most influential control of soil formation. Temperature and precipitation are the elements that exert the strongest impact on soil formation. Variations in temperature and precipitation determine whether chemical or mechanical predominates. They also greatly influence the rate and depth of weathering. The amount of precipitation influences the degree to which various materials are removed from the soil, thereby affecting soil fertility. Climate conditions are important factors controlling the type of plant and animal life present.

Leaching

The depletion of soluble materials from the upper soil by downward percolating water.

Soil profile

A soil profile is a vertical cross section from the surface through all of the soil's horizons and into the parent material. The horizons consist of the O, A, E, B, and C horizons.

Biological activity

Both mechanical and chemical weathering are accomplished by the activities of organisms. Examples include: plant roots wedging the rock apart, burrowing animals breaking down the rock by moving fresh material to the surface, and human mines.

Carbonic acid

Carbon dioxide dissolved in water forms carbonic acid, which readily decomposes many rocks and produces certain products that are water soluble. The dissolving power of carbonic acid plays an important part in forming limestone caverns, such as Carlsbad Caverns in New Mexico.

Chemical weathering (a.k.a decomposition)

Chemical weathering involves a chemical transformation of rock into one or more new compounds.

Chemical weathering

Chemical weathering involves the complex processes that break down rock components and internal structures of minerals. Such processes convert the constituents to new minerals or release them to the surrounding environment.

Climate

Climatic factors, particularly temperature and moisture, are crucial to the rate of rock weathering. One important example from the mechanical weathering is that the frequency of freeze-thaw cycles affect the amount of frost wedging. The best environment for chemical weathering is a combination of warm temperatures and abundant moisture.

Regolith

Earth's land surface is covered by regolith, the layer of rock and mineral fragments produced by weathering.

Differential weathering

Frequently, rocks exposed at Earth's surface do not weather at the same rate. This differential weathering of rocks is influenced by such factors as mineral makeup and degree of jointing.

Talus slopes

Frost wedging is most pronounced in mountainous regions where a daily freeze-thaw cycle often exists. Here, sections of rock are wedged loose and may tumble into large piles called talus slopes that often form at the base of steep rock outcrops.

Time

If weathering has been going on for a comparatively short time, the parent material strongly influences the characteristics of the soil. As weathering processes continue, the influence of parent material on soil is overshadowed by the other soil-forming factors, especially the climate. As a rule, the longer a soil has been forming, the thicker it becomes and the less it resembles the parent material.

Frost wedging

In nature, water works its way into cracks in rock and, upon freezing, expands and enlarges these openings. After many freeze-thaw cycles, the rock is broken into angular fragments. This process is called frost wedging.

Mechanical weathering (a.k.a disintegration)

Mechanical weathering is accomplished by physical forces that break rock into smaller and smaller pieces without changing the rock's mineral composition.

Chemical weathering of quartz

Quartz is very resistant to chemical weathering; it remains substantially unaltered when attacked by weak acidic solutions. After the feldspar crystals turns to clay, some quartz remains in the soil, while much becomes the main component of sandy beaches. These quartz grains may become lithified to form the sedimentary rock sandstone.

Rock characteristics (rate of weathering)

Rock characteristics encompass all of the chemical traits of rocks, including mineral composition and solubility, and any physical features such as joints. The variations in weathering rates are due to the mineral constituents.

Salt Crystal Growth

Rocky shorelines and arid regions are common settings for the salt crystal growth. It begins when sea spray from breaking waves or salty groundwater penetrates crevices and pore spaces in rock. As this water evaporates, salt crystals form and as these crystal gradually grow larger, they weaken the rock by pushing apart the surrounding grains or enlarging tiny cracks.

Soil

Soil is a combination of mineral and organic matter, water, and air (the portion of regolith that supports the growth of plants). About 1/2 of the total volume of a good quality surface soil is a mixture of disintegrated and decomposed rock (mineral matter) and humus, the decayed remains of animal and plant life (organic matter). The remaining 1/2 consists of pore spaces among the solid particles where air and water circulate.

Joints

Some fractures are produced by contraction as igneous materials cool, and still others by tectonic forces during mountain building. Joints are important rock structures that allow water to penetrate to depth and start the process of weathering long before the rock is exposed.

Internal processes

Some parts of Earth's surface are gradually elevated by mountain building and volcanic activity. These internal processes derive their energy from Earth's interior.

Solum (a.k.a true soil)

The O, A, and B horizons in a soil profile. Living roots and other plant and animal life are largely confined to this zone.

Plants and animals

The biosphere plays a vital role in soil formation. The types and abundance of organisms present have a strong influence on the physical and chemical properties of a soil. Soil fertility depends in part on the amount of organic matter present.

Soil Taxonomy

The classification of soils is known as the soil taxonomy. It emphasizes the physical and chemical properties of the soil profile and is organized on the basis of observable soil characteristics. There are six hierarchical categories of classification, ranging from order, the broadest category (12), to series, the most specific category (19,000).

Weathering (external process)

The physical breakdown (disintegration) and chemical alteration (decomposition) of rocks at or near Earth's surface. Weathering occurs when rock is mechanically fragmented (disintegrated) and/or chemically altered (decomposed).

Erosion (external process)

The physical removal of material by mobile agents such as water, wind, or ice.

Parent material

The source of the weathered mineral matter from which soils develop is called parent material and is a major factor influencing a newly forming soil. The nature of parent material influences soils in two ways. First, the type of parent material affects the rate of weathering and thus the rate of soil formation. Second, the chemical makeup of the parent material affects the soil's fertility.

Mass Wasting (external process)

The transfer of rock and soil downslope under the influence of gravity.

Eluviation

The washing out of fine soil components from the A horizon by downward-percolating water.

Chemical weathering of potassium feldspar

The weathering of potassium feldspar generates a residual clay mineral, a soluble salt (potassium bicarbonate), and some silica, which enters into solution.

Topography

Variations in topography can lead to the development of a variety of localized soil types. Many of the differences exist because the length and steepness of slopes have a significant impact on the amount of erosion and the water content of soil. The optimum terrain for soil development is a flat-to-undulating upland surface, where there is good drainage, minimum erosion, and sufficient infiltration of water into the soil. Slope orientation, or the direction a slope is facing, also is significant.

Sheet erosion

Water flowing across the surface carries away the dislodged soil particles. The soil is moved by thin sheets of water, in a process called sheet erosion.

Water in chemical weathering

Water is by far the most important agent of chemical weathering. Although pure water is nonreactive, a small amount of dissolved material is generally all that is needed to activate it. Example: Oxygen dissolved in water will oxidize some materials.

External Processes

Weathering, mass wasting, and erosion are called external processes because they occur at or near the Earth's surface and are powered by energy from the Sun. External process transform solid rock into sediment.

Soil as an interface

When Earth is viewed as a system, soil is referred to as an interface -- a common boundary where different parts of a system interact. Soil forms where the geosphere, the atmosphere, the hydrosphere, and the biosphere meet. Over time, soil gradually evolves to a state of equilibrium w/ the environment.

Mechanical Weathering

When a rock undergoes mechanical weathering, it is broken into smaller and smaller pieces, each retaining the characteristics of the original material, abut increasing the surface area available for chemical attack.

Spheroidal weathering

When angular rock masses chemically weather as water enters along joints, they tend to take on a spherical shape. Gradually the corners and edges of the angular blocks become more rounded. The corners are attacked most readily because of their greater surface area than the edges and faces. This process, called spheroidal weathering, gives the weathered rock a more spherical shape. California's Joshua Tree National Park.

Sheeting

When large masses of igneous rock, particularly granite, are exposed by erosion, concentric slabs begin to break loose. The process generating these onionlike layers is called sheeting. It is thought that this occurs, at least in part, because of the great reduction in pressure, because of the great reduction in pressure when the overlying rock is eroded away, a process called unloading.

Residual soils / transported soils

When the parent material is bedrock, the soils are termed residual soils. By contrast, those developed on unconsolidated sediment are called transported soils. Transported soils form in place on parent materials that have been carried from elsewhere and deposited by gravity, water, wind, or ice.

Erosional agents

Wind, glacial ice, rivers, and waves.