Chapter 4: Weathering, Soil, and Mass Wasting

External Process

Weathering, mass wasting, and erosion are processes that occur at or near Earth's surface and are powered by energy from the Sun.

Internal Process

Processes that derive their energy from Earth's interior.

Weathering

The physical breakdown (disintegration) and chemical alteration (decomposition) of rocks at or near Earth's surface

Erosion

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

Mechanical weathering

accomplished by physical forces that break rock into smaller and smaller pieces without changing the rock's mineral composition. It is broken into smaller and smaller pieces, each retaining the characteristics of the original material.

Chemical weathering

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

Frost wedging

a form of physical weathering that breaks down rocks through the freezing and thawing process. First, water enters the rock through cracks and pores traveling deep within the rock. As the water freezes, it expands into the rock, causing the cracks and pores to grow.

Salt Crystal Growth

When water evaporates, salts precipitate. If this happens in fractures in rock, the growth of the salt crystals can put pressure on the cracks, causing them to grow.

Sheeting

When large masses of igneous rock, particularly those composed of granite, are exposed by erosion, concentric slabs begin to break loose like onion-like layers.

Unloading

The process of removing overlying rock

Exfoliation dome

large, rounded features that form when rocks at the surface expand and fracture due to heat from the sun. The fractures create small domes that are exfoliated (peeled) away from the rock by the wind.

Carbonic acid

An acid that forms from carbon dioxide and water. It is a large contributor to chemical weathering.

Spheroidal weathering

a process of cracking and splitting off of curved layers from a generally spherical boulder, but on a much smaller scale. It occurs when a rectangular block is weathered from three sides at the corners and from two sides along its edges.

Rock characteristics

Encompass all of the chemical traits of rocks, including mineral composition and solubility. Variations in weathering rates due to the mineral constituents can be demonstrated by comparing old headstones made from different rock types

Climate

Climatic factors such as temperature and moisture affect the rate of weathering like in frost wedging. The optimal environment for chemical weathering is warm temperature + abundant moisture

Differential weathering

Masses of rock do not weather uniformly. Responsible for creating many unusual and sometimes spectacular rock formations and landforms

Regolith

a layer of rock and mineral fragments produced by weathering.

Soil

a combination of mineral and organic matter, water, and air—the portion of the regolith that supports the growth of plants.

Soil Texture

refers to the proportions of different particle sizes.

USDA Soil Texture Triangle

is a widely used tool for classifying soil texture based on the proportions of sand, silt, and clay particles.

parent material, time, climate, plant and animals, and topography

5 Controls of soil formation

Parent material

The source of the weathered mineral matter from which soils develop

Residual Soils

Soils developed from bedrock

Transported Soils

soils developed on loose sediment

Humus

a material that no longer resembles the plants and animals from which it is formed.

Slope Orientation

the direction the slope is facing

Soil Horizon

Zone of soils and layers parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath.

Soil Profile

A vertical section through a soil, showing its succession of horizons and the underlying parent material.

Eluviation

Washing out of the fine soil components. As water percolates downward through the E horizon, finer particles are carried away

Leaching

Depletion of soluble materials from the upper soil. As water percolates downward, it also dissolves soluble inorganic soil components and carries them to deeper zones

Zone of Accumulation

B-horizon or the accumulation of the fine clay particles enhances water retention in the subsoil

Solum

Where soil-forming processes are active and that living roots and other plant and animal life

Hardpan

In extreme cases clay accumulation can form this very compact and impermeable layer

Immature soils

Lacks some horizons. Characteristics of steep slopes where erosion continually strips away the soil, preventing full development

Soil Taxonomy

Emphasizes the physical and chemical properties of the soil profile and is organized properties of the soil profile and is organized on the basis of observable soil characteristics

Mass Wasting

the downhill movement of rock and soil material due to gravity.

Angle of Repose

the maximum angle to the horizontal at which rocks, soil, etc, will remain without sliding.

Fall

When the movement involves the free falling of detached individual pieces of any size. Common on slopes that are too steep for loose material to remain on the surface.

Slide

which occur whenever material remains fairly coherent and moves along a well-defined surface. Sometimes the surface is a joint, a fault, or a bedding plane that is roughly parallel to the slope.

Flow

occurs when material moves downslope as a viscous fluid and are saturated with water and typically move as lobes or tongues.

Rock Avalance

rock and debris can hurtle downslope at speeds exceeding 200 kilometers (125 miles) per hour.

Slump

to the downward sliding of a mass of rock or unconsolidated material moving as a unit along a curved surface

Rockslide

occur when blocks of bedrock break loose and slide down a slope

Debris slide

If the material is mostly soil and regolith that slide down a slope.

Debris flow

a relatively rapid type of mass wasting that involves a flow of soil and regolith containing a large amount of water

Mudflow

a relatively rapid type of mass wasting that involves a flow of fine-grained material containing a large amount of water

Lahars

Debris flows composed mostly of volcanic materials on the flanks of volcanoes

Earthflow

When water saturates the soil and regolith on a hillside, the material may break away, leaving a scar on the slope and forming a tongue- or teardrop-shaped mass that flows downslope

Creep

a type of mass wasting that involves the gradual downhill movement of soil and regolith. Example is alternate expansion and contraction of surface material caused by freezing and thawing or wetting and drying.

Solifluction

When soil is saturated with water, the soggy mass may flow downslope at a rate of a few millimeters or a few centimeters per day or per year.

Permafrost

refers to the permanently frozen ground that occurs in Earth's harsh tundra and ice-cap climates.

Active layer

zone above the permafrost which thaws in summer and refreezes in winter.

O horizon

Loose and partly decayed organic matter

A horizon

Mineral matter mixed with some humus

E horizon

Zone of eluviation and leaching

B horizon

Accumulation of clay transported from above

C horizon

Partially altered parent material

Alfisols

Moderately weathered soils that form under boreal forests or broadleaf deciduous forests, rich in iron and aluminum. Clay particles accumulate in a subsurface layer in response to leaching in moist environments. Fertile, productive soils, because they are neither too wet nor too dry. 9.65%

Andisols

Young soils in which the parent material is volcanic ash and cinders, deposited by recent volcanic activity. 0.7%

Aridosols

Soils that develop in dry places; insufficient water to remove soluble minerals; may have an accumulation of calcium carbonate, gypsum, or salt in subsoil; low organic content. 12.02%

Entisols

Young soils having limited development and exhibit properties of the parent material. Productivity ranges from very high for some that form on recent river deposits to very low for those that form on shifting sand or rocky. 16.16%

Gelisols

Young soils with little profile development that occur in regions with permafrost. Low temperatures and frozen conditions for much of the year slow soil-forming processes. 8.61%

Histosols

Organic soils with little or no climatic implications. Can be found in any climate where organic debris can accumulate to form a bog soil. Dark, partially decomposed organic material commonly referred to as peat. 1.71%

Inceptisols

Weakly developed young soils in which the beginning of profile development is evident. Most common in humid climates, they exist from the Arctic to the tropics. Native vegetation is most often forest. 9.81%

Mollisols

Dark, soft soils that have developed under grass vegetation, generally found in prairie areas. humus-rich surface horizon that is rich in calcium and magnesium. Soil fertility is excellent. Also found in hardwood forests with significant earthworm activity. Climatic range is boreal or alpine to tropical. Dry seasons are normal. 6.89%

Oxisols

Soils that occur on old land surfaces unless parent materials were strongly weathered before they were deposited. Generally found in the tropics and subtropical regions. Rich in iron and aluminum oxides, these are heavily leached, hence are poor soils for agricultural activity. 7.5%

Spodosols

Soils found only in humid regions on sandy material. Common in northern coniferous forests and cool humid forests. Beneath the dark upper horizon of weathered organic material lies a light-colored horizon of leached material, the distinctive property of this soil. 2.56%

Ultisols

Soils that represent the products of long periods of weathering. Water percolating through the soil concentrates clay particles in the lower horizons (argillic horizons). Restricted to humid climates in the temperate regions and the tropics, where the growing season is long. Abundant water and a long frost-free period contribute to extensive leaching, hence poorer soil quality. 8.45%

Vertisols

Soils containing large amounts of clay, which shrink upon drying and swell with the addition of water. Found in subhumid to arid climates, provided that adequate supplies of water are available to saturate the soil after periods of drought. Soil expansion and contraction exert stresses on human structures. 2.24%