8.1 Weathering and Soil

External processes

Occur at or near Earth’s surface and are powered by energy from the Sun

Weathering, Mass wasting, Erosion

External processes

Weathering

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

Mass wasting

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

Erosion

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

Mechanical weathering

the process that breaks material into smaller and smaller pieces, each retaining the characteristics of the original material

Frost wedging

continuous freezing and thawing weakens the rocks and making them into smaller pieces

Frost heave

expansion of moist soils during freezing due to growth of ice lenses

Salt crystal growth

expansive force that can split rocks due to growth of salt crystals

Sheeting

the process of unloading and exfoliation

Unloading

a process where great reduction in pressure is experienced by a rock when the overlying rock is eroded away

Exfoliation dome

created from continued weathering that causes the slabs lto separate and spall off

Sheeting

Caused by the expansion of crystalline rock as erosion removes the overlying material

Intense heat from brush or forest fire

it can cause flakes of rock to spall from boulders or bedrock. As the rock surface becomes overheated, a thin layer expands and shatters

Root wedging, Burrow, Human actions

Biological activity

Chemical weathering

involves the processes that break down rock components and internal structures of minerals.

Water

the most important agent of chemical weathering

Acid (carbonic acid)

readily decomposed many rocks and produce certain products that are water soluble

Feldspar will breakdown into residual clay minerals, and quartz will survive the chemical weathering

What will happen to the weathering of granite

Aluminum, silicates, and oxygen, together with water will produce residual clay minerals

What will happen to the weathering of silicate minerals

Spheroidal weathering

Form of chemical weathering that affects jointed bedrock and results in the formation of concentric or spherical layers of highly decayed rock within weathered bedrock

Rock characteristics

The most important factor that influences the type and rate of rock weathering, it encompasses all of the chemical traits of rocks, including mineral composition and solubility

Temperature and moisture

crucial to the rate of rock weathering. Also exert a strong influence on rates of chemical weathering and on the kind and amount of vegetation present

Differential weathering

Masses of rock do not weather uniformly

Soil

A combination of mineral and organic matter, water, and air - that portion of the regolith that supports the growth of plants

Regolith

Covers the Earth’s land surface and the layer of rock and mineral fragments produced by weathering

Humus

The organic component of soil, formed by the decomposition of leaves and other plant material by soil microorganisms

Residual soil

The term for soil when their parent material is bedrock

Transported soil

Soil developed on unconsolidated sediment

Time

an important component of every geological process, and soil formation is no exception

Climate

the most influential control of soil formation. They also influence the rate and depth of weathering

Parent material, Time, Climate, Plants and animals, Topography

Controls of soil formation

Horizons

vertical differences which usually become more pronounced as time passes, divide the soil into zones or layers

O horizon

Consists largely of organic material

A horizon

a largely mineral matter, yet biological activity is high and humus is generally present at up to 30% in some instances

A and O horizon

They compose the topsoil

E horizion

light-colored layer that contains little organic material. Zone of eluviation and leaching

Eluviation

The washing out of the fine soil components as water percolates downward

Leaching

Depletion of soluble materials from the upper soil

B horizon

also known as the subsoil, accumulation of clay transported from above. Zone of accumulation

Hardpan

An impermeable layer formed in extreme cases from accumulation of clay

O, A, E, and B horizioons

The true soil or solum

Solum

it is where the soil-forming processes are active and that living roots and other plant and animal life are largely confined

C horizon

a layer characterized by partially altered parent material

Mature

A well-developed soil profile with relatively stable environmental conditions over an extended time span

Immaturew

Soil that lack horizons altogether and also characteristic of steep slopes where erosion continually strips away the soil, preventing full development

Alfisol

Moderately weathered soil the form under boreal forests or broadleaf deciduous forests, rich in iron and aluminum. Fertile productive soil because they are neither too wet nor too dry

Andisol

Young soils from volcanic ash and cinders deposited by recent volcanic activity

Aridosol

Soils that develop in dry places, may have an accumulation of calcium carbonate, gypsum, or salt in subsoil with law organic content

Entisol

Young soils having limited development and exhibiting properties of the parent material

Gelisol

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

Histosol

Organic soils with little or no climatic implications. Can be found in any climate where bog soil can form. Has peat

Inceptisol

Weakly developed young soils in which the beginning of profile development is evident. In the beginning. Most common in humid climates

Mollisol

Dark, soft soils that have developed under grass vegetation generally found in prairie areas. Humus-rich surface horizon that is rich in Ca and Mg. Excellent soil fertility

Oxisol

Soils that occur on old land surfaces unless parent materials were strongly weathered before they were deposited. Found in tropics and subtropics. Rich in iron and aluminum oxides and heavily leached, poor for agriculture

Spodosol

Soils found only in humid regions on sandy material. Has light-colored horizon of leached material

Ultisol

Soils that represent the products of long periods of weathering, restricted to humid climates

Vertisol

Soils containing large amounts of clay, which shrink upon drying and swell with the addition of water. Found in subhumid to arid climates

Andisol, Histosol, Vertisol

3 orders with unique parent materials

Aridisol, Gelisol, Oxisol

3 orders with unique environments

Entisol, Inceptisol, Ultisol

3 orders by age of development

Alfisol, Mollisol, Spodosol

3 orders by unique vegetative influence

Rills

Tiny channels formed during soil erosion

Gullies

Deeper cuts in the soil as rills are enlarged

Bauxite

A principal ore of aluminum and forms as a result of weathering processes under tropical conditions