Weathering and Soils

Internal processes

Powered by energy from Earth's interior

External processes

Powered by the sun

Weathering

Physical and chemical breakdown of rocks at the surface
Slow and subtle
Response of the planet to the changing environment
Part of the rock cylce

Mass wasting

Transfer of rock and soil downslope through gravity

Erosion

Physical removal of material by mobile agents like water, air, ice, and wind

Mechanical weathering

Physical forces disintegrate the rocks
Increases surface area

Types of mechanical weathering

Frost wedging
Salt crystal growth
Sheeting
Biological activity

Frost wedging

Due to freeze and thaw
Water enters cracks, freezes, and expands

9%

The addition to the volume of water when it freezes

Frost heave

Uplift of the soil surface or foundations caused by freezing of moisture in the soil.

Salt crystal growth / Salt weathering

Salt-saturated water enters rocks, evaporation forms salt crystals which weakens rock
Common in rocky shorelines and arid regions

Sheeting

Pressure unloading
Joints form during the process

Exfoliation domes

Large domed shaped structures usually composed of granite, formed by sheeting.
Formed when rocks begin to fall from a sheeted body.

Biological activity (mechanical weathering)

Through roots, burrowing animals, human impact

Chemical weathering

Components breakdown and internal structure of minerals broken.
Minerals converted to new minerals or carried to another environment

What is the most effective weathering agent?

Water

Spheroidal weathering

Any weathering process that tends to produce a spherical shape from an initially blocky shape.
Angular blocks become rounded as chemical weathering penetrates joints.

What factors affect the rate of weathering?

Rock characteristics
Climate
Differential weathering

Goldich's weathering series

Relative order of silicate mineral decomposition due to weathering
High-temperature minerals (e.g. olivine) weathers first

Soil

A combination of mineral and organic matter, water, and air; that portion of the regolith that supports plant growth.

Regolith

Loose, heterogenous weathered material lying on top of the bedrock.
50% mineral matter + organic matter (> mineral matter)
50% pore spaces

What fills the pore spaces in soil?

Air and water

Controls of soil formation

Parent material
Time
Climate
Plants and animals (biological activity)
Topography

What is the optimal topography for soil development?

Flat and undulating upland surface

How should the slope be oriented in both hemispheres to make it optimal for soil development?

Facing south in the Northern Hemisphere
Facing north in the Southern Hemisphere

In what climate would thickly weathered and leached soils formed by chemical weathering would be common?

Hot and wet climates

In what climate thin soil formed by mechanical weathering would be common?

Cold and dry climates

Eluviation

Washing out of fine soil components

Leaching

Removal of dissolved materials from soil by water moving downwards

What causes the variations in the soil profile?

Composition
Texture
Structure
Color

O horizon

Organic layer
Upper portion = plant litter
Lower portion = partly decomposed humus
Rich with microscopic life

A horizon

Mineral matter + 30% organic matter
High biological activity

E horizon

Zone of eluviation and leaching
Lightly-colored
Little organic matter

B horizon

Zone of accumulation
Washed out material from upper layers deposited here
Clay accumulation increases water retention

C horizon

The least-weathered soil horizon, which always occurs beneath the B horizon and is similar to the parent material.

Hardpan

Crust or layer of hard subsoil encrusted with calcium-carbonate occurring in arid or semiarid regions.
Forms in the B horizon due to clay accumulation.
Prevents good drainage.

Solum

True soil.
Soil-forming processes are very active.
O, A, E, and B horizon.
Plant life confined here.

Mature soil

Complete soil profile.

Immature soil

Incomplete soil profile.
Common in areas of steep topography

Alfisols

High-nutrient soil.
Moderately weathered.
Neither too wet or too dry.
Common in boreal forest.
Rich in Fe and Al.

Aridosols

Desert soil.
Accumulation of calcium, carbonate, gypsum, and salt.
Low organic content.

Andisols

Volcanic soil.
Young. Parent is from a recent volcanic activity.

Entisols

New and young soil.
Still inhibits properties of parent material.
Productive near river, not productive in sandy/rocky slopes.

Histosols

High organic content soil.
Can be found in any climate on where organic debris can form bog soil.
Partially decomposed humus (peat).

Gelisols

Permafrost soil.
Little development.

Inceptisols

Young soil.
Weakly developed; beginning formation.
Common in humid climate.
Hosts native vegetation as forests.

Molisols

Prairie / grassy area soil.
Rich in Ca and Mg in the humus zone.
Forms in boreal-alpine-tropical climate
Hosts hardwood forests with significant earthwork activity.

Oxisols

Tropical forest soil.
Common in tropics-subtropics.
Rich in Fe and Al oxides.
Heavily leached; poor for farming purposes.

Spodosols

Conifer forest soil.
Forms in humid regions and regions of sandy material.
Weathered organic material followed by leached light-colored distinct layer

Ultisols

Low nutrient soil.
Represents long periods of weathering.
Forms in humid (temperate, tropics) climates.
Abundant water, frost-free, heavily leached.
Poor utility.

Vertisols

Expansive clay soil.
Shrink and swell.
Forms in subhumid-arid climates as long as adequate water can be provided.

Do forests have humus?

No because of the high organic activity?

Laterite

Overly leached (Ca and Si removed), useless soils.
Forms in ultramafic parents in humid regions (tropical rain forests).

Pedalfers

Moderately leached.
Forms in mid-latitude forests.

Pedocal

Soil associated with drier regions and characterized by an accumulation of calcium carbonate (caliche) in the upper horizons.

Thick red soils

End stage of extreme chemical weathering.
Severely leached.
Found on rainforests.
Not good for other purposes.

Soil erosion

A natural process.
Begins with sheet flow then followed by rills and gullies.

Is soil erosion rate slower or faster in the past?

Slower due to absence of human activity in the past.

Preventive measures for soil erosion

Windbreaks
Terracing
Plowing (must be parallel to contour)
Tillage
Crop rotation

Secondary enrichment

Chemical weathering + water actions carries away wanted or unwanted material.

Bauxite

Primary ore of aluminum.
From the intense chemical weathering of Al-rich parent rocks in tropical climates