CHP 5 Weathering and Soils

Weathering, Erosion, and Transportation

-Earth's surface is constantly changing
-Rock disintegrates and decomposes-weathering (links to rock cycle, systems)
-move to lower elevations by gravity or eroded away. by water, wind or ice
-Sculpting Earth's physical landscape
- These processes affect the composition of the Earth's atmosphere and produce soil
-Responsible for deterioration of many structure we build

Rocks exposed at Earth's surface are constantly changed by water, air, temperature variations and other factors-weathering

Rocks exposed at Earth's surface are constantly changed by water, air, temperature variations and other factors-weathering

Weathering

The group of destructive processes that change the physical and. chemical character of rocks at or near Earth's surface-prepares rock for erosion and transportation

Erosion

a physical picking up of rock particles by water, ice, or wind

Transportation

the movement of eroded particles by water, ice , or wind

mechanical weathering

Processes that break rocks into smaller pieces without changes to the chemical composition

Chemical weathering

decomposition of rock from exposure to water and atmospheric gases(carbon dioxide, oxygen, and water vapor)

Effects of Weathering

-Destruction of building materials
-Discoloration of surface outcrops
-Production of soil
-Impacts the atmosphere(removes carbon dioxide)
-Creates interesting and unusual rock shapes(spheroidal & Differential)

Spheroidal weathering

occurs where rock has been rounded by weathering from an initial blocky shape. Chemical weathering occurs more rapidly at the corners than along the edges

Differential Weathering

Rocks weather at different rates
ex: Shale (composed of soft clay minerals) tends to weather and erode faster than sandstone (made of hard quartz minerals)

Mechanical Weathering: Pressure Release

Breaks into smaller and smaller pieces, each retaining the characteristic of the original material

4 physical processes leading fragmentation

-Pressure release or joining & sheeting
-Frost wedging
-Salt crystal growth
-Biological activity

pressure release

removal of overlying rock(unloading) allows expansion and fracturing (sheet joints).

JOINTS BREAK LOOSE IN CONCENTRIC SLABS

JOINTS BREAK LOOSE IN CONCENTRIC SLABS

Frost action

mechanic effect of freezing (and expanding) water on rocks. Ex: Frost wedging and Frost heaving

frost wedging

The mechanical breakup of rock caused by the expansion of freezing water in cracks and crevices

frost heaving

The lifting of rock or soil by the expansion of freezing water.

Other Types of Mechanical Weathering

-Biological Activity(plant growth, burrowing animals)
-Thermal variation
-Salt pressure

Plant growth and mechanical weathering

growing roots widen fractures

Thermal Variation

Larger temperature changes fracture rocks by repeated expansion and contraction
ex:desert, forest fire

Salt Pressure

Salt crystal growth as water evaporates in cracks

As mechanical weathering breaks rock into smaller pieces, more surface area is exposed to chemical weathering.

As mechanical weathering breaks rock into smaller pieces, more surface area is exposed to chemical weathering.

Chemical Weathering

-complex breakdown of rock components and internal structures of minerals
-New minerals are formed and may be released to the environments
-This decomposition converts the original rock into substances that are more stable in the surface environment

Main Process of Chemical Weathering

-Dissolution
-Oxidation
-Hydrolysis

(Chemical Weathering Oxygen & Acid)

Role of Oxygen/Oxidation

Chemically active oxygen from atmosphere.
• Iron oxide stains are common result.

(Chemical Weathering Oxygen & Acid)

Role of Acids

hydrogen cations replace other in minerals

Chemical Weathering Oxygen & Acid

- Carbonic acid from atmospheric CO2 dissolved in water, most important source of acid
-Sulfuric, hydrofluoric acids emitted by volcanic eruptions
-some minerals such as calcite may be dissolved by the weak acid
-Human activity such as mining and burning of fossil fuels, produces acids.

Weathering the carbon cycle and global climate

-Present in the atmosphere as carbon dioxide (CO2)gas
-CO2 combines with water to form carbonic acid
-This acid weathers rocks and leads to limestone formation in bodies of water -This product of weathering is returned to mantle by tectonic plate movement
-The CO2 is released back to atmosphere by volcanic eruptions
-The CO2 in the atmosphere impacts global temperature

Chemical Weathering of Feldspars

- most common minerals in crust.
- slightly acidic rain water attacks feldspar.
- clay minerals produced are structurally different from the feldspar rock

Chemical weathering of other minerals

-similar to feldspars, creates clay minerals and dissolved ions
-More complex silicate bonds lead to lower weathering susceptibility

weathering products

-soil is one of the most important products of weathering
-Feldspar and ferromagnesian mineral weather into clay releasing ions in solution
-quartz weathers into sand and silt to ions released

Weathering Products

SOILS ARE OFTEN DEFINED IN THESE TERMS AS "DYNAMIC NATURAL BODIES'' HAVING PROPERTIES DERIVED FROM THE COMBINED EFFECTS OF CLIMATE AND BIOTIC ACTIVITIES

SOILS ARE OFTEN DEFINED IN THESE TERMS AS "DYNAMIC NATURAL BODIES'' HAVING PROPERTIES DERIVED FROM THE COMBINED EFFECTS OF CLIMATE AND BIOTIC ACTIVITIES

Soil

a layer of weathered, unconsolidated material on top of bedrock capable of supporting plant growth and the common soil constituents (clay minerals, organic matter, water, and quartz)

Pedon

a 3-D unit that represents the entire soil body and is the smallest volume of soil that shows all the characteristics/properties of a particular soil
-typically represent 10-100 sq ft of surface area

Polypedon

a group of pedons

Horizonation

process of formation of soil horizons
-results from differential gains, losses,tranformations, and translocations occurring over a period of time

Soil Horizons

O horizon-uppermost layer, organic material

A horizon-dark-colored, rich in organic matter and high in biological activity

E horizon-zone of leaching; fine grained material removed by percolating water

B horizon- zone of accumulation; clays and iron oxides leached down from above; formation of hard pan in wet climates

C horizon- partially weathered bedrock

Soil Formation

Most soils are derived from sediments that are transported by water, ice, and wind and gravity
General Order:
Rock(at a topographic site)-->PM
PM+Biotic Factors+Time-->Soil

factors of soil formation Hint:5

1) Parent material (PM)

2) Climate:rainfall and temperature result in weathering

3) Living Organisms ( specifically vegetation & human activity)

4) Topography/slope

5) Time

(Factors of Soil Formation)

Parent material(PM)

-Nature of parent material
A. Texture and Structure
B. Chemical and Mineralogical Composition
-PM includes glacial deposits, alluvial and marine deposits, residual minerals and rocks

(Factors of Soil Formation)

Climate

rainfall and temperature result in weathering

(Factors of Soil Formation)

Living Organisms

-The type of vegetation affects the soil color and organic matter content, especially in the A horizon
-Trees contribute less organic matter to the soil each year when compared to grasslands.

(Factors of Soil Formation)

Topography *3 parts*

-Elevation-the height above mean sea level
- Slope- the tilt or inclination of the land
- Aspect-the direction the slope is facing

Soils on a steep slope

-when lopes increase soils become shallower, thinner, and have fewer horizons
Why:THE STEEPER THE SLOPE THE GREATER THE RUNOFF AND THE GREATER THE EROSION

(factors of soil formation)

Time

Important in soil formation because it determines the degree to which the other soil forming factors express themselves

Old soils

Soils that experienced intense weathering of parent material in the presence of biotic factors
-well developed profiles
-contains A,E,B horizons

Young Soils

Have weakly developed horizons and may often lack E &B horizons
-lacks horizon development

THE O AND A HORIZONS ARE THE MOST VALUABLE AND THE MOST VULNERABLE TO EROSION

THE O AND A HORIZONS ARE THE MOST VALUABLE AND THE MOST VULNERABLE TO EROSION

How Soil Erodes

-Soil particles are small and are therefore easily eroded (carried away) by water and wind
- water erosion is the most significant type; wind erosion is generally less signifigant.

Rates of Erosion

soil characteristics, climate, slope, vegetation

Consequences of Erosion

- Removal of an essential resource
- Sedimentation of water bodies

Categories of the System(divisions)

1) Orders
2) Suborder
3) Great group
4) Subgroups
5) Families
6) Series
-Grouped based on characteristics of the horizons in the soil profile

Soil orders

1) Entisol (ent)
2) Inceptisol (ept)
3) Spodosol (od)
4) Alfisol (alf)
5) Aridisol (id)
6)Mollisol (oll)
7) Ultisol (ult)
8) Vertisol (ert)
9) Histosol (ist)
10) Oxisol (ox)
11) Andisol (and)
12) Gelisol (el)

World Soil Orders

Entisols (ent)

soils of recent origin an not developed

-occurring as unconsolidated parent materials (sediments and rocks) with no horizons except an A horizon

-found in diverse environments, in steep, rocky settings

-most diverse and most common soils

Utisol

-high contents of aluminum and iron oxides
- soils in wet climates of Southeastern U.S. and Europe
-soil tends to be thick

Oxisols (ox)

-In tropical regions (high temp abundant rainfall)
-soil tends to be highly leached
- soils are called Laterities

Laterites

A red, highly leached soil type found in the tropics that is rich in oxides of iron and aluminum (Oxisols)

-generally nonproductive soil

- If aluminum content is high it forms aluminum ore bauxite

Hardpans

a soil layer that has become hardened through the cementation of soil particles with organic matter or other naturally occurring soil chemicals that root penetration and water movement are restricted

How are hardpans formed?

In wet areas when clay minerals, silica, and iron compounds accumulate in the B horizon eluviation (leaching dissolved and suspended materials) from the overlying E horizon and harden, hardpans are formed

*hardpans are difficult to penetrate and prevent root development*

Leaching

removal of dissolved materials from soil by water moving downwards

Eluviation

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

aridisols

-arid dry climate in western U.S.

-soils tend to have very thin and have very little organic matter

-water in this soils moves upward and contains high levels of salt

-when water beneath the surface evaporates it precipitates and leaves salt within the soil

-very alkaline

-some of these soils build up high levels of calcium and are called pedocals

Pedocals

Soils formed in dry and temperate climate; rich in calcium carbonate

Afisols

In the temperate areas of the U.S with moderate rainfall, soils which form have moderate levels of iron and aluminum and called pedalfers.

Pedalfers

Soils formed in humid, temperate climate; rich in iron-oxide and aluminium oxide; reddish-brown color