Weathering and Soils

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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The Rock Cycle

The Ideal Rock Cycle

• Igneous rocks form when molten rock (magma) invades the crust.
• They break down by weathering and the breakdown products are transported by erosion.
• The breakdown products are deposited to form sedimentary rocks
• Sedimentary rocks are buried, subjected to heat, pressure, and the action of fluids
• Heat, pressure, and the action of fluids change rocks, forming metamorphic rocks.
• Metamorphic rocks may eventually be heated enough to melt, forming a new generation of igneous rocks.

Anything can happen in the Rock Cycle

• Igneous rocks can be subjected to heat, pressure, and the action of fluids to form metamorphic rocks.
• Igneous rocks can be re-melted, forming a new generation of igneous rocks.
• Sedimentary  rocks can be weathered and eroded to form a new generation of sedimentary rocks.
• Sedimentary rocks may eventually be heated enough to melt, forming a new generation of igneous rocks (although technically they'd be metamorphic by the time they melted).
• Metamorphic rocks can be weathered and eroded to form sedimentary rocks.
• Metamorphic rocks can be subjected to heat, pressure, and the action of fluids to form a new generation of metamorphic rocks.

Weathering

Breakdown of Rock near the Surface Due to Surface Processes

• Chemical Alteration (Often Addition of Water)
• • Solution & Leaching
  • Biological Action
  • Hydration
• Mechanical
• • Impact
  • Frost - Wedging
  • Plant Roots
  • Salt Crystal Growth
  • Expansion of Hydrated Minerals

Mass-Wasting

Movement of Large Amounts of Material Downhill under Gravity

• Creep
• Mudflows
• Slump
• Rockfalls
• Avalanches

Surface Area and Weathering

Imagine a cube of rock one meter on a side:

Thus: Divide the edges by 2, total area increases 2 times Divide the edges by 3, total area increases 3 times Divide the edges by n, total area increases n times For a one-meter block crushed into 0.1 mm pieces, the edge length is divided by 10,000, and the area multiplied by 10,000. 10,000 square meters is over 100,000 square feet or 2-1/2 acres.

Surface-Volume Effects

The smaller we subdivide particles, the more surface area is exposed. Things that happen at surfaces, like heat transfer or chemical reactions, happen faster. Some common applications of this fact:

• Crushed ice cools faster than an ice cube
• Granulated sugar dissolves faster than a large lump
• A carburetor or fuel injector sprays a fine mist of gasoline into an engine cylinder

A more serious example:

Flour in a sack in your kitchen is about as inert as a material can get. Grain dust (which is what flour is) dispersed in the air is highly combustible, even explosive. Every year a couple of dozen people in the U.S. die in grain elevator explosions caused by suspended dust. Coal dust in mines can be even deadlier.

How Surface-Volume Ratio Affects Weathering

Hence: corners of blocks weather fastest, then edges, then faces. The interior is immune as long as the surrounding material is intact.

What Determines Soil Type

• Climate
• Vegetation
• Drainage
• Time
• Parent Material
  • Residual - Transported
  • Least Important Factor for Mature Soils

Soil Formation

Young Soils

Strongest Influence Is Parent Material

Mature Soils

Strongest Influences:Climate, Vegetation, Drainage

Processes

Leaching from Surface

• K, Mg, Na
• Ca
• Si
• Al, Fe

Accumulation beneath Surface

• Al, Fe in Humid Climates
• Ca in Arid Climates

Soil Horizons and Profiles

Soil Horizons

Layers in Soil
Not Deposited, but Zones of Chemical Action

Soil Profile

Suite of Layers at a Given Locality

Principal Soil Horizons

O - Organic (Humus) Often Absent
----------------------------------------
A - Leaching
K, Mg, Na, Clay Removed
----------------------------------------
E - Bleached Zone - Present Only in Certain Soils
----------------------------------------
B - Accumulation
Absent in Young Soils
Distinct in Old Soils
Al, Fe, Clay (Moist)
Si, Ca (Arid)
----------------------------------------
C - Parent Material

Limits of Soil Formation

• Balance Between:
  • Downward Lowering of Surface
  • Downward Migration of Horizons
• If erosion rapid or soil evolution slow, soils may never mature beyond a cetrain point.
• Extremely ancient soils may have lost everything movable

Soil Classification

First Systems Russian

• Chernozem (Cherny + Zemlya) Black + Earth
• Serozem (Sery, Gray + Zemlya)
• Podzol (Pod + Zola) Beneath + Ash

Early US Names

• Pedalfer (Al + Fe in Subsoil)
• Pedocal (Ca in Subsoil)

Other Terms

• Laterite - Intensely Weathered Tropical Soils
• Caliche - Calcite - Cemented Arid Subsoil

Soil Classification

This may be the most difficult classification problem in science because of the many factors involved.

Multiple Objectives

Scientific

Genesis & Evolution

Agricultural

• Fertility
• Most Effective Use

Engineering

• Slope Stability
• Expansion and Shrinkage
• Stability of Excavations

Varied Bases for Classification

• Parent Material
• Constituent Material
• Maturity
• Structure
• Climate & Vegetation

"The 7th Approximation"

US Soil Conservation Service
12 Soil Orders

Soils of the U.S.

Weakly-Developed Soils

Entisols

Mostly on young alluvium in the West where climate is dry and chemical weathering is slow. The large area in Nebraska is the Sand Hills, former Pleistocene sand dunes. Some in the East on very young deposits.

Aridisols

In drier parts of the West

Inceptisols

Mostly on mountain slopes where weathering and erosion are about equal. Some on young glacial deposits. Imply somewhat wetter climate than entisols.

Well-Developed Soils

Mollisols

Mostly in the Midwest and West in grasslands, often on wind-blown deposits (loess).

Alfisols

Mostly in the Midwest and East in deciduous forest areas. Some in the West at higher elevations in deciduous forest areas.

Spodosols

Coniferous forest areas in northern Wisconsin, Michigan, New York and New England. Also high mountains in the Northwest with conifer forests.

Ultisols

In the Southeast where warm climates and stable conditions allow extremely long times for soil development. A few areas in the far West. Note in the Oregon Cascades how ultisols occur at low elevations but mollisols at higher, cooler elevations. Still higher, alfisols occur. The final stage of soil evolution, oxisols, do not occur in the continental U.S. but do occur in Hawaii and Puerto Rico.

Soils With Special Characteristics

Vertisols

Occur mostly in flood plains in clay deposits, and along outcrop belts of certain shales where the right clay minerals occur.

Andisols

Form on volcanic deposits in the Northwest.

Histosols

Cold-climate peat bogs in Minnesota and northern Michigan, coastal swamps in the Southeast.

Gelisols

Abundant in Alaska, not found in the continental U.S. except perhaps in a few very high mountain areas.

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Created February 3, 1997, Last Update 23 January 2001

Not an Official UW-Green Bay Site