Weathering and Soils

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

Rock Cycle

The Ideal Rock Cycle

Anything can happen in the Rock Cycle


Breakdown of Rock near the Surface Due to Surface Processes


Movement of Large Amounts of Material Downhill under Gravity

Surface Area and Weathering

Imagine a cube of rock one meter on a side:

surface-volume ratio


  • 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:

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

surface-volume ratio 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

Soil Formation

Young Soils

Strongest Influence Is Parent Material

Mature Soils

Strongest Influences:Climate, Vegetation, Drainage


Leaching from Surface

Accumulation beneath Surface

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

Soil Classification

First Systems Russian

Early US Names

Other Terms

Soil Classification

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

Multiple Objectives


Genesis & Evolution



Varied Bases for Classification

"The 7th Approximation"

US Soil Conservation Service
12 Soil Orders

Little Weathered   Moderately Weathered   Highly Weathered
No B Horizon Weak B Horizon Distinct B Horizon   Very Deep Horizons
Soils Classified on Basis of Horizon Development
Moist Climate
Dry Climate Soils
Deciduous Forest
B Horizon Clay Rich
Conifer Forest
B Horizon Fe-Rich
Prairie Soils
Often Loess
Old Temperate Soils
Old Tropical Soils
Soils With Special Characteristics - Lack of B Horizon Not Related to Maturity
Volcanic Ash
Very young ash soils are so distinctive they have their own soil order.
"Self-mixing" Soils
No B Horizon due to internal churning by swelling and shrinking clays
Organic Soils
No B horizon: A Horizon is organic debris resting on subsurface materials.
Permafrost Soils
No B horizon: A Horizon resting on permafrost.

Soils of the U.S.

soil map of U.S.

Weakly-Developed Soils
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.
In drier parts of the West
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
Mostly in the Midwest and West in grasslands, often on wind-blown deposits (loess).
Mostly in the Midwest and East in deciduous forest areas. Some in the West at higher elevations in deciduous forest areas.
Coniferous forest areas in northern Wisconsin, Michigan, New York and New England. Also high mountains in the Northwest with conifer forests.
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
Occur mostly in flood plains in clay deposits, and along outcrop belts of certain shales where the right clay minerals occur.
Form on volcanic deposits in the Northwest.
Cold-climate peat bogs in Minnesota and northern Michigan, coastal swamps in the Southeast.
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

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