Topic 19: HOW SOILS ARE MADE: Rock Weathering

Weathering - the group of destructive processes that change 
the physical and chemical character of rocks at or near the earth's surface.  
Weathering is distinguished from Erosion which is the 
physical removal of rock particles by an agent such as streams or 
glaciers.

Mechanical Weathering - physical disintegration of rocks.

Frost Action - water can collect in cracks in rock, expand upon freezing,
and can thus break the rock apart.

Abrasion - grinding of rock by friction wear and impact during transportation 
in streams or under glaciers.

Stress release - removal of great weight allows rock expansion, which 
causes sheet joints to form in elastically-inhomogeneous rock.

Temperature -  constant diurnal and annual heating and cooling causes 
exfoliation which is similar to peeling of layers from an onion.

Chemical Weathering - the transformation of rocks and minerals exposed 
to water and air into new chemical products.

Role of Oxygen - rusting of an iron nail exposed to dampness and air 
is a simple example of chemical weathering.  Iron oxide (hematite) is 
a product of weathering.  If water is present iron oxide combines with 
water to form limonite. product of numerous minerals containing iron.

Role of Acid - Natural acids disrupts the atomic structure of many minerals. 
The source of natural acid for rock weathering is dissolved carbon dioxide 
in water.  Acid causes feldspars to alter to clay.

Solution weathering - some minerals such as calcite will dissolve in water.

Soil - the layer of weathered, unconsolidated material on top of bed rock.

Soil Horizons -layers of soil are distinguishable by characteristic physical 
or chemical properties.

A horizon - the zone of leaching characterized by the downward movement
 of water.  In a humid (wet) climate, clay minerals, iron oxides, and 
dissolved calcite are most typically leached downward.

B horizon - the zone of accumulation of material leached downward from 
the A horizon.  This layer is often quite clayey and stained red or brown 
by hematite and limonite.

C horizon - incompletely weathered parent material.

Pedalfer - are marked by effective downward leaching due to high rainfall 
and to the acids produced by the decay of abundant humus.  These soils 
contain a high content of aluminum and iron.

Pedocal - tend to be thin and characterized by little leaching, scant humus, 
and upward movement of soil water beneath the land surface.  Evaporation 
of water beneath the land surface can cause the precipitation of salts 
within the soil and formation of alkali soils.

Caliche - a hardpan soil which forms by calcium carbonate and other salts 
that precipitate in the soil as water evaporates.

Laterite - highly leached soils forming in tropical regions where 
temperatures are high and rainfall is abundant.  These soils are almost 
entirely iron and aluminum oxides.