2. Objectives
• Define weathering
• Identify causes of weathering
– Physical or Mechanical Weathering
– Chemical Weathering
– Biological Weathering
•Discuss soil formation
3. Weathering
Weathering is a set of
physical, chemical, and
biological processes
that alter the physical
and chemical state of
rocks and soil at or near
the earth's surface. http://www.uh.edu/~jbutler/physical/chapter6.html
4. Effects of Weathering
• Loss of atoms and molecules from
weathered surfaces.
• Addition of specific atoms to the
weathered surface.
• Breakdown of one mass into two or more
masses.
5. Chemical Weathering
• Involves the alteration of the chemical
composition of weathered material.
There are different process that result in
chemical weathering including:
– Hydrolysis
– Oxidation
– Carbonation
6. Hydrolysis
Chemical weathering is a process that involves the reaction
between mineral ions and the ions of water (OH- and H+),
and results in the decomposition of the rock surface
by forming new compounds, and by increasing the pH of the
solution through the release of the hydroxide ions.
- Silicate minerals weather by hydrolysis to form clay.
7. Oxidation
Chemical attachment of free oxygen to other
elements and compounds.
Iron-bearing silicate minerals that
undergo oxidation include the
following:
– pyroxene
– amphibole
– biotite
– olivine
Rocks that contain these elements weather by oxidation into
an orange color.
http://wwwrses.anu.edu.au/research/annrep/ar2006/ee/index.php?p=pillans
8. Carbonation
Dissolving of calcium
carbonate (limestone) in
acidic groundwater
- similar to hydrolysis but all the
products are ionic and there is
no residue
- produces bicarbonate (HCO3),
a major part of the dissolved
load of most rivers
- the carbonation of limestone
results in karst topography:
caves, sinkholes, etc.
http://www.khaolaklanddiscovery.com/_images/tours/khao_sok/
old_coral_big.jpg
9. Physical Weathering
• Breakdown of mineral or rock material by entirely
mechanical methods brought about a variety of reasons.
– Frost Wedging
– Exfoliation or Unloading
– Thermal Expansion
– Plant Wedging
http://members.aol.com/rhaberlin/mwstyg.htm
10. Frost Wedging
• Mechanical disintegration, splitting
or break up of rock by the
pressure of water freezing in
cracks, crevices, pores, etc.
• Caused by repeated freeze-thaw
cycle of water in extreme
climates. http://regentsprep.org/Regents/earthsci/units/weathering/frostwedging.gif
11. Exfoliation or Unloading
• Expansion of rock
caused by lifting and
erosion
• Rock breaks off into
sheets along joints
that parallel surface. http://facstaff.gpc.edu/~pgore/Earth&Space/condestr.html
12. Thermal Expansion
• Repeated cooling and
heating of rock.
• Heat caused rock to
expand and cooling
causes rock to contract
• Looks very similar to frost
wedging and salt crystal
growth, but typically
occurs in climates of
extreme weather
changes.
Thermal shattering of sandstone by forest fire.
http://w3.salemstate.edu/~lhanson/gls210/gls210_weath1.htm
13. Plant Wedging
• Plant roots can wedge into
cracks in rocks and produce
enough pressure to split them
• Chemicals produced by the
roots of plants help in the
chemical weathering, as
enzymatic activity digest the
rock as the plant searches for
nutrients in the rock.
http://facstaff.gpc.edu/~pgore/Earth&Space/condestr.html
14. Biological Weathering
• Animals and Plants can
assist in breaking down
rocks into sediment and
soil.
Biological Weathering can be
caused by:
• Roots of trees and other
plants
• Lichens, fungi, and other
microorganisms.
• animals
http://w3.salemstate.edu/~lhanson/gls210/gls210_weath1.htm
15. Soil Formation
• Soil is formed from the
weathering of rocks and
minerals.
• Surface rock breaks down
by weathering and is mixed
with organic material. Over
time it creates a thin layer of
soil.
lh3.google.com/.../s800/IMG_1466-1.jpg
16. Five Soil Forming Factors
1. Parent Material- The primary material from which rock
is formed.
2. Climate- Weathering forces including heat, rain, ice
snow, wind, etc.
3. Organism- all animals living in or on the soil. Ex:
Animals living in the soil will affect how soil is moved
around and decomposition of waste materials.
4. Topography- the location of the soil on a landscape.
Ex: Soil at the bottom of a hill will get more water than
that on the slope.
5. Time- The formation of soil takes years.
17. Topography
Image courtesy of the United States Department of Agriculture, Soil
Conservation Service
http://soil.gsfc.nasa.gov/soilform/parmat.htm
