This document discusses two main types of weathering - mechanical and chemical weathering. Mechanical weathering breaks rock down physically into smaller pieces without changing the chemical composition, such as through ice wedging, exfoliation, thermal expansion, and biotic processes. Chemical weathering breaks rock down through chemical reactions that alter the chemical composition of minerals, such as oxidation and carbonation which involve reactions with oxygen and carbon dioxide respectively. Both types of weathering are the first step in forming soil and sedimentary rock.

1. Weathering
Mechanical
and
Chemical

2. What Caused This?

3. What is Weathering?
• First step to forming soil and
sedimentary rock
– Breakdown of rock into smaller pieces
– The smaller pieces do not move to a new
location until erosion carries them away
– Many types of weathering

4. What is Mechanical Weathering?
• Breakdown of rock into smaller pieces
without any change in the chemical
composition of its minerals
– Sometimes called “physical” weathering
– Rock is torn apart by physical force, rather
than by chemical breakdown
– Smaller pieces do not move to a new
location until erosion carries them away

5. Mechanical - Ice Wedging
• Ice Wedging
– Water fills joints of rocks and freezes
– Water expands 10% when it freezes, pushes
rock apart
– Repeated freeze and thaw cycles over the
years causes rock to break along joint

6. Mechanical - Exfoliation
• Exfoliation or unloading
– Rock breaks off into sheets along joints which
are parallel to the surface
– Caused by expansion of rock due to uplift and
removal of surface material that originally
buried the rock

7. Mechanical - Thermal
• Thermal expansion
– Repeated daily heating and cooling of rock
– Heat causes expansion; cooling causes
contraction
– Different minerals expand and contract at
different rates causing the rock to split

8. Mechanical - Biotic
• Biotic – means life
– Weathering caused by living organisms
– Plant roots act as a wedge and widen cracks
– Other causes of biotic weathering are digging
animals, microscopic plants and animals,
algae and fungi.

9. What is Chemical Weathering?
• Breakdown of rock into smaller pieces because
of change in the chemical composition of its
minerals
– Chemical reactions break down the bonds holding
the rocks together, causing them to fall apart
– Chemical weathering occurs in all types of rock
– Rock reacts with water, gases and solutions

10. Chemical - Oxidation
• Oxidation - oxygen combines with other
elements in rocks to form new types of
rock
– New substances are usually much softer than
original, easier for other forces to break rock
apart
– Causes a “rusting” of the rock, often causes a
color change in the rock

11. Chemical - Carbonation
• Carbonation – Carbon dioxide (CO2) is
dissolved in water making carbonic acid
– Weak acid is formed when CO2 in the air
mixes with rain. This is the same acid found in
soft drinks.
– Acid is too weak to harm plants and animals
but slowly causes feldspars and limestone to
decompose