Weathering
I. Definitions:
the process by which rocks and
minerals break down at or near
the Earth’s surface
Weathering:
Produces soil, releases the
minerals and elements that we
need in our food
Breaks down everything we build.
Erosion: the process by which
moving water, wind, ice, or
gravity carries pieces of
weathered rock away from the
bedrock and deposits them
somewhere else
Weathering
I. Definitions:
Weathering: the process by
which rocks and minerals
break down at or near the
Earth’s surface
Work together to create
sediment which is the raw
material for sedimentary
rocks.
Sediment: unconsolidated
particles created by the
weathering and erosion of
rock, precipitated from
water, or secretions of
organisms.
Weathering
Weathering II. Mechanical Weathering: breaks a mineral or rock into
smaller pieces without changing their chemical makeup
Creates more surface area.
Weathering
II. Mechanical Weathering:
breaks a mineral or rock into
smaller pieces without
changing their chemical
makeup
Creates more surface area.
A. Frost Wedging: one of the
most effective types of
weathering in places where
surface water is abundant
Temperatures fluctuate
around freezing (0C)
Weathering
II. Mechanical Weathering—
breaks a mineral or rock into smaller
pieces without changing their
chemical makeup—creates more
surface area.
A. Frost Wedging
Water expands when it freezes (up
to 9%), e.g., water bottle example
When water gets into cracks and
freezes, it exerts enough force on
the rock to break it.
Enlarges cracks, loosens rock and
may dislodge it.
Top freezes first (in contact with air),
so as water below freezes it can’t
expand up, it expands to the sides
Weathering
II. Mechanical Weathering
B. Crystal Growth: salt
Water may get into crack then
evaporate—growing crystals put
pressure on rock, pushing them
apart.
Important process on coastal cliffs
C. Thermal Expansion &
Contraction: fires or in deserts
Heat causes a mineral’s structure to
actually expand—different
minerals expand by different
amounts
Helps weaken mineral bonds.
Poor conductors of heat, so only
outer portion expands and flakes
break off the surface
Weathering
II. Mechanical Weathering
D. Exfoliation
When land above large
igneous intrusions is eroded
away, the overlying pressure
is reduced so the rock
expands
Expansion is parallel to the
exposed surface
Fractures into sheets parallel to
exposed
In mines, slabs from walls of newly cut tunnels
have been known to explode off the walls
Sheets may break free and fall
from the sloping surface of
the exposed rock
Weathering
II. Mechanical Weathering
D. Exfoliation
When land above large igneous
intrusions is eroded away, the
overlying pressure is reduced
so the rock expands
Expansion is parallel to the
exposed surface
Fractures into sheets parallel to
exposed
Sheets may break free and fall
from the sloping surface of
the exposed rock
Weathering
II. Mechanical Weathering
E. Biological
Roots grow into fractures, as the
plant grows, the roots expand, put
force on the rock and expand the
cracks
Weathering
II. Mechanical Weathering
F. Abrasion
1. Desert wind sand-blasting
Wind blown particles collide into
each other and rocks on ground,
grind away & break off pieces
2. Glaciers scraping
Rocks in base of glaciers scrap
against underlying rock, grind
away bedrock and themselves
3. Streams scraping
Sediment transported by streams
scrapes against the bottom of the
stream bed
Weathering
II. Mechanical Weathering
Angular Boulders
decomposes and rounds
Weathering
III. Chemical Weathering:
Alters the composition of rocks
and minerals, usually through
chemical reactions involving
water
Water is the most important
factor controlling the rate of
chemical weathering!
A. Dissolution: dissolving of ions
1. water dissolves ions: water is
polar so it can dissolve ions
(NaCl) and carry them away
a. in solution, ions dissolved in
water
b. precipitated, when the water
leaves the ions behind to form
solid compounds
Weathering
III. Chemical Weathering:
Alters the composition of rocks
and minerals, usually through
chemical reactions involving
water
Water is the most important
factor controlling the rate of
chemical weathering!
A. Dissolution: dissolving of ions
1. water dissolves ions: water is
polar so it can dissolve ions
(NaCl) and carry them away
Weathered and
unweathered limestone
Weathering
III. Chemical Weathering:
Alters the composition of rocks
and minerals, usually through
chemical reactions involving
water
2. Water may react with other
compounds in the environment
to form acids that then
dissolve rock
a. H2O + CO2  H2CO3
carbonic acid
b. CaCO3 + H2CO3 
Ca+2 + 2 HCO3
bicarbonate ions
c. ions then carried away in water
and they leave holes in the
rock
d. creates caves: water in cracks,
cracks grow
Weathering
III. Chemical Weathering:
Alters the composition of rocks
and minerals, usually through
chemical reactions involving
water
B. Oxidation—forms rust
Positive ions in a mineral combine
with oxygen to form an oxide
Example: iron in a mineral
combines with oxygen in the
atmosphere
4Fe+3+3O2  2Fe2O3 (hematite
= rust)
Weathering
III. Chemical Weathering:
C. Hydrolysis: ions from water (OHor H+) replace other ions in minerals
to form new minerals
Feldspars (Na-plag, Ca-plag, K-spar)
60% of minerals in crust = clay
Example:
2 KAlSi3O8 + 2H+ 9H2O 
Al2Si2O5(OH)4 + 4 H4SiO4 + 2K+
K-spar + clay (kaolinite) + silicic acid
In solution
Silicic acid comes into play when
creating (cementing) sedimentary
rocks
Potassium transported by water =
nutrient for plants
Clays accumulate at surface of earth as
soils or wash out to sea as mud
Weathering
IV. Factors affecting weathering
A. Climate: water drives all chemical
weathering
1. wet  more chemical weathering
2. hot (dry)  more mechanical
weathering (heat helps break bonds)
B. Organisms: burrow and churn up
the surface exposing unweathered
minerals to the atmosphere
C. Time: more time = more weathering
D. Composition of minerals: some
minerals more resistant to weathering
than others
Early formed minerals (high temp)
weather faster
Later formed minerals (lower temps) are
more stable near Earth’s surface
Weathering
1. What are the two types of weathering?
Weathering
Weathering