Geology 12
Presents
Sedimentary Rocks
Weathering and Erosion
• Weathering: the physical breakdown
(disintegration) and chemical alteration
(decomposition) of minerals and rocks at
of near the Earth’s surface
• Erosion: removal of weathered material
(by water, wind, or ice)
•
•
3 Types of Weathering:
1. Physical/Mechanical Weathering:
– Physical forces break rocks into
smaller pieces without changing
chemistry
– 4 kinds
a) Frost action/ice wedging: water
seeps into cracks, freezes, expands
(9%) and pushes rock apart. Most
effective in areas where temperature
fluctuates above & below 0’C
animation link
Ice wedging
cliff
Joints = rock
fractures
Talus = debris
accumulation
3 Types of Weathering
 1.
Physical
(Mechanical)
Weathering:
–a) Ice Frost
Wedging
Ice Frost
Wedging
Talus = fallen
rock debris
Talus
cone
• b) pressure release: many minerals/rocks
are formed under immense pressure.
When they are uplifted and exposed
through erosion, the pressure is greatly
reduced. The rock expands forming layer
of fractures (joints)
Sheet joints
• Exfoliation: slabs of rock break along
sheet joints and fall off
Exfoliation dome
before
after
• Popping/rock bursts: sudden pressure
relief in deep mines
rock explodes
like a grenade injuring/killing miners
 B)
Pressure Release
Exfoliation Dome: Chief
• c) Thermal expansion & contraction:
rock is a poor heat conductor; in deserts
where the temperatures rise and fall
30’C/day, the outside of the rock heatsexpands and cools-contracts animation
day
night
+ dark minerals get hotter than white
minerals
C) Thermal Expansion and Contraction
• d) Salt crystal growth: salts in rock
crevices grow. Expand prying the rock
apart like ice wedging.
• Road salt in roads & bridges, inside of
concrete foundation.
D) Salt Crystal Growth
• 2. Biological Weathering = activities of
organisms
• a) animals: burrow, tramp, bring materials
to surface for more weathering, help
transmit gases and water to depths, etc.
Animation
• b) plants: roots wedge rocks apart
•
Animation
2. Biological Weathering
 A)
Animals:Moles
2. Biological Weathering
 A)
Animals:ants
2. Biological Weathering
 A)
Animals:clams
 B:Plants:root
wedging
 B:Plants:
root wedging
Lichens
B) Plants
Root Wedging
B) Plants
Root Wedging
• 3. Chemical Weathering: chemical
alteration of rock/parent material
• 3 kinds
• a) Solution: a solid dissolves (halite,
calcite) and is usually helped by CO2 in
water which forms carbonic acid (also in
plant roots)
• Results in hard water, caves & karst
topography (sink holes).
3. Chemical Weathering

A) Solution
Sinkhole
• b) oxidation: reacts with oxygen to form
oxides (rust).
• Ferromagnesians (olivine, pyroxene
amphibole & biotite) combine with O2 for
form hematite or limonite.
• Hi sulfur/sulfide coal
• Iron sulfides (pyrite FeS2)
acid rain/runoff
sulphuric acid + iron oxide
 B)
Oxidation
Chevy: like a rock all right!
• c) Hydrolysis: cations (metals) in minerals
are replaced by H+ from water
• Ex: Potassium (K) Fs + water
• Ex: Plagioclase Fs + water
clay + K + silica
clay + (Na, Ca) + silica
 C)
Hydrolysis
Kaolinite clay
• Terminology:
• Differential Weathering: rocks that
weather at different rates produce uneven
surfaces
Resistant cap rock
plateau
Mesa/butte
Hodo/
pinnacle rock
Differential Weathering
Hard Caprock
• Spheroidal Weathering: reactangular rocks
weather into roundish rocks because
corners are attacked on 3 sides, edges on
2 sides, but flat planes on 1 side
• Result: rounded (& smaller) rocks
Spheroidal Weathering
Resistance to Weathering
poor
good
Higher temperature minerals weather most easily (olivine,
calcium-rich plagioclase)
Lower temperature minerals weather least (quartz)
That’s all folks!
 Do
Ws 5.1
+ look at Unit 1 Review WS