Weathering
Weathering Lectures
1. Weathering vs. Erosion
2. Joints: Setting the Stages
3.
4.
5.
6.
Physical (Mechanical) Weathering
Chemical Weathering
Products and Forms Made by Weathering
Weathering Landscapes
1. Weathering vs. Erosion
Common Error Made in K-12 Earth
Science Teaching: these are synonyms
No!
Weathering – chemically dissolving candy
or physically crunching candy –
breakdown in place
Erosion – moving pieces (dissolved or as
fragments)
1st - Weathering =
decay in place
2nd - Detach = break
off
3rd Erode = move
1st – decayed
2nd – detached
3rd – eroded
The balance between weathering
and erosion defines the landscape
In deserts – transport is faster
Humans can upset
the balance and
accelerate
erosion. So
when transport
(detachment and
erosion)
becomes faster
than weathering,
landscapes are
not sustainable.
s
2. Joints: Setting the Stage
Joints – fractures (that allow water to
penetrate and weather the rock)
Not what you are thinking!
2. Joints: Setting the Stage
Joints dependent on lithification
(sedimentary & foliated metamorphic
rocks) – where fractures occur along
bedding planes or along foliations
Joints independent of lithification (all rock
types) – all other fractures
Joints dependent on Lithification
• Sedimentary
Rocks – where
fractures occur
along bedding
planes
Joints dependent on lithification
• Can also be
Foliated
Metamorphic
Rocks – where
fractures occur
along bedding
planes or along
foliations
Joints dependent of lithification
Rocks can be cracked by cooling, leading
To columnar jointing
Columnar
Jointing
From contraction
after lava flow cools
Joints independent of lithification
Rocks can be
cracked by
Tectonic
Stresses
or faulting/folding
stresses the rock
Joints can even fracture
sedimentary strata from
regional tectonic
pressures
Sandstone
fractured
across bedding
planes
Can have both together
• Along bedding (& foliation) weaknesses
and cutting across
3. Physical (Mechanical) Weathering
•
•
•
•
•
•
•
Abrasion
Frost weathering
Pressure release weathering
Salt Weathering
Thermal Expansion/Contraction
Wetting/Drying
Root pressure
Abrasion
•
Often see abrasion by sediment or soil
transport by water over rock… this both
weathers (decays) and erodes by abrasion
(like sand paper)
Anthropogenic Abrasion
•
carving
chiseling,
bullet
impacts
Frost Weathering
Check out these links
Click here to see a
fun movie clip
about frost
weathering
Click here to see a 360
degree view of fractured
rock in Antarctica by frost
weathering
Breaks rock along small fractures
Breaks rock along large fractures
& produces jagged alpine topography
Aesthetic, so used in commercials
Pressure Release
pressure release shells
Sometimes called “exfoliation” (like exfoliating skin) – incorrect usage
Erosion
removes
overburden &
shells pop off
as pressure is
released
Glacial Erosion Great Way to
Generate Pressure Release
Salt Weathering
Salt is common
in deserts
Salt is common
along coasts
Mechanics of Salt Weathering
•
Salt Crystal Growth: Extreme
pressures in cracks and rock pores are
caused by salt crystal growth from
solution. There are varying causes of
and extents to which salt growth occurs.
•
Thermal Expansion: During
temperature fluctuations, salts
trapped in pores may expand to a
greater degree than the
surrounding rock minerals.
•
Hydration: The hydration of various salts
causes expansion & contraction, pushing
apart the silicate host minerals
Caverns (tafoni)
Base of rock
notching (basal
weathering)
Wedging
tafoni – larger
cavernous forms
(Goudie and Viles, 1997:168)
Alveoli – smaller cavernous forms:
alveolization is the process
Alveolization
can occur
from more
than salt
weathering,
including
insects & bird
activity!
Weathering along bases of rocks:
basal notches
Salt Efflorescence
Salt subflorescence
(salt under the surface)
Salt subflorescence
(salt under the surface – usually see when
look at undersides of eroded flakes)
fissuresol wedging &
calcrete wedging
Order from innermost crack out:
calcrete – white calcium carbonate
iron film – orange clays and iron
rock varnish- outer edge black clays & manganese/Iron
Wedging can split small rocks
and giant boulders – carbonate & dust washes
away – leaving iron film and rock varnish
Thermal Expansion/Contraction
•
Thermal Expansion/Contraction
A bit of moisture & sudden heat
makes the rock pop –
typically flakes (fingernail thick)
but sometimes scales (thicker)
“flaking”
Wyoming
rock art
site
Wetting/Drying
Vermillion Cliffs
Root Pressure creates
weaknesses
Simple
Model of
the
Process
• More than
roots can
affect
rock.
Plants
growing
next to
rock art
can burn
and wind
movement
can
abrade
Imagery seen in this presentation is courtesy of
ASU faculty, Paradise Valley and Mesa CC
faculty, students and colleagues in other
academic units, individual illustrations in
scholarly journals such as Science and Nature,
scholarly societies such as the Association of
American Geographers, city, state governments,
other countries government websites and U.S.
government agencies such as NASA, USGS,
NRCS, Library of Congress, U.S. Fish and
Wildlife Service USAID and NOAA.