Download Ch 16 Weathering, Erosion and mass wasting

Ch 16
1.
2.
3.
4.
5.
Weathering, Erosion and
mass wasting
Rock Cycle, weathering, and erosion
Mechanical weathering
Chemical Weathering
Soil
Mass wasting
Rocks are a mix of
minerals
A rock is classified by its:
1) TEXTURE
 Size of mineral crystals
2) COMPOSITION
 Minerals present
3) Process of formation
What type of rocks are made from
previously weathered rocks?
a.
b.
c.
d.
Igneous
Metamorphic
Sedimentary
B and C
There are 3 types of rocks:
divided according to process of formation
Igneous: crystals solidify from melted rocks
Metamorphic: reactions of minerals from pre-existing rocks
changes in temperature or pressure
Sedimentary: derived from physical disintegration or
chemical dissolution of pre-existing rocks
Ch 16 Weathering, erosion, and
mass wasting
Weathering: Changes that occur in sediments and
rocks when exposed at the earth’s surface
– Changes are physical and chemical
– Atmosphere, hydrosphere, and biosphere all
contribute
Weathering, erosion and soil
Weathering is extremely boring from
a spectator point of view
But the product of weathering and
erosion are spectacular
Tim Davis/Photo Researchers
Weathering is critical for LIFE ON EARTH
Only small fraction of
Earth’s land surface
has bedrock
Most of Earth’s land surface
is covered by SOIL
 the product of weathering
What is soil made of?
a.
b.
c.
d.
Mineral matter, from weathered rocks
Gases, from the atmosphere
Water
All of the above
Weathering is a fundamental component of
the ROCK CYCLE
Weathering and
Only small fraction of erosion
Earth’s land surface
has bedrock
Rocks
Sediments
Lithification
and diagensis
Weathering and erosion operate together
EROSION: incorporation and transport of material
by water, wind, ice, etc
 weathered rock, sediment, or soil
Factors Controlling Rates of Weathering
SLOW
Rock type
Minerals present
Rock structure
High SiO2
climate
Cold
dry
Vegetation and
animals
Soil
limited
Massive
None
Bare rock
FAST
Low SiO2
and carbonates
fractures
warm
wet
extensive
soil cover
Joint-controlled Weathering
Jeff Foott/DRK
Fig. 6.11
How do you calculate a weathering
or erosion rate
Distance
= rate
Only small
fraction
of * time
Earth’s land surface
200 km = 100 km/hour *erosion
2 hours
has bedrock
Rate = distance / time
If I run a 10 km in 0.75 hours, at what
rate am I running?
weathering
How do you
calculate an
erosion rate
Only small fraction of
Earth’s land surface
has bedrock
weathering
erosion
Rate = distance / time
?
= 20 meters / 5 m.y.
Rate = 4 meters per m.y.
Time = distance / rate
Weathering and erosion operate together
Weathering: Changes that
occur in sediments and rocks
when exposed at the earth’s
surface
EROSION: incorporation and
transport of material by water,
wind, ice, etc
 typically weathered rock
Two main types of weathering
Mechanical weathering: physical forces break rock into smaller
pieces
 no change in chemical composition
Chemical weathering: chemical transformation of rock into one
or more new components
Mechanical weathering
1. Frost — water expands by 9% when it
freezes: freezing expands cracks in rocks
2. Thermal expansion — differential thermal
expansion of minerals creates stress in rocks
3. Organic activity — tree roots, animal
burrowing, insects, micro-organisms
4. Unloading — decrease in pressure, rocks
expand
Boulder Fractured by Frost Action
Michael Hambrey
Fig. 6.13
Role of Organisms in
Weathering
Peter Kresam
Fig. 6.12
Mechanical Weathering Changes
the Surface to Volume Ratio
Fig. 6.5
Chemical weathering
• minerals formed in the earth’s
interior are not stable at the surface.
• For silicate minerals, stability varies
with Si02 content
– High Si02 most stable
– Low Si02 least stable
When you get to the beach…….
nothing left but quartz
Chemical weathering
1. Solution or Dissolution
2. Oxidation
3. Hydrolysis
1. Weathering by solution
(i.e., in water)
•Breakup of minerals into ions in solution
•NaCl (halite) is simple example
•calcite (limestone) common carbonate= CaCO3
CaCO3 + H2CO3 = Ca2+ + 2HCO3-
Rainwater is
acidic
(pH = ?)
In soil, organic
activity introduces
additional acids
Fig. 6.6
Limestone (CaCO3) weathers faster than granite
due to dissolution
Weathered
Limestone
Ric Ergenbright
Fig. 6.10
2. Oxidation provide Color
to the Desert Landscape
4Fe + 3O2  2Fe2O3
Betty Crowell
Fig. 6.9
What type of weathering
produces clay minerals?
a. mechanical
b. oxidation
c. Dissolution
d. hydrolysis
3. Hydrolysis
Hydrolysis: reaction between mineral elements and
the hydrogen ion (H+) of water
•Primary mechanism for weathering silicates
•acids in water are critical
Analogy of hydrolysis: making coffee
fresh grounds + water = coffee + residue
(a solution)
K-feldspar + water = K+ + kaolinite
(a clay mineral)
Weathering and Making coffee
Fig. 6.4
Weathering and soil
Soil: Mixture of altered mineral matter, organic
matter, air, and soil water
Regolith: a layer of broken pieces of rock and
slightly altered rock
Bedrock: unaltered rock of any kind
Soil Profile: vertical differences in
soil properties called horizons
There are many, many
different types of soils
why?
Called “parent material”
Fig. 6.17
Weathering and Soil Formation is complex
Fig. 6.16
Soil loss/erosion intermittent
 example, dust bowl 1930’s
MASS WASTING:
The work of gravity
Which location is at greatest
risk for debris flows?
A. Ridge top.
B. Open land.
C. Bottom of a steep
mountain channel.
D. Middle of a broad
mountain valley.
Gros Ventre Slide, Wyoming
Mt Huascaran, Peru
(before 1970)
Mt Huascaran, Peru
(after 1970)
Towns buried
by debris
avalanche
Map showing alpine
debris flows trigger by
July 25, 1999
thunderstorm, central
Front Range, Colorado
(Source: Jonathan W. Godt
and Jeffrey A. Coe U.S.
Geological Survey OpenFile Report 03-050)
I-70, Western Colorado, May 2003
Carabelleda, Venezuela, December 1999
Mass wasting and
landform development
• Mass wasting = downslope movement of
rock, regolith, & soil under the direct
influence of gravity
Mass wasting and
landform development
• Role of mass wasting
• often follows weathering
• mass wasting (and running water) produce
stream valleys
Mass Movement Depends on
Nature of Material
Angle of Repose:
the maximum angle at which a pile of unconsolidated
particles
can rest
Weathered shale
forms rubble at base
of cliff
Weathered shale
forms rubble at base
of cliff
Angle of Repose
The effect of water
on mass wasting
Mass Movement Depends on
Water Content
Surface tension in
damp sand increases
cohesion
Mass Movement Depends on
Water Content
Surface tension in
damp sand increases
cohesion
Dry sand is
bound only by
friction
Mass Movement Depends on
Water Content
Surface tension in
damp sand increases
cohesion
Dry sand is
bound only by
friction
Saturated sand
flows easily
because of water
in pores
Controls and triggers
of mass wasting
• Important factors include
• The role of water
– Diminishes inter-particle friction
– Water adds weight
Classification of
mass movement
is based on
dominant
material, fluid
content, and
velocity of
movement.
Rockfall
A very rapid mass movement in
which newly detached blocks of
rock suddenly fall from a steep
slope or cliff.
Rockfall
<- Rockfall
in Zion
National
Park
http://www.youtube.com/watch?v=fzRhLs5GkYs
Rock avalanche
Types of
Unconsolidated Mass Movement
Unconsolidated Flows
Creep
Debris slump
Debris flow
Increased
velocity
Soil Creep
The downhill movement of soil
and other debris, typically at rates
of about 1 to 10 mm/year.
Evidence of Creep
Debris flow
A fluid mass movement of rock fragments
supported by a muddy matrix. Peak speed
can be >10 m/s!
Debris flow
Debris flow videos
http://www.youtube.com/watch?v=r6Lt0oPJFwA
http://www.youtube.com/watch?v=8mKC3eID074
Central Apennines, Italy
Small debris flow
Monterey Park, greater LA
Winter 1980
Photos by Douglas M. Morton
U.S. Geological Survey
Debris flows can carry big rocks
… as in Malibu, CA, 2004
Rocky Mountain National Park
Which location is at greatest
risk for debris flows?
A. Ridge top.
B. Open land.
C. Bottom of a steep
mountain channel.
D. Middle of a broad
mountain valley.
Map showing alpine
debris flows trigger by
July 25, 1999
thunderstorm, central
Front Range, Colorado
(Source: Jonathan W. Godt
and Jeffrey A. Coe U.S.
Geological Survey OpenFile Report 03-050)
Possible Triggers for Mass Movement
• over-steepened slope:
– erosion at base
– volcanic ash
– excavation (manmade)
• increased water content:
– intense rainfall
– rising water table (e.g. behind dam)
• earthquakes
Effect of Tectonic Setting
• high relief, steep slopes
• fractured, tilted rocks
• frequent earthquakes (triggers)
Aftermath of 1999 Chi Chi earthquake, Taiwan (photo Jan 2002)
Ways to Reduce Losses Due to
Landslides Include:
• avoid construction in areas prone to
mass movement
• build in a way that does not make
naturally stable slope unstable
• engineer water drainage to prevent
strata to become water saturated and
prone to fail