3/22/2014
Weathering and Erosion
Outline for next couple weeks
Earth’s subsystems or “cycles”
Weathering
(breakdown of rock)
Mechanical: physical breakdown (disintegration)
of rocks and minerals
Chemical: chemical alteration (decomposition) of
rocks and minerals
Erosion
The physical removal and transport of material by
agents such as water, wind, and ice.
1. Weathering and Erosion
2. Surface water systems (rivers)
3. Groundwater systems
4. Groundwater in-class activity (grade)
Weathering is the disintegration or
decomposition of a rock
Mass Wasting
(special case of erosion)
Downslope movement of rock, regolith, and soil
under the direct influence of gravity. Does not
require medium such as water or wind - just
gravity-driven.
Erosion is the removal and transport of
the decomposed rock that results from
weathering
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3/22/2014
Weathering comes in two varieties:
H.Rae online blog: weathered cobbles, Eel River, Mendocino, CA
Mechanical weathering
Chemical weathering
Freeze/Thaw (frost wedging) - water expands by 9%
when it freezes
Unloading-related expansion - pressure release;
exfoliation
Thermal expansion – temperature-related expansion
and contraction of minerals creates stress in rocks
Biological activity - tree roots and micro-organisms
Mechanical (physical)
weathering
Mechanical abrasion - grain-to-grain contact during
transport
Acid mine drainage
Frost wedging and jointcontrolled weathering
Biological Activity
(example: roots)
Unloading-related expansion - pressure
release; exfoliation
Weathering comes in two varieties:
H.Rae online blog: weathered cobbles, Eel River, Mendocino, CA
Chemical weathering
Mechanical (physical)
weathering
Acid mine drainage
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3/22/2014
Chemical weathering processes
can chemically alter to form a
completely new mineral
Hydrolysis – Oxidation – Dissolution
Feldspar
hydration
weathers to clay
(not all silicates weather to clay)
oxidation
Mafic rocks
oxidation
dissolution
Carbonate rocks
weather to iron oxide
totally removed
The minerals can actually dissolve . . .
hydrolysis
Mafic (Fe & Mg-rich) minerals are more
susceptible to chemical weathering than are
felsic (Si-rich) minerals
Calcite is especially prone
to dissolution
Quartz is resistant to chemical weathering,
forming sand, but all other common silicate
minerals break down chemically to form clay
minerals, the stuff of mud
Because chemical reactions can occur only
on rocks exposed to water, joints are
particularly important in accelerating this
process
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3/22/2014
Weathering
(breakdown of rock)
Wind
erosion
Mechanical: physical breakdown (disintegration)
of rocks and minerals
Chemical: chemical alteration (decomposition) of
rocks and minerals
Erosion
The physical removal of material by agents such as
water, wind, and ice.
Mass Wasting
(special case of erosion)
Water erosion
Downslope movement of rock, regolith, and soil
under the direct influence of gravity. Does not
require medium such as water or wind - just
gravity-driven.
Glacial erosion
Clicker Question #1
A flash flood removed the material
from underneath this road. This is an
example of:
Jamestown, CO during 2013 flood
A. Mechanical
weathering
B. Chemical
weathering
C. Erosion
Clicker Question #2
The dark rock in the center of the photo is a
remnant of what the entire rock once was.
The red/white material surrounding it is
much softer than the original rock. This
surrounding material represents:
A. Mechanical
Weathering
B. Chemical
Weathering
C. Erosion
Weathering
(breakdown of rock)
Mechanical: physical breakdown (disintegration)
of rocks and minerals
Chemical: chemical alteration (decomposition) of
rocks and minerals
Erosion
The physical removal of material by agents such as
water, wind, and ice.
Mass Wasting
(special case of erosion)
Downslope movement of rock, regolith, and soil
under the direct influence of gravity. Does not
require medium such as water or wind - just
gravity-driven.
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3/22/2014
I-70, Western Colorado, May 2003
Can be rapid
(meters/sec)
Rockfall
in Zion
National
Park
Face of “Old Man in the Mountain”,
New Hampshire, fell off in 2003
Figure 6.A
Slides
movement of
material along
a planar failure
surfaces
Slides
movement of material
along a planar
failure surfaces
Rockslide along I-40 in Pigeon
River Gorge, NC, July 1997
Weathered shale
forms rubble at base
of cliff
Bedding plane
Slides on Fountain Formation Sandstone,
Boulder, CO, during 2013 floods
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3/22/2014
Mass Movement Depends on
Nature of Material
Weathered shale
forms rubble at base
of cliff
Angle of Repose
Angle of Repose:
the maximum angle at which a pile of unconsolidated particles
can rest
Which house is more likely to slide into the
valley?
Factors affecting mass
movement
Steepness of slope
Slope composition
Water
Vegetation
Triggers (floods, earthquakes, etc.)
a. The blue one on the left b. The pink one on the right
c. Equal chance
d. Neither will
d. Don’t know
Why?
Identify features in this landscape that represent
1) Erosion (what medium?)
2) Mass wasting
3) Chemical weathering
a.
b.
c.
d.
e.
Because it is close to the valley bottom
Because the bedding planes make sliding easier
the hill it sits on is steeper
b and c
Because it is pink
Mudslide in Boulder
Canyon Sep 2013
https://www.youtube.com/watch?v=
eL06PWjOFxo
Next time: Surface Water
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