Weathering and
Mass Movement
Dynamic Equilibrium Model
Uplift creates potential energy of position
(disequilibrium)
Sun provides heat energy
Hydrologic cycle provides kinetic energy
Atmosphere and crustal reactions provide
chemical energy
Landforms constantly
adjusted toward equilibrium
1. Equilibrium Stability
2. Destabilizing Event: ‘geomorphic threshold’
eg. lava flow, tectonics, heavy rainfall, forest
fire,deforestation, climate change)
3. Period of Readjustment
4. New Condition of Equilibrium Stability
Material loosened by weathering, eroded and
transported
Agents of erosion must overcome friction
before downslope movement occurs
Slopes are often convexo-concave
Convex at the top (waxing slope and free face)
Concave at the bottom (debris slope and waning
slope lead to pediment in the depositional zone)
Slope Mechanics and Form
Weathering processes disintegrate rock
into mineral particles or dissolve them
into water
Two forms:
1.
2.
Smaller fractures throughout
(large)
Joints and fractures increase
weatherable surface area
Limestone bedrock, Kansas
Photo: J.S. Aber, 1977
Weathering Factors
1. Rock Composition and Structure
Jointing increases exposed surface area
Some rocks more soluble (eg. limestone) than
others (eg. granite)
2. Wetness and Precipitation
Promotes chemical and physical weathering
3. Temperature
Promotes chemical weathering
4. Freeze-thaw cycles
Volume increase of H2O upon freezing mechanically
splits rock: humid continental, subarctic, polar and
alpine environments
Weathering Factors
5.
Hydrology (Soil water and Groundwater)
Water promotes chemical weathering within the
parent material
6.
Geographic Slope Orientation
Affects exposure to sun, wind and precipitation
7.
Vegetation
Acids from organic decay add to chemical
weathering; shields rock and soil; roots hold soil
together on steep slopes but split jointed bedrock
8.
Time
Rock broken & disintegrated: no chemical alteration
Surface area for chemical weathering increases
Freeze-thaw weathering
•H2O increases in volume by 9% upon freezing
•Repeated freezing and thawing breaks rocks apart
•Humid continental, subarctic, polar and alpine env’ts
Frost wedging pushes portions of rock apart.
The loosened, angular rock falls from cliffs in steep areas
and accumulates downslope, forming talus slopes
Talus Slopes
POTHOLES
Dry weather: moisture drawn upward to rock surfaces
Dissolved minerals crystallize.
Crystals spread mineral grains apart (esp. sandstone)
Opened spaces available to water and/or wind erosion.
Minerals absorb water and expand
Stresses rock – grains forced apart
•Overburden removed through weathering
•Pressure released - heave for millions
of years
•Layers of rock peel off in curved slabs
“pressure-release jointing”
•Exfoliation (sheeting) leaves massive, arch
and dome-shaped features
Exfoliation
Dome
Half Dome,
Yosemite
National
Park, USA
Exfoliation in Granite
Exfoliation in Granite
The decomposition of rock minerals
1. Combine with oxygen or carbon dioxide in the air
2. Dissolve or combine with water
Forms of Chemical Weathering:
Hydrolysis and dissolution
Minerals combine with water and/or carbonic acid in
a reaction to the mild acids in precipitation water
Disintegration etches/erodes/softens/removes rock
Complete dissolution: magnesium-rich olivine
Mg2SiO4 + 4H2CO3  2Mg2+ + 4HCO3- +H4SiO4
Partial dissolution: plagioclase feldspar
kaolinite clay
CaAl2Si2O8 + 2H2CO3 + H2O  Ca2+ + 2HCO3- + Al2Si2O5
Water can dissolve 57 natural elements and
many of their compounds – “universal solvent”
The H2CO3 in precipitation water reacts with rock
minerals containing Ca, Mg, K and Na
Minerals dissolved into H2O (eg. CaCO3)
Washed away in rainwater
Karst topography: sinkholes, tower karst and
stalagtites/stalagmites
Florida Sinkhole
Karst
Topography
Stalactite and
Stalagmite complex
CaCO3(s) + H2O(l) + CO2(aq) → Ca(HCO3)2(aq)
Karst and Limestone Regions
Oxidation in Rock
Oxygen oxidizes metallic
elements to form oxides
(eg. iron oxide, Fe2O3)
More susceptible to
further chemical
weathering
Urban Chemical Weathering
Any unit movement of a body of material propelled and
controlled by gravity. Slopes and gravitational stresses
are always involved
Physical and chemical weathering weaken rock near the
surface, making it susceptible to mass movement
Angle of repose:
Slope achieved at equilibrium as grains flow downslope
Driving force:
Gravitational forces. The greater the slope angle, the
greater the likelihood of mass movement.
Resisting force:
Cohesiveness and internal friction
Slope Mechanics and Form
Types of Mass Movements
1. Rockfall
rock falls through air and hits a surface
pile of irregular, broken rocks results
2. Landslides (translational or rotational)
sudden movement of cohesive mass of bedrock
or regolith
3. Debris avalanche
faster than landslide since water or ice fluidize
the debris
These three definitions
- rock, debris and soil
can be overlapping
Debris avalanche /
Landslide
4. Flows (formed due to increased moisture content)
5. Creep (persistent, gradual mass movement)
Very slow movement of individual soil particles due to
freezing and thawing, wetting and drying, temperature
changes and animal disturbance
Long after a a debris avalanche / flow
Soil Creep Effects
Effects of Lahar
(special form of earthflow)
Drainage Basin
Drainage basin/catchment/watershed:
•Defined by the ridges – every stream has a basin
•A drainage basin collects water, which is delivered to
a larger basin, creating larger streams
Continental Divide:
•The line separating subcontinental-scale watersheds
Water and sediment usually terminate in oceans
Internal drainage
Basins in which water does not terminate in an ocean
(evaporation or subsurface drainage)
http://atlas.gc.ca/site/english/maps/freshwater/distribution/drainage
Drainage Basins
Red: selected
drainage
basins for first
order streams
(collection of
red areas should
fill the yellow
area but some
streams not
represented)
Yellow:
larger drainage
basins for river
•Determined by dividing the total length of all
streams by the area of the basin
•Higher density in humid areas
Arrangement of channels is determined by:
•Slope
•Rock resistance to weathering
•Climate
•Underlying bedrock
•Subsurface hydrology
1.
Dendritic
Tree-like
Efficient: branch
length minimized
2.
Rectangular
Faulted and jointed
landscapes
Directs streams along
right angle turns
3.
Trellis: Forms where resistance of bedrock
varies or along a folded landscape
Folds create parallel large streams,
capturing runoff from smaller streams
and joining into larger rivers at right angles
Rivers may
predate
folding with
erosion
faster
than uplift
4. Radial Drainage
Streams flow from
central peak or dome
5.
Annular Drainage
Occurs in domes with
concentric patterns of
rock strata
6. Parallel drainage
Similar to dendritic, but
steep slopes cause
branches to appear
parallel to one another
7. Deranged Drainage
In areas with disrupted
surface patterns
No clear drainage geometry
Common in zones of glacial
deposition
Streamflow Terminology
Flow velocity: A measure of how fast a stream
moves downstream (v in m s-1). It depends on the
discharge, slope, size and shape of the channel.
Discharge: The amount of water flowing through
a cross section of a stream (Q in m3 s-1). Fluctuates
seasonally and diurnally Q = f (wdv)
Capacity: The amount of sediment that can be
carried by a stream (m3 s-1 or kg s-1). Capacity
increases with discharge.
Competence: The maximum particle size that can
be carried by the stream (related to flow velocity)
Fluvial Transport
Sedimentary load
Total amount of sediment carried by a stream
1. Bedload Coarse particles (eg. sand), which have high
settling velocity. Sediments are transported near the
streambed, kept loose by turbulence and particle interaction.
2. Suspended load: Particles are in the water column,
sorted by weight (larger particles near the bottom).
The higher the discharge, the higher the suspended load.
3. Washload: Fine particles with low settling velocity,
which travel at the same speed as the flow.
Almost independent of discharge.
Meandering Stream Profile
Itkillik River, Alaska
Meandering Stream
Development
Floodplain Features
Entrenched
Meanders
Ultimate and Local Base Levels
Stream Longitudinal Profile
Stream
Velocity and
Discharge
Nickpoint
Retreat of Niagara Falls
Alluvial
Terraces
Ganges River
Delta
Nile River Delta
Streamflow Measurement
Urban Flooding
Stream Channel Flood Response
September 15, 1941
Which of the following terms refers to
the maximum particle size that can be
entrained and transported by a river?
a. Competence
b. Capacity
c. Discharge
d. Suspension load
Which of the following terms refers to
the maximum particle size that can be
entrained and transported by a river?
a. Competence
b. Capacity
c. Discharge
d. Suspension load
Because of the shift from erosion to
deposition at a point site on a floodplain
as a stream meanders, which of the
following tends to be true about the
resulting floodplain sediment profile?
a. Coarser sediments on inside of curve
b. Finer sediments on the bottom of profile
c. Finer sediments on outside of curve
d. Finer sediments on top of profile
Because of the shift from erosion to
deposition at a point site on a floodplain
as a stream meanders, which of the
following tends to be true about the
resulting floodplain sediment profile?
a. Coarser sediments on inside of curve
b. Finer sediments on the bottom of profile
c. Finer sediments on outside of curve
d. Finer sediments on top of profile
Following urban development, how
does streamflow response to a heavy
precipitation event change?
a. Higher peak flow, occurring sooner
b. Higher peak flow, occurring later
c. Lower peak flow, occurring sooner
d. Lower peak flow, occurring later
Following urban development, how
does streamflow response to a heavy
precipitation event change?
a. Higher peak flow, occurring sooner
b. Higher peak flow, occurring later
c. Lower peak flow, occurring sooner
d. Lower peak flow, occurring later
Coastal Processes and Landforms
Erosional and depositional landforms of coastal areas are
the result of the action of ocean waves.
Erosional Landforms
Sea Cliffs
Wave-cut Notches
Caves
Sea stacks
Sea arches
Depositional landforms
Beaches
Barrier Spit
Baymouth Bar
Lagoon
Tombolo
Wavelength
Distance from one wave crest to the next
Wave height
The distance between trough and crest
Wave period
The time taken for two crests to pass a given point (remains
almost constant)
=V*P
The wavelength, , is the product of its velocity and period.
Wave Properties
The energy source for both coastal erosion and sediment
transport are the ocean waves generated by the frictional
effect of the winds incident on the ocean surface
(1) Kinetic Energy:
The motion of the water within the wave.
(2) Potential Energy:
Due to the position of water above the wave trough.
Wave energy increases with wind speed and fetch
Wave motion
(a) Ocean depth > ½ the wavelength
- waves not affected by ocean floor
(b) Ocean depth < ½ the wavelength
- wave height increases and wavelength decreases
The wave becomes more peaked
“Breakers” form
Breaking of waves: conversion
of potential to kinetic energy
Work done on the shoreline
Wave Refraction
Straight shoreline
- drag exerted by the ocean floor causes waves to break
parallel with the shoreline.
The direction of travel of a wave varies as it approaches an
indented coast.
Crests approaching the headlands experience the drag of
the ocean floor first, which causes:
1. Increase in wave height
2. Decrease in wavelength
3. Decrease in velocity
Decrease in
wavelength as
waves approach
a shoreline
Transport of Sediments by Wave Action
Rock particles are eroded from one area and deposited
elsewhere. Wave refraction affects this process.
Beach Drift:
Swash and backwash rarely occur in exactly opposite
directions
Upward movement occurs at some oblique angle
Backward movement occurs at right angles to the beach.
This creates lateral movement of particles (beach drift)
Rip Currents:
Rip currents form when
waves are pushed over
sandbars.
The weight of excess water
near the shore can ‘rip’ an
opening in the sandbar,
causing water to rush seaward.
Source: NOAA web site
Longshore Currents:
Spring Tide
Tides enhanced
during full Moon
and new Moon
Sun-Moon-Earth
closely aligned
Neap Tide
Tidal effects of
Moon and Sun
not additive
Influence of Perigee, Apogee, Perihelion and
Aphelion on the Earth’s Tides
Stronger for perigee and perihelion
Erosional Coastal Landforms
Along rugged, high-relief, tectonically-active
coastlines
Sea cliffs
A tall, steep rock face,
formed by the undercutting
action of the sea
Wave-cut notches
A rock recess at the foot of a sea cliff where the energy
of waves is concentrated
Sea Caves
Caves form in more erosive sediment when the rock does not
fully collapse in a deeply-notched environment
Wave-cut platform
Horizontal benches in the tidal zone extending from the
sea cliff out into the sea
If the sea level relative to the land changes over time
(becoming lower with respect to the land), multiple wave
cut platforms (terraces) result
1
2
3
4
5
6
http://www.rgs.edu.sg/events/geotrip/cliff.html
Barrier Spit
Material transported by
littoral drift deposited along
ridge, extending outward
from a coast in an area with
weak offshore currents
If the spit grows to completely
block an embayment, it is
called a bay barrier or
baymouth bar
A lagoon is a body of water
behind the barrier
Puget Sound, WA
Bay Barrier
Near Eureka, CA
Frost Island, WA
A tombolo occurs when sediment deposits connect the
shoreline with an offshore sea stack or island
http://www.geog.ouc.bc.ca/physgeog/contents/11m.html