CHAPTER 10
Weathering, Karst
Landscapes, and
Mass Movement
This chapter examines the forces/processes that
“wears away or rearranges” landforms
Text calls this denudation (I used to lecture these
forces as degradation) – forces that collectively are
“levelers” of the Earth’s surface
--- weathering; mass movement; erosion;
transportation; deposition
Paradoxically, in conjunction with forces of transport,
reducing agents are also aggradational forces
building (deposition) the Earth surface
Dynamic Equilibrium Approach to Landforms
In conjunction with tectonic forces,
degradation and aggradation forces are
part of a 4.5 billion year old Earth system
… surface building and reduction
[We have previously discussed the end to this system and the
ultimate “leveling” of the Earth surface]
This is the text’s dynamic equilibrium model
Equilibrium
Stability
Destabilizing
Event
New Condition
of Stability
Period of
Adjustment
Reducing forces are atmospheric and gravitational in
origin and tied to geology and weather and climate
Taken together these elements define a dynamic
equilibrium state for the Earth surface
A change in one or more of these elements may
destabilize and trigger a geomorphic threshold
… a point at which there is enough energy to
overcome resistance against movement
Weathering Processes
Processes by which rocks at or near Earth
surface are physically or chemically
disintegrated or decomposed in preparation
for removal and transportation
Weathering is the breakdown of bedrock
(parent rock material) without
transportation
regolith is the term for surface/near surface
material acted upon by weathering
With enough time, even the most massive of
Earth materials will be decomposed and
transported
*Weather does not by itself level the
landscape. It breaks rock material so that
denudation (degradation) can take place
Weathering is traditionally divided by process agent
into:
(1) Physical (or Mechanical) Weathering
[generally dominate in drier/cooler climate regions]
(2) Chemical Weathering
[generally dominate in warmer/wetter climates]
Both are primarily surface phenomena
(but may occur as deep as 600’ below ground)
Variations in weathering depth will depend
on:
(1) structure and composition of rock
(2) climate
(3) configuration of the land surface
(4) vegetative cover
Physical Weathering
- Physical / mechanical weathering is
disintegration without chemical alteration
--- Important:
(1) Smaller rock material easier to transport
(2) “ “
“
“
“ encourages greater
chemical weathering by exposing more rock
material
Examples of physical weathering
(1) ice wedge
(2) frost wedge / frost heave
(3) salt-crystal growth
(4) uneven expansion of rock material in a
conglomerate or in the ultimate expansion of
high-pressure-formed igneous plutons
(5) root growth / burrowing by animals
Chemical Weathering
Chemical weathering is decomposition and decay of
the chemical structure of rock
Prepares rock material for transport in several ways:
(1) may form “new” softer rock material
(2) by combining with water / through chemical
change, No. 1, may create new structures more
easily stressed / fractured
(3) may create acidic compounds
Principal chemical weathering processes
(1) oxidation
(2) hydration and hydrolysis
(3) carbonation and solution
--- “chemical erosion”
Different rock types are affected by different
weathering processes and at different rates
Under differing geologies and geographies,
this can lead to a highly diversified visual
landscape
Karst Topography and Landscapes
-
Limestone is abundant on the Earth
[look at Fig. 10.13 at global distribution of limestone / look
especially at eastern U.S. distribution]
- It is a very soluble rock
- Where limestone is present, distinctive and
spectacular landscapes may be produced
Karst
- term given to any landscape having
landforms evolved from solution of
limestone
… approx 15% of Earth surface
- term derived from the Krs (Karst) plateau of
the Dalmatian Coast of Croatia
Karst, cont
Synonymous with limestone strata, it takes more than
limestone to create karst landscape … many
regions have karst features – true karst is rare
(1) the limestone formation must contain 80% or more
calcium carbonate
(2) joints “cracks and conduits” must be present in the
limestone
(3) there must be an aerated zone between ground and water
table
(4) vegetative cover must be available (for organic acids)
Limestone Landscapes
-
-
Groundwater is a weak carbonic acid containing
CO2
Limestone is primarily calcium carbonate (CaCO3)
Together they form calcium bicarbonate Ca(HCO3)2
CaCO3+H2O+CO2-----Ca(HCO3)2
Limestone Landscapes, cont
Created is a landscape of bumpy, pitted surface
topography, poor surface drainage, and well
developed solution channels
We would expect to find (Fig 10.17):
(1) sinkholes (solution sinkholes)
(2) dolines
(3) caves / caverns
--- collapse sinkholes
--- valley sinks
(4) haystack hills (hums)
Limestone Landscapes, cont
Limestone landscapes are as well known for
underground features
--- travertine (dripstone)
--- stalactites
--- stalagmites
--- “soda straws”
--- etc
Mass-Movement Processes
Natural systems are noted for their attempts to seek
steady state or equilibrium
Gravity is an equalizing force that attempts to create
this equilibrium
- massive amounts of material are moved
downslope by gravity
- moved downslope as far as the defined angle of
repose
Mass Movement
In addition to gravity, any action that:
(1) increases shear stress
or
(2) decreases friction between slope
material particles
will increase the potential effects of
gravity and slope
Mass Movement
[Fig 10.19]
collective term for any of the various forms of
gravity-induced downhill material transportation
- can either be natural or man-induced in origin
- massive and rapid vs slow and constant
- actual volume of global mass movement is
unknown
- many actions taken by man can either delay or
accelerate these processes
-
Mass movement processes are classified:
(1) movement fast or slow?
- a continuum of “subjective generalities”
slow (long-term observation – to – fast
(occurrence over extremely short period)
(2) type of material moved?
- range of loose Earth surface material in
unstable (or potentially unstable)
location
--- ice, snow, debris,soil, rock, mud
(3) kind of motion?
- ways that slope material moves in
response to gravity