Geography 140
Introduction to Physical Geography
Lecture: Introduction to Gradational Processes
V. Gradational forces in the earth's crust
A. In the last section, we went over those forces which tend to build up
the earth's crust and to increase topographic contrasts of all sorts.
These tectonic forces included vulcanism, faulting, and folding, all
produced by plate tectonic forces.
B. In this section, we review those forces which tend to even out the
surface of the continents. These forces break up rock materials,
sometimes altering them chemically, erode them, transport them, and
deposit them. The overall effect is for erosion to wear down high
places or prominences and for deposition to build up low spots and
other low energy places.
C. Collectively, these forces are called "gradation," and they comprise
fluvial (flowing water), glacial (moving ice), coastal (waves), and
æolian (wind) processes, together with weathering and mass
wasting. This lecture will discuss weathering and mass wasting.
D. Weathering
1. Refers to any process, which tends to break up rock materials into
smaller and smaller clasts (or bits) and/or alter them chemically.
Weathering can be subdivided into mechanical weathering and
chemical weathering.
2. Mechanical weathering involves mechanical forces working to
disintegrate rock materials. Compression, tension, and shearing
stresses are applied to the rock, eventually overcoming its shear
strength, which results in deformation and failure.
a. A rock's resistance to shear stress comes from:
i. The cohesion produced by electrostatic and magnetic bonds
between minerals making up the rock.
ii. The frictional resistance provided by the roughness of
individual rock grains and crystals in a rock. Roughness
causes the grains and crystals to interfere with one
another's motion, locking them in place.
iii. Any cementing agents between rock grains, such as calcium
carbonate.
iv. Water in the rock's pore spaces, which plays a
contradictory rôle, depending on how much moisture
there is in the rock.
a. It can enhance shear resistance up to a certain point,
because its surface tension acts as a sucking force
helping to hold the rock together.
1. Surface tension is that film at the surface contact
between water and air, which can actually support
water strider insects as they walk on water, and
which makes water bead up on surfaces it doesn't
adhere to very well (such as a well-waxed car).
2. This suction force produces negative pore pressure,
and it dominates the rôle of water when there's
just a moderate amount of water in the rock, that is,
as long as there is still some air together with the
water in those pore spaces to form those water-air
tension surfaces.
b. Water can, however, weaken shear resistance once all the
pores in the rock are saturated with water. Any
additional water now exerts positive pore pressure,
pushing outward in each of those tiny little spaces
among the rock grains. This reduces friction by
providing lubrication, making it easier for the rock to
shear apart along some particularly weak internal
discontinuity, some small crack or fracture.
b. Examples of mechanical weathering agents:
i. Frost wedging or cryofracture, in which liquid water
introduces itself within the cracks and crevices of a rock
and then freezes. Water expands about 9 percent when it
freezes, and this wedges the rock apart still further
through tension stress at the bottom of the cracks. When
the ice melts, the water works its way even farther into
the rock's extended cracks and crevices. This form of
mechanical weathering is common in cold, humid climates and
seasons where water freezes at night and melts in the
daytime.
ii. Salt crystallization, which involves water dissolving salts
as it moves within rocks and then evaporating from their
surfaces. The water evaporates from the surface of the
rock, and that draws water deeper inside the rock up to the
surface, because water molecules are drawn to other water
molecules. This tendency for water molecules to follow
other water molecules is called capillary attraction. The
water goes to the surface of the rock and evaporates, but
the salts it carries cannot evaporate. This means the salts
crystallize in surface openings in the rock. As these
precipitated crystals grow, they wedge the rock apart.
iii. Root wedging, in which a plant's roots or rootlets find a
crack or other open space in rock and grow into it, wedging
it open further. You often see this in urban environments,
when tree roots wedge up sidewalks and curbs.
iv. Expansion/contraction of minerals in the rock with
temperature changes. This is sometimes called insolation
weathering, because it is common in the hot, dry climates,
which see dramatic temperature changes with the rise and
setting of the sun. This creates strong expansion and
contraction at the surface of the rock, which is less
pronounced a short distance in from the surface of the
rock. This can create exfoliation fractures paralleling
the surface (the rock fails in sheets) or crumbling if the
rate and magnitude of volume changes differs markedly among
different minerals in the rock.
v. Unloading, in which erosion removes a rock's "overburden,"
releasing the pressure of the materials above bearing down
on the rock. As pressure decreases, rock can expand and so
fracture off in sheets, that is, exfoliate.
vi. Slaking of clay minerals by water can also stress a rock,
if the rock contains shrink-swell clay minerals, such as
montmorillonite and vermiculite. These clays contain
micro-fine sheets of silica in them. As the water hydrates
the clay, it pushes these sheets of silica apart, which
puffs up the volume of the mineral. Going on inside a
rock, this generates expansion stresses when the rock is
wet and stresses associated with contraction when the rock
dries out.
c. Mechanical weathering, then, is most common in high elevations,
high latitudes, and deserts, where the forces behind them (e.g.,
evaporation, hot-cold temperature alterations) are likeliest to
operate.
d. Landscapes that are predominantly mechanically weathered are
typically angular and rugged, beautiful in a spectacular sort of
way (e.g., Canadian Rockies, Sierra Nevada, Andes, Monument
Valley).
![[ Monument Valley, AZ, USGS, Peter Kresan ]](http://pubs.usgs.gov/gip/deserts/cover4.gif)
![[ Grands Tetons, C.M. Rodrigue ]](https://home.csulb.edu/~rodrigue/jps/art/tetons50.jpg)
3. Chemical weathering involves rock decomposition or rotting due to
chemical reactions.
a. Examples of chemical unions that result in weathering:
i. Hydration entails chemical union of a mineral with water,
which is incorporated in its molecular crystal lattice.
This results, obviously, in chemical alteration and it also
generates a microscopic scale tensile stress in the rock.
Iron oxides are susceptible to this.
ii. Hydrolysis is also a chemical union of certain rock
minerals with water, not with regular water H2O
but with its dissociated ions (those electrically
imbalanced atoms and molecules we keep bumping into):
H+ and OH-. Plagioclase feldspar in
granite is particularly susceptible to hydrolysis, giving
us "DG" or decomposed granite and kaolinite clay.
iii. Oxidation is chemical change due to union with oxygen
(e.g., rust of iron). In mafic rocks, ferrous iron oxides
can be further oxidized into ferric iron oxide, which
destabilizes the mineral's crystal lattice and the rock in
which it's incorporated.
iv. Reduction is chemical change due to the removal of oxygen
or the addition of hydrogen. Iron oxide can be reduced
back to elemental iron by carbon monoxide, for instance.
v. Carbonation involves the presence of carbonic acid in
surface or ground water. Carbonic acid is formed when
carbon dioxide builds up in soils due to plant and
microbial respiration and combines with water:
H2CO3. It is mildly acidic and combines with calcium
carbonate or CaCO3 (e.g., limestone) to form calcium
bicarbonate (Ca(HCO3)2), which is easily dissolved
in water and carried off. Limestone and marble are, thus,
apt to dissolve in humid climates with a lot of plant cover
on the ground over these rocks.
b. Chemical weathering prevails then in warm, humid climates, which
provide conditions that increase the rate of these chemical
reactions.
c. Chemical weathering tends to produce a gently rounded landscape,
pretty but not spectacular, such as much of the Southern
Appalachians.
![[ Shenandoah Valley, Southern Appalachians, Rahul
Mishra, George Mason University ]](http://mason.gmu.edu/~rmishra/images/Shenandoah/Valley2.jpg)
d. There is one quite spectacular landscape, however, that can,
indeed, result from chemical weathering. This is karst.
i. Karst involves limestone beds in a warm, humid climate with
a dense vegetative cover.
ii. Groundwater gets into the limestone, traveling along the
cracks paralleling the beds, turning the calcium carbonate
into calcium bicarbonate and schlepping it off.
iii. The rock beds develop openings, which widen into caves and
often quite elaborate cavern systems (the largest cavern
system in the New World is found in our own Puerto Rico
[well, PR is an American commonwealth, which is a euphemism
for colony]: The Río Camuy cavern system in
northwestern PR). This cavern system is well worth a trip
to the island, folks!
iv. The caverns widen and develop flowstone features:
![[ flowstone in a cavern, D. Bunnell ]](http://www.goodearthgraphics.com/virtcave/stalmite/tolkien.jpg)
a. Stalactites are rods of flowstone that hang down from
the ceilings of the caves as water drips or evaporates,
leaving the dissolved mineral behind:
Stalactites hang tight to the ceiling!
b. Stalagmites are mounds of flowstone on the floors of
caverns directly under the stalactites:
Stalagmites mound up on the floor!
c. Columns form when stalactites reach down to the
stalagmites building up below below them, and they fuse.
d. Curtains are sheets of flowstone.
v. Eventually, the caverns get so big that there are cave-ins,
creating sinkholes or dolines on the surface above
(sometimes pretty catastrophically: There have been cases
in Florida where a home or a car has been undermined and
slid in).
![[ big urban sinkhole, USGS ]](http://www.uwec.edu/jolhm/EH/Below/Images/sinkhole%20BIG.jpg)
![[sinkhole swallowing house, USGS ]](http://www.earlham.edu/~scheuer/sinkhole_diagram_2.JPG)
vi. With enough of this activity, sometimes a spectacular karst
landscape results: A bizarre, almost vertical collection
of baguette-shaped mountains towering over narrow valleys
that used to be the floors of caverns long ago. You've
seen those Chinese landscape paintings of surreal mountains
in the mist? You thought that was artistic license? Nope
-- it's an accurate rendition!
![[ Chinese karst photograph, GlobalEd, The China
Project ]](http://www.globaled.org/chinaproject/teachingmaterials/lesson_65_Pic%206.jpg)
E. Mass wasting refers to the transport of weathered materials through
the action of gravity alone, that is, not requiring erosion and
transport in flowing water, ice, or wind or wave action. This movement
may be sudden and catastrophic, or nearly imperceptible.
1. Rockfall is the dropping of mechanically weathered rock down the
face of a cliff.
a. It forms an apron of broken rock at the bottom, called a "talus
slope" or "scree."
b. This slope of broken rock material retains an angle reflecting
the material's friction strength: High friction strength
permits high angles of repose.
c. Friction strength is the strength given to a soil or rocky
material by the frictional resistance created as the edges of
its constituent particles lock into one another.
2. A debris avalanche is a very sudden slope failure that moves at
tremendous speed. The material may have been destabilized by an
earthquake, which releases the normal downward stress on it exerted
by gravity. When normal stress is suddenly relieved, the mass
loses cohesion, so that it takes very little shear stress to set it
in motion downslope.
a. This is particularly apt to happen when all pore spaces among
the rock or soil grains are saturated, at which point the pore
water begins to exert positive pressure on the surrounding rock
grains. At this point, it's the water itself which is bearing
normal stresses instead of grains of rock or soil locked
together around their rough edges, and this amounts to
lubrication.
b. The whole mass then breaks past the liquid limit, meaning it
becomes completely incoherent and fluidized and then flows just
like a liquid.
c. Each rock particle is on its own, lubricated against the
frictional resistance of other particles and, in fact, when the
individual particles smack into one another during the chaos of
the avalanche, their bouncing off one another actually enhances
the buoyancy and fluid behavior of the whole pack of debris.
d. Debris avalanches are extremely hazardous because of their
tremendous speeds (try 300 km/hr!) and lack of time to warn and
evacuate people.
e. There was an awful incident of this type, when a 1970 earthquake
triggered a debris avalanche of 100 million cubic meters of
debris down the side of Nevado Huascarán, the tallest
peak in the Andes. This avalanche dropped over 4 km vertically
and 16 km horizontally is just a few minutes, burying a city
named Yungay, killing 18,000 people in an instant.
3. Landslide is the sudden sliding of large, coherent masses of rock
or soil down a steep slope as a unit. This can take a variety of
forms, depending on the slope angle and the angle of any
discontinuity or weak plane in the material with respect to the
slope angle.
a. Planar slides involve reduction of cohesion along a
discontinuity that roughly parallels the slope angle or is
steeper than the slope angle. So, the still coherent material
above the failure plane just shears off and down the slope.
b. Wedge slides involve a more complex geometry, featuring two
discontinuity planes that intersect with one another. The line
marking their intersection roughly parallels or is steeper than
the slope angle and, again, a reduction of cohesion along this
line can free a wedge of coherent material to slide down slope.
The gouge in the slope created by this mass movement is
typically V-shaped (where the two discontinuity planes crossed
one another).
c. Circular slides involve loose materials with little cohesion. A
slide may start high up on a steep slope and, as it moves
downslope, it destablilizes otherwise cohesive materials farther
down at lower slope angles and these fail, too. The result is a
concave gouge in the slope.
4. Slumping, occurs when soil, weathered rock, or weak bedrock becomes
saturated and then oozes down a slope.
a. All pores in these materials are filled with water, which then
exerts positive pore pressure outward on the rock or soil
material, but the amount of water (and pore pressure) is only
enough to push the material past the plastic limit but not all
the way to the liquifaction limit.
b. So, it flows as a soft solid, a plastic flow (kind of like the
flow in the æsthenosphere or in glaciers).
c. This kind of mass movement creates a typical scar on a slope,
with a concave, stepped area at the top where the slump detached
(the scarp zone, for those step-like features), a gouged track
marking its path downslope, and a convex, bulging area at the
end where the material piled up (the toe, because it sort of
looks like a sore toe!).
d. You see a lot of these in the hills of Southern California but
you have to look closely to see the older ones, because they get
covered with vegetation eventually.
5. Solifluction is a type of small-scale slumping in tundras.
a. Top layers of permafrost (permanently frozen ground) melt in
summer and saturated soil oozes downslope.
b. The slumps may be only a few centimeters or a couple meters in
length.
c. Solifluction produces a lumpy or stepped slope, which is very
characteristic of tundra environments.
d. At the bottom of the slopes, there may be pooled water (it can't
drain downward into the soil because of the ice below, and it
commonly can't find a stream to begin flowing to the sea,
because tundra environments had their drainage patterns deranged
during the Pleistocene ice ages. Perfect mosquito habitat.
6. Mud and debris flows occur when soils become saturated.
a. Positive pore pressure fluidizes them and they then flow
downslope as though they were water ("no-one here but us water
molecules").
b. Soon, they thicken enough to re-assert normal stress, which
raises the threshold required for shear stress to keep them
going against frictional resistance.
c. Back in 1925 near Jackson Hole, Wyoming, a massive flow moved
some 37 million cubic meters down one side of the Gros Ventre
River canyon and actually back up about 30 meters on the other
side of the river! This dammed up the river for a few years!!
7. A lahar is the same thing as a mudflow, but it involves mud created
by the melting of snow or glaciers on a volcano that has erupted.
a. Again, you get sudden saturation of rock, soil, and new ash and
tephra, and down it comes.
b. This is the sort of mass movement that took place around Mount
St. Helens in 1980, where some of the flows travelled over 20 km
down the Tuttle River valley (wiping out that poor idiot, Harry
Truman, who refused to evacuate Spirit Lake where he ran a
resort, explaining that he had talked to the mountain and "she"
had said "she" wasn't going to blow -- watch out for the advice
of mountains you think you hear talking to you!!!).
c. A truly horrible example took place in Colombia back in 1985,
when the Nevado del Ruiz volcano blew. Civil authorities did
not take geologists' warnings seriously and, so, people were not
effectively evacuated from the hazard zone. Sure enough, a
monstrous lahar swept down and killed more than 23,000 people in
the town of Armero. This tragedy was particularly controversial
because of the inadequate government response to clear
geological warnings.
8. A nuée ardente is another volcano-related mass movement.
a. It's a glowing avalanche of extremely hot volcanic gas,
boulders, and ash, which moves with extremely high speed down
the side of an erupting volcano, if that volcano is effectively
plugged at the top and, so, erupts out a side vent.
b. Again, this is an incoherent mass of independently moving
debris, fluidized this time by the erupting gas, which accounts
for its tremendous speed.
c. This is the mass movement that destroyed everyone in the town of
St. Pierre in Martinique (Caribbean) back in 1902, except for a
single man, who was a prisoner in the town jail. Ironically,
this sole survivor of the 30,000 residents of St. Pierre was
scheduled for execution the day after the catastrophe!!! Talk
about "Twilight Zone"! The nuée ardente crossed the 8 km
from Mt. Pelée to St. Pierre in less than a minute!
Here's a photograph taken right during the process:
![[ nuée ardente, Martinique, 1902 ]](http://www.iml.rwth-aachen.de/Petrographie/Pelee.jpg)
8. Soil creep is the much less exciting, extremely slow, almost
imperceptible motion downhill of soil particles on a hill (it
sounds to me like the ultimate in geographical epithets, "you soil
creep, you!"). It is most active closest to the surface and less
prevalent deeper in a slope's soil. What happens is some sort of
vibration or perhaps soil expansion/contraction due to
hydration/dehydration or even just diurnal temperature changes
causes soil particles to "jump" a microscopic distance upward.
They go up at right angles to the slope but descend straight down
under the influence of gravity, thus producing a tiny net motion
downslope.
9. For a lot of these, the Mohr-Coulomb equation is a helpful
framework for visualizing what's going on in a slope. It states
that:
S = C + N (tangent F), where S = shear stress
C = cohesion
N = normal stress (gravitational
stress held in balance with
intertia)
F = angle of internal friction
a. Shear stress is that lateral stress that we see along strike
slip faults: one side trying to move one way and the other side
trying to move in the opposite direction along a plane.
b. Normal stress is everyday gravity. The higher the downward
stress of gravity, the greater the shear stress has to be to
release materials from inertia to slide.
c. The amount of shear stress needed to move masses in zero gravity
is the inherent cohesion of the material. It is greater in
solid, unfractured materials and in materials with a lot of
frictional resistance to moving. It is less in saturated
materials because of positive pore pressure.
d. Internal friction governs the angle of repose of a pile of
material: Higher internal friction permits steeper sided piles,
and lower interal friction means not much of a slope angle can
be supported against gravity.
10. Mass wasting can be an extremely dangerous hazard, causing massive
loss of life, as in the Nevado del Ruiz lahar and the Nevado
Huascarán debris avalanche, and the hillsides of Southern
California are prone to slumps, flows, and slides. It is a good
idea to investigate the mass movement history of any home you are
considering, above and beyond real-estate disclosure requirements.
a. If you would like to check out the earthquake-triggered
landslide or liquefaction hazards where you live, work, and
(ahem) study now, have a look at the California Geological Survey
hazard zones website. It shows a small scale map of Southern
California, which you can click on to get to your community (and
you can click on those to see really large scale maps of your own
locale). Morbidly fascinating.
b. A really angry victim of the Anaheim Hills Landslide of 1993
put together an absolutely amazing website detailing the
disaster, the effects it had on him and his neighbors, the
inadequate disclosure by local government before the hazard,
some really bad zoning changes that may have triggered the
landslide, some fabulous photographs and animations (brownie
points to whoever finds the photo of the constant street
resurfacing program and an unfortunate rat that got in its way),
and some pretty intense invective. This site illustrates the
possibilities for effective political action offered by the web.
Well, th-th-th-that's all, folks, for weathering and mass wasting. And here
you thought mass wasting meant the parties after finals week!
Know the two styles of weathering (mechanical and chemical), examples of each,
and the kinds of climates you would expect to find one dominant over the other
in shaping the landscape. Also, know the basic appearances of landscapes
dominated by one or the other weathering style.
Understand karst processes and landscape features.
Know what mass wasting is and how its several subtypes work (and the book
details more). Have a basic understanding of the Mohr-Coulomb equation of
material failure and how it might apply to each of the mass wasting examples.
Document and © maintained by Dr.
Rodrigue
First placed on web: 11/29/00
Last revised: 07/08/07