Revised 8 / 06 (Monroe 6th ed.)
Including...
Introduction
Factors that Affect Erosion
Classification of Mass Movements
Rockfalls
Slides
Flows
Complex Movements
Solifluction
Creep
Conditions Contributing to Mass Wasting
Stabilizing Slopes to Minimize Mass Wasting
Clarification of terms
Weathering: the breaking and/or decomposition of rock at or near the surface
Erosion: Movement of material downslope under the influence of gravity
a.k.a. Mass Movements: large scale events (slides, rockfalls, etc.)
Term gives me trouble (EXPLAIN)
I prefer Mass Wasting
A term we can all identify with
It's also a much broader term
Includes the more subtle movements of material
Creep, etc. (see below)
Transport: river systems move weathered and eroded material to the beach
Refer to Strickler's 3rd Law of GeoFantasy
Rivers act like conveyor belts
Deposition: the accumulation of rock debris
Lithification: conversion of sediments into rock
And we're back into the rock cycle
As usual, water is the key
Water acts as catalyst and lubricant in all steps of the process
Climate
Rain and runoff
Freeze and thaw
Saturation of surface materials
The amount of contained water in the ground
Directly affects runoff
Type of material at and near the surface (Monroe: fig. 14-1, pg. 427)
Porous sedimentary rock vs. massive igneous rock
Solid vs. fractured
Vegetation
This one can get a bit confusing
Roots can strengthen slopes
Add to stability
Roots can break up soil and rock
Add to weathering and accelerate erosion
Degree of weathering
Duh!
Steepness of slope
DIGRESS TO: Angle of Repose
Earthquake activity
Seismic events can set off all kinds and sizes of mass wasting events
Human disturbance
Becoming increasingly common as we build on steep and/or unstable slopes
And as we expand into unstable areas
DIGRESS TO: stability and geologic time
ALL THE ABOVE are influenced (and in many cases controlled) by water
Truly amazing stuff!
Most of the divisions are pretty obvious (Monroe: Table 14-2, pg. 432)
As usual, the transitions can get fuzzy
Can be nearly imperceptible to extremely spectacular events
Classification generally based on type and rate of movement
Vertical to near vertical movement of rock debris
Can include single rocks to massive avalanches (Monroe: fig. 14-8, pg. 433)
Common in alpine regions where ice wedging is prevalent
EXAMPLE: Glacier Point rockfall, Yosemite Valley (1996)
Result in "talus slopes" at base of cliff (Monroe: fig. 15-7b, pg. 464)
Can result in flooding if they fall into standing water (lakes or fjords)
Broad term covering several types of downslope movement
Often the result of human disturbance
Oversteepened slopes
Building pads, roads (lots of examples all over the place)
Hong Kong disaster
The model for the slide in "Noble House" by James Clavell
Rock slides
Can be rock or soil
Slippage generally planar
Often develop in areas where the bedrock layers parallel the surface (Monroe: fig. 14-12, pg. 438)
Water seeps between bedding and lubricates the upper portion
Often set off by under-cutting or earthquakes (Monroe: fig. 14-11, pg. 435)
Rotational slides (Monroe: fig. 14-10, pg. 434)
Concave upward slippage surfaces
Fairly characteristic form
Abrupt scarp at head of slump
Lesser scarps downslope
Surface rotated backwards between scarps
Panama Canal: 73 million cubic yards
Pushed up bottom to make an island
Flows - several broad categories
In general:
Relatively common
More fluid than slides
Often occur at the toe of a slide
Additional water downslope lubricates material and it "flows"
Earthflows (Monroe: fig. 14-17, pg. 442)
Faster than slumps or creep, slower than mudflows
Generally related to the relative proportions of water and debris
Shape is similar to slumps (with headwall scarp and toe at bottom)
The excess water makes the main portion of the flow more fluid
Does not retain internal cohesion like a slump
Mudflows (Monroe: fig. 14-15, pg. 441)
Additional water increases lubrication
In general they move farther and faster than earthflows
Add more water and you get a creek
Some in Nebraska claim that the Platte River is just a very fluid mudflow
"Too thin to plow, too thick to drink"
Up to 80% sand, silt, and clay
The remaining 20% is rock and debris carried along in the flow
Occur in all climates
Possibly more common in arid lands
Excess material and seasonally heavy rainfall
Debris flows
Similar to mudflows, but with bigger pieces
Volcanic mudflows
Like the debris flow from Mt. St. Helens
Rockfalls --> slides --> earthflows --> mudflows
Increase water content, decrease slope upon which it can move
Complex movements - combination of processes
Lots of examples
Can be very destructive to people and structures which get in the way
Gohna, India: 1893
Probably the largest in recorded history
4.7 billion cubic yards!
Dammed a river (900' high, 3000' across)
Lake formed was 800' deep
The dam lasted for 2 years
British engineers predicted to within 10 days when the dam would fail
Gave them time to "prepare" downstream
Quite a flood when it finally let loose
366 million cubic yards of water in 4 hours
Estimated at 686,250 ft^3/sec!
Sent a 250' high wall of water downstream
Nevado Huascarán, Peru: May 31, 1970 (Monroe: Fig. 14-24, pg. 446)
Large earthquake shakes loose 50 million cubic yards of ice, snow, and rock
Free-fall 3000 feet
Then down-valley at speeds up to 200 mph (how fast can YOU run)
Over-topped ridges over 400 feet high
Roared into the town of Yungay - killed more than 20,000 residents
Continued downslope to Ranrahirca where it buried 5,000 more
Only part of Yungay not buried was "Cemetery Hill"
92 people survived by running to the top
One survivor states (with minor editing) "As we drove past the cemetery the car began to shake it was an earthquake. We stopped the car and got out to observe the damage around us. We saw several homes near to us collapse from the shaking. The quake lasted for 30 to 45 seconds. When it was over I heard a great roar coming from Huascarán. Looking up I saw a cloud of dust and it looked like a large mass of ice and rock was breaking loose from the north peak. My immediate reaction was to run for the high ground of Cemetery Hill. Part way up my friend fell and I turned to help her back to her feet.
"The crest of the wave (of debris) had a curl, like a huge breaker coming in from the ocean. I estimated the wave to be at least 250 feet high. I observed hundreds of people running in all directions, many towards Cemetery Hill. All the while, there was a continuous loud roar and rumble. As I reached the top and turned, I saw a man about 10 feet down the hill who was carrying 2 small children. The debris flow caught him and he threw the 2 children toward the hilltop to safety, but the debris flow swept him away. The same wave also swept away 2 women near to him, and I never saw any of them again. It was the most horrible thing I have ever experienced and I will never forget it."
Solifluction (Monroe: fig. 14-20, pg. 444)
Slow movement of saturated surface sediments
Common in permafrost regions during summer thaw
Creep - affects the topmost layer of soil/debris
Anything which disturbs the surface of the land causes creep
Ice needles, people, wind, rain drops, ants, bunny farts, anything!
Results in the upper surface "creeping" downslope
This is happening everywhere, and all the time
Lots of evidence (Monroe: fig. 14-22 pg. 445)
Pistol butt trees common in forested mountains
Cracked foundations, sidewalks, walls
Tilted power poles
Seems like no big deal, but
Radius of earth = 3963.5 miles = 20,927,280 ft.
Surface area of a sphere = 4 X pi X r2 = 5.5 X 1015 ft2
Total area above sea level (29%) = 1.6 X 1015 ft2
Divided by 9 ft2 per yard2 = 1.77 X 1014 yds2
If it is all involved to a depth of 0.0001 yard
1.77 X 1014 yds2 X 0.0001 yard = 1.77 X 1010 yds3
Assume that 10% is involved in any given minute
1.77 X 1010 yds3 times 0.10 = 1.77 X 109 yds3 per minute!
Or 2.55 X 1012 yds3 per day (2.55 trillion cubic yards)
Most books stress large rockfalls and mudslides, and say that they "stagger the imagination." So does creep!!
Gravity acting on material "in an unstable condition"
In general terms, ANY slope is unstable
Due to the persistence of gravity
DIGRESS TO: erosion vs. deposition - no place is safe
And the lubricating properties of water
"Water is the hidden devil in the ground"
As we discussed at the top, water is the key to most (if not all) mass wasting processes
Lubricant
Catalyst
Add mass/weight
Lots of ways to add water to a slope
Precipitation
Watering a lawn
De-vegetation
Clear-cut a forest
Build a parking lot
Vegetation acts as water pumps which extract moisture from the soil and return it to the atmosphere
Evapotranspiration
Dam a river
Saturates the banks
EXAMPLE: Prado Dam / Chino Hills Airport
EXAMPLE: Vaiont Dam, Italy
DIGRESS TO: Geologic cross-sections
Instability can occur naturally (Monroe: Point Fermin, pg. 436)
Remember, the earth is working REAL HARD to maintain a balance and will re-adjust the surface as needed to maintain slope stability
And the earth is good at this (it's been practicing for a very long time)
This is why things aren't moving all over the place all the time
The earth is maintaining a very precarious balance between friction, lubrication, and the tenacious force of gravity
In most cases, human development disturbs this delicately balanced situation
Any construction involving the movement of surface materials will result in a disruption of slope stability
Lots of examples of how human activities contribute to slope failure
Road building (Monroe: fig. 14-4, pg. 429)
Housing developments
Shopping malls
Dams
True GeoFantasy
Kind of hard to win this battle against gravity and water
Remember Strickler's 4th Law of GeoFantasy
Lots of methods have been tried
Reduce angle of slope (Monroe: fig. 14-27/28, pg. 449)
This involves removing vegetation and disturbing the slope
Generally not a good idea
Drain excess water (Monroe: fig. 14-26, pg. 448)
Surface and/or subsurface drainage systems
Cover the area so it won't soak up any water
"Pave paradise and put up a parking lot"
All sorts of retaining walls have been tried (Monroe: fig. 14-29/30, pg. 450)
Rock, concrete, rip-rap
Every effort is only successful in the short term
Water always wins! Strickler's 4th Law of GeoFantasy
The best solution lies in educating the public
And population control so we can stay out of the worst areas
And a healthy respect for karma and joss
I am absolutely convinced that these will fail in the long run (especially the population control)
And we will be stuck with living, working, and playing in areas which are fundamentally unsafe
Lots of potential for employment in geology as we continue to expand into these marginally safe (to blatantly unsafe) areas
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