Revised 8 / 06 (Monroe 6th ed.)

Erosion and Mass Wasting of Weathered Materials (Ch. 14)




Factors that Affect Erosion

Classification of Mass Movements




Complex Movements



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


Factors that affect erosion


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)

Rock vs. soil

Porous sedimentary rock vs. massive igneous rock

Solid vs. fractured


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


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!


Classification of Mass Movements

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

To summarize: in general terms...

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!!


Conditions Contributing to Mass Wasting

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



Add mass/weight

Lots of ways to add water to a slope


Watering a lawn


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


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



Stabilizing Slopes to Minimize Mass Wasting

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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