Surface Processes








Decomposed Granite


Falls, Slides, and Flows

Complex Movements




General Review: a regional view

Earth has 2 levels

Earth has 2 types of rock

Earth has 2 processes

Construction and Destruction

Over the course of geologic time these 2 are in balance

Tectonics: the study of earth processes which result in the creation and deformation of magma and rock

This builds the earth as we know it (videodisc of mountains)

The destructional processes are covered in "The Hydrosphere"

This stuff can be mighty impressive

Carve valleys (get videodisc - rivers and glaciers)

Wash to sea (beach shots with sand/gravel)

The basic idea: the earth takes what it builds and puts it in the ocean (overhead)

But first the earth has to break it up because the mountains are too big to move in one piece


Surface Weathering

Mechanical (physical) weathering

Smaller pieces / same stuff

Fracturing and Abrasion

Like the difference between a hammer and sandpaper

Results in clastic sediments

Sands to conglomerates

Lots of methods

Rocks in river

Rocks falling

Ice - the big one

Cold areas with large daily temperature fluctuations

Ice wedging: expands 10% upon freezing

Can exert up to 4,900,000 lbs./in^2!!

Makes angular fragments and surfaces

Biological weathering

Tree roots can grow into cracks - force them apart

Biologic acids can "eat" the minerals

Chemical weathering

Over the course of geologic time everything gets dissolved (overhead)

Minerals dissolve when exposed to water and heat

Smaller pieces AND different stuff

Water need to be able to touch the minerals

Surface area VERY important!


High temperatures also act as a catalyst

Hot humid areas best for chemical weathering

Makes rounded fragments and surfaces

Do a lab on this tomorrow

Chemical weathering lab

The 3 types can work together

It's like a cycle (remember the hydrologic cycle? Same idea)

Story of Decomposed Granite (or What's really happening below your feet)

Biologic / physical (EX: tree roots in cracks)

Biologic / chemical (EX: root acids)

Chemical / physical (EX: granite)

Physical / chemical (EX: Fracture and abrasion lead to inc. surface area)

Abrasion and "rock flour"

Chemical / biological (EX: leads to soil formation)

And back to biologic weathering


Erosion of weathered materials

Movement of material downslope under the influence of gravity

Water acts as catalyst and lubricant

Factors that affect erosion include


Saturation of surface materials

Type of material at and near the surface


Degree of weathering

Steepness of slope

DIGRESS TO: Angle of Repose

Earthquake activity

Human disturbance

Becomming increasingly common as we build on steep and/or unstable slopes

Can be nearly imperceptible to extremely spectacular events

Rockfalls-vertical to near vertical

Common in alpine regions where ice wedging is prevalent

Result in Talus Slopes at base of cliff

Can result in flooding if they fall into a lake

Slides - broad term covering several types of downslope movement

Often the result of human disturbance

Lahars - volcanic mudflows

Like the debris flow from Mt. St. Helens

Complex movements - combination of processes

Nevado Huascarán, Peru: May 31, 1970

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 >400 feet high

Roared into the town of Yungay - killed >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."


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

Pistol butt trees common due to creep

Seems like no big deal, but

Radius of earth = 3963.5 miles = 20,927,280 ft.

Surface area of a sphere = 4 pi r^2 = 5.5 X 10^15 ft^2

Total area above sea level (29%) = 1.6 X 10^15 ft^2

Divided by 9 ft^2 per yard^2 = 1.77 X 10^14 yds^2

If it is all involved to a depth of 0.0001 yard

1.77 X 10^14 yds^2 X 0.0001 yard = 1.77 X 10^10 yds^3

Assume that 10% is involved in any given minute

1.77 X 10^10 yds^3 times 0.10 = 1.77 X 10^9 yds^3 per minute!

Or 2.55 X 10^12 yds^3 per day

Most books stress large rockfalls and mudslides, and say that they stagger the imagination. So does creep!!