GEOLOGY 12
SURFACE PROCESSES I
CHAPTERS 5, 18 & 19 NOTES
WEATHERING, EROSION AND MASS MOVEMENT
Name __________________
I. WEATHERING VS EROSION
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Weathering is the breakup of rock mainly from exposure to the atmosphere.
Erosion is the moving or transport of earth materials by moving natural agents.
The four agents of erosion are water, wind, ice and gravity.
TYPES OF WEATHERING
A. Mechanical Weathering – physical changes due to…
1. Biological Effect is the action of plants and animals – lichen, tree's roots, ants,
earthworms, etc. Also known as Biological Weathering
2. Unloading of overlying rock and soil which causes:
a) joints - long curved breaks in sheet rock
b) exfoliation - when large sheets of loosened rock break away from an
outcrop.
3. Ice Wedging - freezing water forces rocks apart (see diagram p.4)
a) talus – steep slopes of broken rock at base of mountains or cliffs
b) ice heaving results in potholes
B. Chemical Weathering – chemical changes mainly from the action of rainwater,
oxygen, carbon dioxide, and acids from plant decay
(i.e. AKA the biological effect).
1. Hydration - chemical union with water. First they expand and then they crumble
into powdery clay, e.g. feldspar, hornblende, and augite.
2. Oxidation - the chemical reaction of oxygen with other substances. Minerals
containing iron are especially prone to oxidation forming rust or iron oxide. e.g.
magnetite, pyrite, hornblende, augite, biotite from hematite (red) or limonite
(brown).
3. Carbonic Acid - Carbon dioxide dissolves in water forming carbonic acid which
leeches (dissolves) elements like K, Na, Mg, and Ca out of feldspar, hornblende,
augite, and biotite to change them into clay. It dissolves calcite completely leaving
caverns, etc.
4. Acid Rain - CO2 and sulphur compounds (from industry) combine with rainwater to
produce carbonic acid and sulphuric acid. It increases chemical weathering.
Chemical weathering occurs most quickly at the edges and corners of rock outcrops. This
rounds the rock and is called spheroidal weathering.
Weathering occurs more quickly in a hot humid climate.
RESISTANCE TO WEATHERING
In accordance with Bowen’s Reaction Series (discussed in Chapter 3) quartz is most
resistant and weathers slowly making pebbles and sand grains. Mechanical weathering
breaks many minerals into large fragments and then chemical weathering breaks them
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into fine clays. Most igneous and metamorphic rocks weather more quickly in wet
climates.
Sedimentary rocks differ in their resistance to weathering depending on the cements
holding their particles together. When these break up, their quartz fragments remain as
boulders, pebbles, and sands. Quartzites, well-cemented sandstones and conglomerates
(with silica cement) are among the longest lasting of all rocks. Shales, the weakest, split
easily between layers and crumble into clay.
Marbles and limestones resist mechanical weathering well but are especially prone to
weathering by acids in water due to their calcite content. Moist climates affect these but
limestone is one of the most durable in a dry climate.
II. SOIL
STAGES OF SOIL FORMATION
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Soil - loose, weathered rock material in which plants with roots can grow. Always
contains some organic material. Soils differ in color, mineral composition and amount
of organic material.
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Regolith - any loose earth material covering bedrock.
Soil formation is the process by which rocks are broken down into progressively smaller
particles and mixed with decaying organic material. Bedrock begins to disintegrate as it is
subjected to freezing-thawing cycles, rain, and other environmental forces. The rock
breaks down into parent material, which in turn breaks into smaller mineral particles.
Biological material adds to the chemical breakdown of the rock material. As soil continues
to develop, layers called horizons form.
The A-horizon, nearest the surface, is usually richer in organic matter, while the lowest
layer, the C-horizon, contains more minerals and still looks much like the parent material.
The soil will eventually reach a point where it can support a thick cover of vegetation and
cycle its resources effectively. At this stage, the soil may
feature a B-horizon, where leached minerals collect.
MATURE SOIL PROFILE
A-horizon - topsoil, contains most organic materials (humus)
B-horizon - subsoil, more clay, less organic material (humus)
C-horizon - mostly rock fragments sitting on bedrock
TYPES OF SOIL
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Residual soil - where the soil is made up of weathered
bedrock of that area.
Transported soil - is soil that is made up of material that has
been brought from other areas by winds, rivers, or glaciers.
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SOIL TYPES AND CLIMATE
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Pedalfers - ped for soil, al for aluminum, fer for ferrous (iron), rich in clay, iron oxides
and quartz fragments, poor in soluble minerals because usually in high rainfall areas
(> 65 cm/ year.)
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Pedocals - cal for calcium, generally rich in calcium because rainfall is less than 65 cm
per year; less clay.
III. MASS MOVEMENT
Gravity is an aid to weathering and causes steep slopes to weather more rapidly than the
gradual slopes. Movements of loose earth materials are collectively called mass
movement.
CAUSES OF MASS WASTING AND SOIL EROSION
Mass movements occur in areas of heavy rainfall and steep slopes. Volcanic eruptions
and earthquakes may also trigger large-scale mass movements. (i.e. Mt. St. Helens,
Columbia, SA 1985 – 25000 deaths)
A. Climate
1. Climate is an important factor for it affects the rate and type of weathering.
Weathered materials become susceptible to mass movement under the influence
of gravity.
2. The hot and humid climate in the summer promotes chemical weathering.
3. Dry winters and wet summers result in alternate drying out and soaking of
weathered materials.
B. Rainwater
Occasional and sudden heavy downpours are particularly serious problems since the
amount of runoff water often cannot be effectively and quickly drained away.
C. Rock Structure
Granite is easily weathered by physical and chemical processes and is susceptible to
mass wasting.
D. Terrain
1. Rapid movements, such as landslides and slumping, are common along steep
slopes.
2. Slow movements, such as soil creep, are common along gentle slopes.
E. Deforestation
1. Bare areas without vegetation due to human activities such as clearing of trees,
excavation for building sites, mining, quarrying, logging, trampling.
2. Hill fires, especially in dry periods, also contribute to deforestation.
F. Excavation
Excavation work to construct roadways and the steepening of slopes for building
weaken the geological support structure.
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G. Vibrations
Vibrations are caused by dynamiting during construction or quarrying or earthquakes.
TYPES OF MASS WASTING
Mass wasting may be classified according to the rate, direction and nature of movement.
1. Slow Movement - common along gentle slopes (as gentle as 5 degrees)
a. Soil Creep
♦ Soil creep is a very slow, often not discernable.
♦ downslope movement of soil particles or fine weathered materials under the
influence of gravity.
♦ It may be caused by presence of rain water which increases the weight of the
soil mass and reduces friction by trampling and burrowing.
♦ Evidence of soil creep includes trees with curved or bent trunks, tilted or
displaced posts, and accumulation of material against stone walls on the
upslope side.
b. Solifluction and Frost Creep
♦ Solifluction is a very slow, viscous flow of materials saturated with meltwater
over a gently sloping frozen ground.
♦ Frost creep is a slow mass movement caused by the lifting of soil surface by
frost action which is common in cold areas with temperatures fluctuating above
and below 0 degrees Celsius.
2. Rapid Movement - common along steep slopes
a. Landslide and Slumping
♦ Landslide or slide is a rapid and sudden movement of weathered materials
down a slope under the force of gravity. It is common on steep slopes.
♦ If the slope has a curved surface, the movement is rotational with a backward
tilt, and the slide
is called a slump or slumping.
b. Rock Slide
♦ Rock slide is the rapid and sudden downward movement of rocks or boulders
along a steep slope under the influence of gravity.
♦ It often results in road blockages, endangering lives and seriously interrupting
communication
and transportation.
c. Mud Flow and Earth Flow
♦ These are rapid and sudden downslope movement of waterlogged soil particles
in the form of a stream.
♦ Streamlike mudflows are produced in arid regions after sporadic, highly
localized and intense rains or due to snowmelt.
♦ They may bury surface features and vegetation, sometimes adjacent
settlements. For smaller scale mud flow, they may also block the drainage inlets
and cause flooding.
3. Vertical Movement
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a. Rock Fall
♦ It is the rapid, sudden, vertical and free falling of rocks or debris from a cliff.
b. Subsidence
♦ Subsidence is a sudden, rapid vertical movement of materials due to
undermining. It is caused by lack of support and vibrations.
EXAMPLES:
Landslide is a general term used to describe the sudden
down-slope movement of soil, rock and organic materials
under the influence of gravity. It also describes the landform
that results. (i.e. Hope Slide, Frank Slide)
Creep is the slow imperceptible movement of soil
and is noticeable wherever there are leaning fence
posts, crooked steps, sagging sidewalks and
cracked driveways. (i.e. Wilkinson Road Jail
stonewall)
Talus or scree slopes are formed from the rock
fragments that gather at the base of cliffs.
Slumps occur when small
block of land tilt or “slouch”.
(i.e. HWY #1 near
Thetis Lake)
Mudflows result from the rapid movement of water
saturated masses of clay and silt.
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IMPACT OF MASS WASTING AND SOIL EROSION
Soil erosion is a serious problem. It can undermine structures and paved areas, ruin the
environmental and aesthetic attraction of planted areas and clog downstream areas with
excess sediment.
1. Threat to Life and Property
There are several serious incidents of landslides and rock slides every year. They
cause loss of life and property. In a minor incident they may block only one lane of a
road, but in severe cases entire blocks of buildings collapse.
2. Environmental Impact
♦ Loss of vegetation
Mass wasting and soil erosion result in the loss of surface topsoil which is essential
for vegetation. As a result, more areas become barren.
♦ Scars and Gullies
In areas where topsoil and vegetation are removed, bare spots form scars in the
landscape. Gullies form deep grooves on weathered slopes through rain action and
mass wasting in areas with little or no vegetation. Intense gullying cuts up the
landscape into large-scale gullies and ridges and destroys the area. Gullying is
common in the bare, granitic areas.
MEASURES TO CHECK MASS WASTING AND SOIL EROSION
Since the impact of mass wasting, especially landslides, is so severe, some measures
can be taken to prevent accidents. See pages 412 to 416 for diagrams and additional
explanations. Similar diagrams are often found on the Provincial Exam.
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Chunam plaster or cement can be spread on steep slopes.
Gradients of steeper slopes could be reduced.
Potentially unstable rocks or soil masses should be removed.
Retaining walls or structures can be built to hold back possible slides.
Steep slopes should be inspected regularly, especially during periods of intense or
prolonged rainfall.
More surface drainage channels and ditches can be constructed.
Subsurface drainage systems may need to be constructed or improved.
Legislation can restrict development and building in dangerous zones.
Vegetation can be planted on steeper slopes.
Instruments can be installed to check slope instability, providing early warning in areas
of concern.
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IV. WIND AS AN AGENT OF EROSION
Wind is most effective on dry materials found in arid regions. Smaller particles are easier
to move. The terms sand, silt, clay and pebble/gravel refer to the size of particles. Wind
can move masses because it has energy from the Sun.
Particle Sizes (in mm)
boulder
cobble
pebble
sand
silt
clay
colloids
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>256 mm
64 – 256 mm
2 – 64 mm
0.0625 – 2 mm
0.0039 – 0,0625 mm
0.0005 – 0,0039 mm
< 0.0005 mm
Particle size is only important because it indicates MASS.
“Sand” refers to particle size, NOT composition.
require > 18 km/h winds to move them
bouncing action (saltation) causes scouring
most action < 1 m above ground level
usually composed of quartz but may be composed of calcite,
gypsum, quartz, mica, magnetite, obsidian or shells
- features formed by quartz sand → toadstools and ventifacts
Sand Grains -
Silt/Clay -
too soft to abrade most rocks
can lift very high and form dust clouds, even up into upper atmosphere
form very uniformly textured, compact deposits
sorted by energy of the wind
EROSIONAL EFFECTS OF WIND
1. Desert Pavement - removal of the fine products of erosion by wind action (deflation)
leaves surface of pebbles and boulders (ie Badlands of S. Dakota + SE Alberta + SW
Sask., Sahara + Gobi deserts, high deserts of Mongolia)
2. Blowouts - hollow depressions caused by wind action. Reaching the water table stops
this action and forms an OASIS. Vegetation also stops further action.
3. Loess - windborn deposits, highly sorted, fine soils, yellowish silt, angular particles
found in China, N. Europe, N. Central USA from 1 m. – 100 m. thick
4. Sand Dunes
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formed when wind-carried sand piles up against an obstruction
usually made of quartz sand
white sands Monument, N. Mexico is made of gypsum sand
Bermuda has dunes of white, coral-derive calcite sand
other sands: mica, magnetite, shells
shape depends on supply of sand, wind strength, vegetation
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Label the following:
Dune Migration
Types of Dunes
1. Barchan Dunes - steady wind, limited sand, cusps point downwind
2. Transverse Dunes - steady winds, abundant sand, series of ridges across the wind
3. Parabolic Dunes - form around blowholes from the material blown out, vegetation
anchors horns so cusps face upwind, oasis may form in blowout if it meets water table
4. Longitudinal Dunes - strong winds, one direction, thin sand cover, run parallel to wind
direction
5. Beach Dunes - usually traverse or parabolic
ASSIGNMENT:
Reference Pages: Chapter 5 p.86-93, 97-103
Chapter 18 p.385-393
Chapter 19 p.400-416
Complete the following worksheets and activities.
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Chapter 5 WS
Chapter 18 WS
Chapter 19 WS
Landslide Exercise (hand in for marks)
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IMPACT OF MASS WASTING AND SOIL EROSION