WEATHERING & EROSION
weathering = change in physical form or chemical
composition of rock materials exposed at earth’s surface
TWO MAIN TYPES
mechanical weathering = physical processes break down
rock into smaller pieces
1. does not change chemical compositions of rocks and
minerals
2. also called disintegration or
physical weathering
3. common agents include ice,
plants, animals, gravity,
running water, wind
4. main factors that cause processes are temperature and
pressure
5. processes:
a. exfoliation = curved sheets peel
away from exposed bedrock
1) joints = long, curved cracks
parallel to surface
2) overlying rock gradually erodes
3) also called pressure release fracturing
b. ice wedging = water freezes in cracks,
expands, and forces rock apart
1) also called frost wedging
2) volume of water expands by 10%
when frozen
3) each time ice thaws and refreezes it goes further into
rock widening crack
4) common at high elevations and climates where
temperature varies above and below freezing
c. salt cracking = salts that are dissolved in water found in
pores of rock will crystallize when water evaporates
causing widening of cracks and pushes grains apart
d. abrasion = wearing and grinding
of rock surfaces by
friction & impact
- produces rocks with smooth,
rounded appearance
e. root pry = seed germinates in
rock crack, pressure from
developing root enlarges crack
f. digging activities of burrowing animals expose new rock
surface
g. thermal expansion and contraction
1) at surface rocks warm and cool as air temperature
changes
2) rock expands when heated and contracts when it is
cooled
3) if heated rapidly surface will expand faster than interior
which may cause fracture
4) hot object will crack when cooled too rapidly
chemical weathering = processes break down rock by
changing its chemical composition
1. also called decomposition
2. caused by water, acids, bases,
oxygen, and other substances
3. all other factors being equal, rate of reaction doubles with
each 10oC increase in temperature
4. processes:
a. hydrolysis = chemical reaction of water with other
substances
1) water can dissolve minerals as
it passes through rock and then
deposit different substances in
their place
2) leaching = when water carries
dissolved minerals to
lower layers of rock or soil
b. carbonation = minerals react with carbonic acid
1) carbon dioxide from air dissolves in
water to create a weak carbonic acid
2) speeds up hydrolysis
3) can produce underground caverns
4) results in formation of clay minerals
c. oxidation = chemical reaction of oxygen with other
substances
1) usually turns useful material to waste
2) attacks iron-bearing minerals
3) causes mineral to rust or turn
reddish in color
d. acid precipitation = nitrogen and sulfur oxides in air
combine with water
1) more corrosive than pure water
2) dissolves minerals faster
e. plant acids
1) humic acid = produced by plant roots
and decaying plants
2) usually lichens and mosses that
grow on rocks
spheroidal weathering = occurs more
quickly at corners and edges of rock
outcrops and boulders causing them
to become rounded
RATES OF WEATHERING depends on:
1. composition of rock: determines resistant to being broken
down
a. determined by materials that holds fragments of
sediments together
b. sedimentary rocks are generally more easily weathered
than igneous or metamorphic rocks
c. quartz least affected by chemical and mechanical
weathering
2. amount of exposure
a. amount of time exposed: longer exposure results in
more weathering
b. amount of surface area: more are exposed, more
weathering
1) increased by fractures and joints
2) weather more rapidly
3. climate: variables include precipitation, temperature, and
evaporation
a. greatest effect due to rainfall and freezing and thawing
produced by alternating hot and cold weather
b. rate of chemical reactions increase as temperature
increases
c. very rapid in warm, humid areas
d. slower in hot, dry areas and very cold climates
4. topography = elevation and slope of surface
a. colder at higher elevations so ice wedging often
increases
b. steep slopes allow fragments to be pulled downhill by
gravity and washed away
c. as materials move down slope exposes new surfaces for
weathering to occur
EROSION
erosion = process that removes and transports weathered
sediments from one location to another
1. four main agents: gravity, running water, glaciers, wind
2. have several things in common:
a. carry sediments only when have enough energy of
motion
b. all drop load of sediments when
energy of motion decreases =
deposition
3. gravity causes rock fragments/materials
to move down slopes
4. water has more power to move large particles of
weathered material than normal winds
a. greatest when water is moving rapidly
b. can carry more material over longer distances before
depositing
5. watershed = network of streams that drain an area as
water works its way to ocean
a. rill erosion = caused by running water in small channels
on the side of slopes
b. gully erosion = forms a deep and
wide channel
c. streams and rivers carry billions of
metric tons of sediments and
weathered materials to coastal
areas every year
6. glacial erosion
a. effects are large-scale due to ability to
scrape and gouge out large sections as
move
b. able to carry huge rocks and piles of
debri over large distances
7. wind erosion
a. effects areas of limited precipitation and high temperatures
most
b. usually very little vegetation in place to hold soil in place
c. wind blown particles can damage natural structures and
human structures
d. windbreaks = trees or other plants grown perpendicular
to direction of wind as a wind barrier
8. plants, animals, and humans add to erosion by moving soil
location, digging burrows, building highways, etc.
EROSION OF LANDFORMS
geomorphology = study of earth’s forms or landforms
geomorphic provinces = areas with distinctive landforms and
topography
Ex: Hawaiian Province; Appalachian Provinces
landscapes = physical features of the Earth’s surface found in
an area
landforms = features that make up the shapes of the Earth’s
land surface
1. result of two opposing processes
a. one bend, breaks, and lifts earth’s crust making elevated
landforms
b. one is the wearing action of weathering and erosion
2. three major types:
a. mountains = steep landforms of very high elevations
b. plains = large, relatively flat areas that do not rise far above
sea level usually
1) makes up 1/2 of all land in U.S.A.
2) very small differences in elevation
3) areas of low relief
c. plateaus = high elevation flat surfaces
1) rise more than 600 meters up to more than 1500 meters
above sea level
2) areas of nearly horizontal rocks that
have been uplifted by forces within Earth
3) most are located inland, few near
the ocean
4) those next to an ocean end in a cliff
5) many are dry, nearly desert areas
EROSION OF MOUNTAINS
1. youthful mountains = those that are being lifted
a. rugged, have sharp peaks and deep, narrow valleys
b. usually rises faster than it is eroded
2. mature mountains (intermediate) = no longer rising
a. rounded peaks and gentle slopes
3. old mountains = reduced to a low, almost featureless
surface near sea level
a. peneplain = low, almost level surface usually with low,
rolling hills
1) have been mistaken for plains
2) rocks beneath have been tilted
and folded by tectonic forces
b. monadnocks = knob of rock that
protrudes above a peneplain
4. playa = dry desert lake bed
a. in arid regions
b. could be a low
area between
mountains
EROSION OF PLAINS AND PLATEAUS
1. plains
a. characterized by broad rivers and streams
b. plants are mostly grasses
c. types:
1) coastal plains = low, flat areas along the coasts
a) called lowlands due to low elevation
b) marsh = grassy wetland that is usually flooded with
water
2) interior plains = low flat areas found inland on a
continent
a) somewhat higher above sea level
b) have good soil for farming
3) high plains = those along the foot of the Rocky
Mountains
a) rise over 1500 m above sea level
2. plateaus
a. young = deep stream valleys separating broad, flat
regions
b. mature = eroded into rugged hills and valleys
c. landforms created by erosion and weathering
1) mesas = small, table-like areas
a) flat topped hills
b) covered by layer of resistant rock protecting soft
rock beneath
2) buttes = small, narrow
topped formations
a) have steep slopes
b) smaller than mesas
WEATHERING AND SOIL
1. regolith = layer of weathered rock fragments mixed with
clay and sand
2. bedrock = solid, unweathered rock that lies beneath the
regolith
a. often referred to as parent rock
3. soil = complex mixture of minerals, organic material, water
and gases
a. result of chemical and biological activity over long periods
of time
b. soil-forming process begins
when weathering breaks
solid bedrock into smaller
pieces
4. humus = dark, organic material
a. made of pieces of decaying plants and animals
b. light and spongy, readily absorbs water
c. soils rich in this will usually be dark colored
d. holds in soil nutrients and makes them available to
plants
e. part of any fully developed soil
SOIL EROSION
1. occurs worldwide and is normally slow
2. deposited elsewhere as transported soil
3. accelerated erosion
a. overgrazing = occurs when livestock eat every bit of
grass off land
b. desertification = desert formation
1) damaged by overgrazing in areas that receive little
rain
2) changes wind patterns and drives away moisture
laden clouds
3) happening on
every continent
c. clearing of forests for farming
1) soil in tropical rain forest depends on nutrients
provided by forests
2) when cleared farmers deplete nutrients from soil in a
few years
d. gullying = furrows plowed up and down slopes allowing
faster water movement creating larger furrows
e. sheet erosion = strips away parallel layers of topsoil
exposing subsoil
1) continuous rainfall
2) wind during dry periods
3) “dust bowl” = poor farming practices
and drought resulted in topsoil
being carried away by the wind
creating dust storms
SOIL COMPOSITION
1. three main types of material: include clay, sand and silt
2. amount of each present determined by composition of
parent rock
a. clays form from feldspars
b. sandy soils form from granite and rocks with a lot of
quartz
c. length of time it takes for soil to form also determined by
parent rock
d. proportion of minerals in soil
and in parent rock may not
be the same
3. loam = most fertile type of soil which is a mixture of
sand, clay, silt, and generous amounts of organic
matter
4. residual soil = has a bedrock as its parent material
a. form in place by gradual weathering of parent rock
b. layer of different colors and textures form
5. transported soil = weathered mineral grains carried away
from location of parent rock by water, wind, or
glaciers
a. horizons are poorly seen or absent
b. different composition than the bedrock where it is
deposited
Loamy soil
soil profile = vertical section/sequence of soil layers
1. commonly present in residual soils
2. horizons = several distinct layers seen in undisturbed soil
3. O horizon = uppermost layer of mature soil named for its
organic component
a. contains litter = organic matter that has fallen to ground
and started to decompose but still retains its original form
b. also has humus with a small proportion of minerals
4. A horizon = second layer
a. generally gray to black
b. has humus and organic materials
c. most fully evolved layer
d. topsoil = mixture of organic and small rock materials
1) includes both O and A horizons
e. zone from which surface water leaches minerals =
zone of leaching
5. B horizon = third layer
a. also called the subsoil
b. has little or no humus, fewer organisms
c. lighter in color (usually red or brown)
d. some plant roots reach this layer
e. clay gathers here
f. zone of accumulation = dissolved ions and clay
accumulate
6. C horizon = partially weathered bedrock
a. first stages of mechanical and chemical change
b. no humus present
c. very little organic matter
d. lies directly on unweathered parent rock
7. R horizon = unweathered bedrock
soil-forming factors = control how soil develops as parent
material weathers
1. parent rock
a. whether texture is resistant or easily weathered
b. provides nutrients to soil
2. time
a. young soils - weathering of feldspar and other minerals
may be incomplete
b. mature soils have more clay content
c. new materials deposited mix and change the
composition and texture
3. climate
a. determines weathering processes
b. 5 basic climate types based on Koppen climatic
classification system
c. 2 most important factors are temperature and moisture
d. these affect:
1) physical weathering processes and determine
2) microenvironmental conditions for soil organisms,
3) plant growth,
4) rates of decomposition,
5) soil pH, and
6) chemical reactions occurring
in soil
THE KOPPEN CLIMATIC CLASSIFICATION SYSTEM
Climate
Type
A
B
Description
Tropical moist climates: all months have
average temperatures above 18oC(64oF).
Dry climates: with deficient precipitation during
most of the year.
C
Moist midlatitude climates with mild winters
D
Moist midlatitude climates with cold winters
E
Polar climates: with extremely cold winters and
summers
4. topography affects thickness of soil in areas
a. on steep slopes rainwater washes away most topsoil
which will produce thin, coarse, & infertile soils
b. valley floor (lowlands) has deepest and richest soil
c. aspect = orientation of slope with respect to compass
direction
1) sun affects side of hill causing water to evaporate
therefore there are fewer plants and weathering
occurs more slowly [soil development slowed]
2) south-facing slopes receive more sunlight which
allows more vegetation growth & in turn allows for
thicker soils than slopes facing other directions
SOIL TYPES
1. polar soils (arctic climates)
a. formed mostly by mechanical processes
b. form at high elevations and high latitudes
c. no distinct horizons, soil thin (some only few cm deep)
and made up of rock fragments
d. some surface areas poorly drained and boggy
e. bottom layers constantly frozen = permafrost
2. temperate soils (temperate climates): temperature range
between cool and warm, rainfall not excessive
a. support forests, grasslands, and prairies
1) grasslands: rich in humus, rich fertile soils
2) forests: less deep, less fertile soil; have aluminumrich clays and iron oxides
3) prairies: drier, supports only grasses and bushes
b. specific amount of rainfall in area determines type of
vegetation that will grow
c. three major horizons can be several meters thick
d. pedalfer soils = contain clay, quartz, and iron compounds
1) in areas that receive more than 65 cm of rain a year
e. pedocal soils = contain large amounts of calcium carbonate
1) in areas that receive less than 65 cm of rain a year
2) soil less acidic and very fertile
3. desert soils (desert climates)
a. rainfall less than 25 cm per year, very dry so weathering
slow
b. soil thin, often have high levels of accumulated salts
c. vegetation has to be adapted to surviving long periods of
drought
d. profiles seldom more than a few centimeters from top to
bottom
e. fertile when watered
4. tropical soils (humid tropical climates)
a. thick soils develop rapidly due to high temperatures and
heavy rainfall
b. profile may be 3 meters thick
c. laterites = soil with iron and aluminum minerals that do
not dissolves easily in water
d. rain leaches minerals from A horizon, thin humus layer
covers B horizon
e. heavy rains wash away most of
topsoil keeping layer
thin and relatively infertile
f. characteristic red color due to
oxidation of iron and aluminum
SOIL TEXTURES
1. classified by size of their particles
2. relative proportions of these particles determine soil texture
3. affects capacity to retain water and ability to support plant
growth
4. process to determine texture:
a. place soil sample along with water in clear jar and shake
jar well
b. allow particles to settle
c. sediments will sort as they settle;
determine and estimate percentage
of settled clay, silt, and sand
d. use this information along with
textural triangle (p.173, Fig. 7-24) to
determine soil texture
5. characteristics:
a. clays: less than 0.002 mm in diameter
1) very fine particles, compact easily, forms large dense
clumps when wet
2) low permeability to water therefore upper layers
become waterlogged
3) holds positively charged ions
b. gravel (coarse) = unconsolidated mixture of rock
fragments or pebbles
c. loams: best soils for crops; holds water but does create
waterlogging
d. sand: 0.06 to 2 mm; water flows through too fast for most
crops, only good for crops needing low amounts of water
e. silt: 0.002 to 0.06 mm; sedimentary material of very fine
particles intermediate in size
soil fertility = how well can support growth of plants
1. affected by availability of minerals and nutrients, number
of microorganisms present, amount of rainfall available,
topography, and level of acidity
2. natural and commercial fertilizers can be used to replace
minerals
3. legumes can be planted and grown to help replace
nitrates
4. limestone can be added to reduce
acidity
5. adding compost, organic mulch,
and peat moss help maintain fertility
SOIL CONSERVATION
1. growing cover plants to protect topsoil and hold in place
2. contour plowing = plow soil in circular bands that follow
the shape of land
3. strip-cropping = crops are planted in alternate bands
a. often used with contour plowing
b. together can reduce soil erosion by 75%
4. terracing = constructing step-like ridges that follow shape
of sloped field
5. crop rotation = plant one type of
crop one year and
different type next year