WEATHERING

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WEATHERING

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WEATHERING & EROSION weathering = change in physical form or chemical composition of rock materials exposed at earth s surface TWO MAIN TYPES – PowerPoint PPT presentation

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Title: WEATHERING

1
WEATHERING EROSION

weathering change in physical form or chemical
composition of rock materials exposed at
earths 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 earths forms or
landforms
geomorphic provinces areas with distinctive
landforms and
topography
Ex Hawaiian Province Appalachian Provinces
landscapes physical features of the Earths
surface found in
an area

landforms features that make up the shapes of
the Earths
land surface
1. result of two opposing processes
a. one bend, breaks, and lifts earths 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
27

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

33
THE KOPPEN CLIMATIC CLASSIFICATION SYSTEM
Climate Type Description
A Tropical moist climates all months have average temperatures above 18oC(64oF).
B 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
34

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 aluminum-
rich 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