Transcript Chemical Weathering

Chapter 5
Weathering = breakdown
of rock and minerals

2 Types of weathering
Mechanical
Chemical

Mechanical Weathering =
physical forces breakdown
rocks into smaller pieces
without changing the
mineral composition
Frost Wedging
Unloading (exfoliation)
Biological Activity
 Increases the surface area of the
rock

The mechanical breakup of
rock caused by the freeze-thaw
cycles of water in the cracks of
rocks.
 When water freezes, its volume
expands
The freeze-thaw cycles break
rock into pieces
Cracks are formed from water
freezing, expanding, and
breaking the asphalt or concrete
Sections of rock that are wedged
loose may tumble into large
piles called talus which typically
form at the base of steep rocky
cliffs
Most common in mountain
regions of middle latitudes
Exfoliation- reduced
pressure on igneous rock
causes it to uplift and allows
slabs of outer rock to break
off in layers

Plants , burrowing animals
and humans affect the
weathering of rocks.
 Roots
 Construction
 Animals digging into rock
Occurs when the chemical and
mineral composition of a rock
have been changed
 WATER is the most important
agent of chemical weathering
Water absorbs gases from the
atmosphere and chemically
react with minerals :
 EX: Water + CO2 + Calcite =
Carbonic Acid
breaks down limestone
 EX: H2O + O2 + iron + = rust
 EX: H2O + sulfur gas OR nitrogen gas =
Acid Rain
 Evidence of chemical weathering
Mechanical weathering affects
the rate of chemical weathering
 Mechanical weathering breaks rocks
into several pieces, which
INCREASES the surface area of
exposed rock, allowing faster
chemical weathering to take place.
3 things that affect the rate of
weathering
 Rock characteristics (chemical
composition)
 Climate
 Surface Area (amount) of exposed
rock
 Rock’s mineral composition affects
weathering
 Granite is resistant to chemical
weathering
 Marble chemically weathers very
quickly
 Marble is formed from calcite which
easily dissolves in acid
 Affected by temperature and
moisture
 Chemical weathering is most
effective in areas with high
temperatures and an abundance
(lots) of moisture
 Tropical- high temperatures + high
moisture = lots of chemical
weathering
 Polar- low temperatures + low moisture
= little or no chemical weathering
 Deserts- high temperatures + low
moisture = very slow chemical
weathering
Climate with most favorable
weathering conditions has
high temperatures &
lots of moisture
Cracks allow water to seep into
the rock
 The more the crack & crevices,
the greater the weathering
Soil Composition
4 major components of soil
 45% Mineral Matter (mineral broken
down rock)
 25% Water
 25 % Air
 5% Organic Matter or dead stuff
(decayed remains of organisms)
Soil texture is determined by particle size
 sand (large)
 silt
 clay (small)
 Loam - a mixture of all three sizes
 Best suited for supporting plant life
Parent Material
Time
Climate
Organisms
Slope
 Source of mineral matter in the soil
 TOPSOIL- PARENT MATERIAL IS THE
MINERAL MATTER
 SUBSOIL- PARENT MATERIAL HAS BEEN
DEPOSITED FROM ELSWHERE AND
WEATHERS
Important to ALL
weathering
the longer the soil has
been forming the thicker
it becomes
 Major effect on soil formation
 Variations in temperature and
precipitation influence rate, depth,
and type of soil
 *change weathering in your notes to “soil” )
 Hot & wet climate may produce a thick layer of
chemically weathered soil
 Dry & cold climates may produce only a thin layer
of chemically weathered soil
 Influence the soil’s physical and
chemical properties
 Plants are the main source of organic
matter in the soil
 They also aerate the soil (provide air to
the soil)
 Steep slopes erode quickly and have
little or no soils
 Flat slope has little erosion and
produces quality soils.
 North slopes are cooler and
wetter (poorer soil)
 South slopes are warmer and
drier (better soil)
*** soil forms best in hot/moist, tropical climates ***
 Soil profile shows all of the soil
horizons in a cross section
 Soil varies in composition, texture,
structure, & color at different depths
 Variations divide the soil into zones
called horizons
Horizons
 Mature soils have three distinct soil
horizons
 A Horizon
 B Horizon
 C Horizon
 A Horizon = known as topsoil
 consists of organic matter, insects, fungi &
microorganisms
 B Horizon = known as subsoil
 Contains:
 fine clay particles washed out of A horizon, by
water that filters through pore spaces
 Lower limit of most plant roots & burrowing
animals
C Horizon = falls between the B
horizon and unaltered parent
material
 Contains partially weathered
parent material
 Unaltered parent material
 The bedrock that has NOT gone
through any weathering
Loss of fertile topsoil
 major problem in United States
We are eroding much more soil
now than we have in the past
Cross out “water carries tiny pieces of _______
All the way to
Rate of Erosion
 Depends on soil characteristics, climate &
slope, and the amount of vegetation
 Accelerated by human activity
 Removal of natural vegetation by:
 Farming
 Logging
 Construction
Water erodes soil faster than
wind
 Exception with unprotected spoil.
EX: 1930’s Dust Bowl
 Reservoirs fill up with sediment
 Sediments are contaminated by
pesticides and fertilizers
 Planting rows of trees called
windbreaks
 Prevents wind erosion
Terracing hillsides

Prevents water & wind erosion
Soil erosion is accelerated
by removing protective
natural vegetation
Human activities such as farming,
logging, and construction
increase erosion greatly = soil is
being eroded faster than it can be
created

EX: rivers are becoming
clogged with eroded sediment
from farmland = interfere with
river transportation, increase
pesticide levels in water, fill up
reservoirs, and increase
flooding