Chapter 5- Weathering and
Soil
Earth Science
6th Grade
Bell work Ch5 L.1
• Weathering: the mechanical and chemical
processes that change objects on Earth’s
surface over time
• Mechanical weathering: When physical
processes naturally breaks rocks into smaller
pieces
• Chemical weathering: Changes the materials
that are part of a rock into new materials
• Oxidation: Combines the element oxygen
with other elements or molecules
Lesson 1- Weathering
• How does weathering break down or
change rock?
• How do mechanical processes break
rocks into smaller pieces?
• How do chemical processes change
rocks?
Launch lab (work in pairs)
• Each pair gets 12 “rocks” (chocolate candies), 3
plastic cups and a jar with a lid. NO, YOU MAY NOT
EAT THE CANDY.
• Place four rocks in cup and the other eight in the jar.
Fasten lid tightly
• Shake jar vigorously 300x
• Remove half of the pieces and put them in second
plastic cup
• Replace lid and shake another 300x
• Put remaining pieces in third plastic cup
• ANSWER: compare and contrast rocks from each
cup. What do you think caused the rocks to change?
• TURN IT IN
Weathering and its effects
• Weathering: the mechanical and chemical
processes that change objects on Earth’s
surface over time
– Also changes Earth’s surface
– Break, wear, abrade and chemically alter rocks
and rock surfaces
• Weathering breaks rocks into smaller and
smaller pieces= sediment
– Sand, silt and clay (largest=sand, smallest=clay)
• Can change chemical makeup of a rock,
sometimes makes it easier to break
Mechanical Weathering
• When physical
processes naturally
breaks rocks into
smaller pieces
• Chemical makeup
is NOT changed,
just change the size
Mechanical Weathering
• Examples of mechanical
weathering
– Intense heat (like from forest fire)
causes nearby rocks to expand
and crack
– Ice wedging= water enters cracks
in rocks and when the water
freezes (at 0C), it expands and that
widens the crack.
– Abrasion= grinding away of rock by
friction of impact. A strong current
in a stream can carry rock
fragments downstream. Fragments
hit each other and grind each other
down into smaller and smaller
pieces.
Mechanical Weathering
• Examples of mechanical
weathering
– Plants= Can grow into the
cracks of rocks, roots
absorb minerals making the
rock weaker. The stems and
roots get wider and pushes
on the cracks making the
rock break.
– Animals= animals that live in
the soil create holes where
water an go and weather
rocks. They can also help
break down rocks as they
dig.
Mechanical Weathering
• Surface area: increases when
something is broken in smaller
pieces
– Amount of space on the outside of
an object
– Rate of weathering depends on
surface area exposed to
environment (for chemical
weathering too)
• Clay is 100x smaller than sand,
so it has an increased surface
area
– Means that weathering has greater
effect. Soil with small particles is
affected more than soil with larger
particles.
Chemical weathering
• Changes the
materials that
are part of a rock
into new
materials
– Changes the
composition and
the size
Chemical weathering
• Water and chemical
weathering
– Most substances dissolve in
water
– Minerals in rocks dissolve
slowly in water
• Dissolving happens when
minerals in a rock break
into smaller parts in a
solution
– Example: salt (sodium
chloride) breaks down into
individual sodium and
chloride ions when
dissolved in water
Chemical weathering
• Dissolving by acid=
increases rate of
chemical weathering
more than rain or
water.
– Attracts atoms away
from rock minerals and
dissolves them in acid
– Enter air naturally
(volcanoes) or form
pollutants (burning coal)
Chemical weathering
• Use pH to tell if a solution
is acidic (0-7), basic (7-14)
or neutral (7).
– Vinegar= 2-3 so it’s an acid
– Rain water= 5.6 so it’s
slightly acidic. Reacts with
carbon dioxide to form weak
acid, can dissolve rocks.
– Acid rain= 4.5 so it’s more
acidic than normal rain.
Happens when burning coal
produces sulfur oxide which
reacts with rain water.
Oxidation
• Combines the element
oxygen with other
elements or molecules
– Usually oxygen comes
from the air
• Addition of oxygen
produces an oxide
– Iron oxide is a common
oxide= hematite
– Aluminum’s oxide=
bauxite
Oxidation rate
• Not all parts of the
iron-containing rock
oxidize at the same
rate
• Part exposed to
oxygen oxidizes faster
than part not exposed
to oxygen
• Example: Iron oxideoxidized part turns red
(minerals are different
than minerals in the
center of the rock that
are not oxidized)
What affects weathering rate?
• Similar rocks can
weather at different
rates. Why?
– Environment helps
determine weathering
rate
• Both types of
weathering depend on
temperature and water
What affects weathering rate?
• Mechanical weathering
– Occurs fastest in environments with lots of
temperature changes
– Cycles of freezing and thawing or wetting
and drying
• Chemical weathering
– Occurs fastest in warm, wet places.
– Fastest in regions near equator
What affects weathering rate?
• Rock type
– Made of many minerals. Most easily weathered
mineral in a rock determines the rate (only as
strong as the weakest link)
– Low hardness= easier to undergo mechanical
weathering
– More mechanical weathering= more surface area
exposed
– More surface area exposed= more chemical
weathering
• Holes in a rock= easier to weather
HOMEWORK Ch5 L.1
• Vocabulary words on flash cards
– Memorize for quiz
• Lesson review questions p.155 #1-10
• Outline lesson 1
• Quiz!
Bell Work Ch5 L.2
• Soil: a mixture of weathered rocks, rock fragments,
decayed organic matter, water and air
• Organic matter: the remains of something that was
once alive
• Pore: Small holes and spaces in the soil
• Decomposition: the process of changing, once-living
material into dark-colored organic matter
• Parent material: Starting material of soil
• Climate: Average weather of an area
• Topography: Shape and steepness of the landscape
• Biota: All of the organisms that live in the region
• Horizon: Layers of soil formed from the movement of
the products of weathering
Lesson 2- Soil
• How is soil created?
• What are soil horizons?
• Which soil properties can be observed
and measured?
• How are soils and soil conditions related
to life?
Launch lab p.158 work in pairs
•
•
•
•
•
•
Place a cup of soil in a jar with a lid
Add a few drops of liquid soap
Add water until almost full
Firmly attach lid
Shake for one minute
Observe contents of jar after 2 minutes, after
5 minutes.
• ANSWER: How many different layers did your
sample form? What do you think makes up
each layer?
• TURN IT IN
What is soil?
• Soil is a mixture of
weathered rocks,
rock fragments,
decayed organic
matter, water and
air.
– Half the volume is
solid materials
– Life on Earth
depends on soil
What is soil?
• Fragments seen in
Ch5. L1 don’t
become good soil
until animal and
plants live in them.
• Plants and animals
add organic matter
to the rock fragments
– Organic matter= the
remains of something
that was once alive
Pores
• Small holes and spaces
in the soil
– Enable water to flow
into/out of the soil
– Size depends on
particles that make up the
soil
• As particle size increases=
pore size increases
• Can have water and air
in them, the amount
varies.
The organic part of soil
• Organic part of soil= solid part that was
once part of an organism.
– Pieces of leaves, dead insects, waste
products
– Roots die, leaves and twigs fall to the
ground and organisms in the soil
decompose these materials for food
Organic part of soil
• Decomposition: the
process of changing, onceliving material into darkcolored organic matter.
• Organic matter gives soil
important properties
– Dark color of soil absorbs
light
– Organic matter holds water
and provides plant nutrients
– Organic material holds
minerals together in
clusters= keeps pores open
for movement of water and
air
Inorganic part of soil
• Inorganic= materials
that have never been
alive.
• Mechanical/chemical
weathering of rocks
into fragments forms
inorganic matter in
soil
• Classify soil
according to size of
fragments
Inorganic part of soil
• Rock fragment sizes= boulders, cobbles, gravel, sand,
silt or clay.
– Sand- feels rough
– Silt- feels smooth
– Clay- feels sticky
• Large particles have large pores, they drain rapidly.
Small pores retain more water in the soil.
MiniLab p.159
• Observe soil sample, record sizes of
particles.
• Add a little bit of water to moisten the soil
• Rub the moist soil between your fingers
• Record observation
• Determine if sample is mostly sand, mostly
silt or mostly clay based on observations.
• ANSWER: what texture does your soil have
(when wet/dry)? What other properties did
you observe?
Formation of soil
• Many kinds of soil that form depend on
five factors= factors of soil formation
– Parent material
– Climate
– Topography
– Biota
– Time
Parent material
• Starting material of soil
– Rock/sediment that weathers
and forms soil
– Bedrock= rock that weathers
to form soil in the same
location where rock was
formed.
– Soil can be made from rocks
that were carried by
wind/water to another
location
• Particle size and type of
parent material can
determine properties of soil
Climate
• Average weather of an
area
– Precipitation and daily
and average
temperatures are
measures of climate
• Warm, wet climates can
form soil quickly
• Also places where lots
of freezing/thawing
occurs make soil
quickly
Topography
• Shape and steepness
of the landscape.
– Determines what
happens to the water
that reaches the soil
surface
• Flat landscape= most
water is absorbed into
soil
• Steep landscape= most
water runs downhill (can
carry soil with it)
Biota
• All of the organisms
that live in the region
– Help speed up the
process of soil formation
– Some form passages for
water to move through
– Most soil organisms
involved in
decomposition of
material that form
organic matter
• Weathering is
constantly acting
on rock and
sediment
– Soil formation is
constant, slow
process.
• Soil is still young
after 1,000 years
• Mature soil
develops layers
– New soil forms on
top of old soil
– Each layer has its
own
characteristics
Time
Horizons
• Layers of soil formed from
the movement of the
products of weathering
• Each layer has
characteristics based on the
kind of material it contains
• Three horizons common to
most soils
– A-horizon, B-horizon, Chorizon
• O-horizon= top organic layer
• R-horizon= unweathered
bedrock layer
Three common horizons
• A-horizon
– What you see when you
dig a shallow hole
– Darkest usually because
contains most organic
matter
• B-horizon
– Mostly clay deposited
from upper layer by water
seeping through A layer.
• C-horizon
– Layer of weathered
parent material (rock or
sediments)
Soil properties and uses
• Observing and measuring
soil properties
– Feeling the soil allows you to
estimate how much sand, silt
and clay is present
– Color is easily observed and
shows how much organic
matter the soil contains
• In the lab:
– Determine pH, nutrient
content to see if the soil is
suitable for farming
Soil properties and uses
• Soil properties that support life
– Nutrients in soil come from decaying and
weathering
– How well plants grow can tell you about the
nutrient content
– If add fertilizer, then it doesn’t really matter
• Soil formation takes thousands of years so
replenishing nutrients takes a long time
Soil types and locations
• The amount of
horizons and
amount of
weathering
depends on
location where soil
is forming
• Not all soil is alike
HOMEWORK Ch5 L.2
• Vocabulary words on flash cards
– Memorize for quiz
•
•
•
•
Lesson review questions p.165 #1-10
Outline lesson 2
Study for quiz
OPTIONAL Extra Credit p. 169-173 all.
You MUST copy down the entire
question and the answer. Only answers
will not be accepted.
Soil lab p.166-167
• Compare types of rock, climate and topography of Minnesota,
Colorado and Florida. Record similarities/differences.
• Examine soil profile from each location. Record
similarities/differences.
• Draw sample profiles and label A,B,C horizons
• How does each horizon relate to the factors of soil formation?
• Choose one location and model the profile using materials
provided. Label location and horizons.
• How are soil profiles affected by soil-forming factors?
• Were any profiles missing from any locations? Why might one
be missing?
• Was one of the horizons thicker in any of the profiles? Why?
• How does a soil profile relate to soil-forming factors?
• TURN THIS IN