Processes of Change
EQ:
EQ:
How does weathering
occur?
Weathering
• Describe three ways
abrasion occurs in nature.
• List three things that
cause chemical
weathering of rocks.
• Describe the similarity in
the ways tree roots and
ice mechanically weather
rock.
• Describe five (5) sources
of chemical weathering.
Weathering
• Weathering is the process by which rock
materials are broken down by the action of
physical or chemical processes.
• Mechanical weathering is the breakdown of rock
into smaller pieces by physical means. (ice,
wind, water, gravity, plants, animals)
• Chemical weathering is the process by which
rocks break down as a result of chemical
reactions. Water, weak acids, and air can cause
chemical weathering.
6 Agents of Mechanical Weathering
• 1. Ice – water seeps into cracks during warm
weather. When the temperature drops, the
water freezes and expands, causing the ice to
push against the sides of the crack. This causes
the crack in the rock to widen.
– Abrasion – the grinding and wearing away of rock
surfaces through mechanical action of other rock or
sand pebbles.
– The three ways that can cause abrasion are wind,
water, and gravity.
Three Causes of Abrasion
• 2. Water – as rocks and pebbles roll along
the bottom of flowing water, they bump and
scrape against each other, causing these
rocks to become rounded and smooth.
• 3. Wind – wind blows sand and silt against
exposed rock eventually wearing away the
rock’s surface.
• 4. Gravity – rocks grind against each other
during a rock slide, creating smaller and
smaller rock fragments. Anytime one rock
hits another rock, abrasion takes place.
Plants and Animals
• Some plants (#6) can easily break rocks. The
roots grow through existing cracks in rocks.
• The growth causes the root to expand, forcing
the crack to widen. The force can eventually
split the rock apart.
• 7. Animals that live in the soil (moles, prairie
dogs, insects, worms, gophers), cause a lot of
weathering. By burrowing in the ground, these
living creatures brake up soil and loosen rocks
to be exposed to further weathering.
5 Agents of Chemical Weathering
• Common agents of chemical weathering are
water, acids, and air.
• These agents weaken the bonds between
minerals grains of the rock.
• 1. Water – can cause rock to be broken down
and dissolve. Can take thousands of years to
take place.
• 2. Air – the process of oxidation is a chemical
reaction in which an element (iron) combines
with oxygen, causing rust.
– (Weak Acids) - acid precipitation, acids in
groundwater, acids in living things.
Three Sources of Weak Acids
• 3. Acid Precipitation – rain, sleet, or snow that
contains a high concentration of acid.
Normal precipitation is acidic, acid
precipitation contains more acid than normal.
• 4. Acids in Groundwater – carbonic acid or
sulfuric acid reacts with rocks in the ground,
causing a chemical reaction, eating away at
the rock.
• 5. Acids in Living Things – Lichens produce
acids that slowly break down rock.
Summary
• Ice wedging is a form of mechanical weathering
in which water seeps into rock cracks and then
freezes and expands.
• Wind, water, and gravity cause mechanical
weathering by abrasion.
• Animals and plants cause mechanical
weathering by turning the soil and breaking
apart rocks.
• Water, acids, and air chemically weather rock by
weakening the bonds between mineral grains of
the rock.
Rates of Weathering
• What is differential
weathering?
• How does surface
area affect the rate of
weathering?
• How does climate
affect the rate of
weathering?
• Why do mountaintops
weather faster than
rocks at sea level?
Differential Weathering
• Differential weathering is a process by
which softer, less weather resistant rocks
wear away and leave harder, more
weather resistant rocks behind.
• Hard rocks weather more slowly than
softer rocks.
The Shape of Rocks
• Weathering takes place on the outer surface of
rocks.
• The more surface area exposed to weathering,
the faster the rock will be worn down.
• As the surface area increases, the rate of
weathering also increases.
• If a large rock is broken into smaller pieces,
weathering of the rock happens much faster.
• The rate of weathering increases because a
smaller rock has more surface area to volume
than a larger rock.
• More of the smaller rock is exposed to the
weathering process.
Weathering and Climate
• The rate of chemical weathering is faster
in warm, humid climates than cold, dry
climates because of oxidation.
• Oxidation happens when the temperature
is higher and when water is present.
• Water increases the rate of mechanical
(physical) weathering (ice wedging).
• Repeated changes in temperature (freeze,
thaw, freeze, thaw) is a major factor in
mechanical weathering.
Weathering and Elevation
• Mountaintops weather faster than rocks at sea level
because they are exposed to more wind, rain, and ice
than rocks at sea level or lower elevations.
• The increase in wind, rain, and ice increases the effects
of mechanical and chemical weathering. This increase
in elevation causes peaks of mountains to weather
faster.
• Gravity affects the rate of weathering:
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Steepness
Rainwater
Removal of sediment exposes new rock to weathering
Abrasion
Increased surface area of mountain
Summary
• Hard rocks weather more slowly than softer
rocks.
• The more surface area of a rock that is exposed
to weathering, the faster the rock will be worn
down.
• Chemical weathering occurs faster in warm,
humid climates.
• Weathering occurs faster at high elevations
because of an increase in ice, rain, and wind.
From Bedrock to Soil
• What is soil (loam)
formed from?
• What is bedrock?
• What is soil structure?
• What is humus?
• What are soil horizons?
The Source of Soil
• Soil is a loose mixture of small mineral
fragments, organic matter, water, and air
that can support the growth of vegetation.
• Bedrock is the layer of rock beneath soil.
• Parent rock is the rock formation that is
the source of soil.
• Wind, water, and movements of glaciers
can transport or move soil from one place
to another.
Soil Properties
• Soil is made from different-sized materials.
• Soil texture is the soil quality is based on
the proportions of soil particles.
• Soil texture can influence the ability of
water move through the soil.
• Soil structure is the arrangement of soil
particles.
Soil Properties
• Some soils are rich in nutrients, some are
poor in nutrients.
• A soil’s ability to hold nutrients and to
supply nutrients to a plant is describe as
soil fertility.
• Humus is the dark, organic material
formed in soil from the decayed remains of
plants and animals.
Soil Horizons
• Soil horizons are the horizontal
layers of soil.
• The top layer of soil is called
topsoil, containing more humus
than the other layers of soil, rich in
nutrients plants need to be healthy.
• Good topsoil is necessary for
farming.
Summary
• Soil (loam) is formed from the weathering
of bedrock.
• Soil texture affects how soil can be worked
for farming and how well water passes
through it.
• The ability of soil to provide nutrients so
that plants can survive and grow is called
soil fertility.
Soil Conservation
• Why is soil important?
• How can human
activity affect soil
erosion?
• What are three
important benefits
that soil provides?
• List five methods of
soil conservation.
Soil Conservation
• Soil can be endangered, just like plants
and animals.
• It takes thousands of years for soil to form,
it is not easy to replace.
• Soil conservation is a method to maintain
the fertility of the soil by protecting the soil
from erosion and nutrient loss.
The Importance of Soil
• Soil provides minerals and other nutrients
for plant life.
• All animals get their energy from plants.
• Soil also provides a place for animals to
live.
• The region a plant or animal lives is called
a habitat.
• Soil holds water for plants to get the
moisture or nutrients they need.
The Processes of Change
Lesson 18
Processes of Change (5)
• Weathering and erosion wear down,
deposition fills in Earth’s surface.
• Weathering is the slow wearing away or
breaking down of objects exposed to
Earth’s atmosphere
• Two kinds of weathering act on Earth’s
surface
– Mechanical weathering
– Chemical weathering
Mechanical Weathering (4)
• When objects are broken down into small
pieces but their chemical makeup doesn’t
change
• Wind and moving water are two main
causes of mechanical weathering
• Repeated changes in temperature (freeze,
melt, freeze, melt again)
Chemical Weathering (5)
• Material of an object is changed
• Produces underground caverns
• Statue of Liberty needed repairs because
of chemical weathering
• Examples:
– Rust
– Acid rain
Erosion (5)
• The natural moving of material from one
place to another
• Erosion transports weathered rock material
• Causes of Erosion:
– Moving water
– Gravity
– Wind
– Glaciers (moving rivers of ice)
– Waves
Deposition (5)
• Land torn down in one place is “deposited”
in another place
• Gravity can cause a landslide moving
mud, rock and soil down a hill
• Wind erosion can move sand and deposit
it in another area
• Glaciers (rivers of ice) scrape rocks off the
land and moves them downhill
Deposition (5)
• Glaciers will stop moving and even retreat and
cut a steep U-shaped valley in the land
• Erosion caused by mountain rivers form Vshaped valleys
• Hurricanes create waves that erode beaches
and cliffs
• Breaking of waves on a beach can wear it away.
The larger the waves, the faster is the rate of
erosion.