Weathering, Erosion and Soil
Formation
What is weathering?
Weathering is the process by which
rock materials are broken down
Mechanical Weathering
• The breakdown of rock
into smaller pieces by
using physical forces.
• Agents:
• Ice
• Water
• Wind
• Gravity
• Plants
• Animals
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Chemical Weathering
The process by which
rocks break down as a
result of chemical
reactions.
Agents:
Water
Weak acids
Air
ICE
• Frost Action: the
alternate freezing and
thawing of soil and rock.
• Ice wedging: when water
seeps into cracks during
warm weather, then
freezes and expands
during cold weather.
• The ice pushes against
the sides of the crack,
causing the crack to
widen.
Abrasion
• Abrasion is the grinding and wearing away of rock surfaces
through the mechanical action of other rock or sand particles.
• There are three forms of abrasive weathering:
• Water
• Wind
• Gravity
Water
• Water is the most
powerful agent of
mechanical weathering.
• When rocks and pebbles
roll along the bottom of
a swiftly flowing bodies
of water, they bump and
scrape each other.
• These rocks/pebbles
eventually become
smaller, rounder,
smoother as a result of
this friction
Wind
• When wind blows
sand and silt against
exposed rock, the
sand will wear away
the rock’s surface.
• These rocks have
been shaped by
blowing sand.
• These rocks are
called ventrifacts
Gravity
• Abrasion also
happens when rocks
fall on one another.
• Rocks grind against
each other as they
tumble, creating
smaller and smaller
rocks.
Plants
• Plants often send
their roots into to an
existing crack in a
rock.
• As the plant grows,
the expanding root
becomes so strong
that the crack widens
and the rock splits!
Animals
• Animals can cause a lot
of weathering!
• Burrowing
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worms
Ants
Mice
Coyotes
rabbits
• Burrowing moves soil and
exposes fresh surfaces to
weathering
• Some types of tropical
worms can move an
estimated 100 metric tons
of soil per acre in a year.
Review
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Name three things that can cause abrasion
Wind
Water
Gravity
• What is the most powerful agent of weathering?
• Water
• Describe the similarity of how ice and tree roots mechanically
weather rock
• Both ice and tree roots can force cracks in rocks to expand
Chemical Weathering
• The process by which
rocks break down as
a result of chemical
reactions is called
chemical weathering.
• Common agents
• Water
• Weak acids
• Air
Water
• Over thousands of
years, water can
dissolve even the
hardest rocks
• Usually it is by way of
rain, sleet or snow
with a high acid
content
Acid
• Acid can chemically
weather rocks in
different ways
• Acid precipitation
• Acids in groundwater
• Acids in living things
Acid precipitation
• Rain, sleet or snow that that contains a high concentration of
acids is called acid precipitation.
• All precipitation is naturally acidic, but acid precipitation has
higher levels of acid.
• This higher level of acidity can lead to very rapid weathering
• Causes:
• Volcanoes
• Air pollution (burning fossil fuels)
Acids in groundwater
• Some acids, such as carbonic
and sulfuric acids occur
naturally in groundwater.
• If these acids come in contact
with certain rocks, such as
limestone, a chemical
reaction occurs.
• Over a long period of time,
the limestone dissolves,
forming caverns
• Stalactites
• Stalagmites
Acids in living things
• Lichens, which
consist of fungi and
algae living together,
produce acids that
slowly break down
rock.
Air
• Oxygen in the air
reacts with
elements, such as
iron, to chemically
weather objects
• Called oxidation
• Water is not
necessary, but
speeds up the
process
• RUST
Mechanical vs. Chemical Weathering