Download Weathering and Erosion

…shaping, carving, and wearing landforms
Weathering and Erosion
Weathering and Erosion drive the rock cycle…
Weathering – process
of breaking down
materials (rock and
minerals).
Erosion – process of
moving sediments from
one place to another,
usually by wind, water,
gravity, or a combination
of these forces.
Given enough time, weathering and erosion can transform the
land by carving valleys, tearing down cliffs, and flattening
mountain ranges.
2
How do you know if a rock is being weathered?
Does it look like its starting to crumble?...have cracks?...color
changing?...How is this happening?
Types of Weathering:
Mechanical Weathering
- The substance undergoes a
physical change.
- Process of physically breaking
down materials without altering
the chemical composition of its
components.
Chemical Weathering
- The substance undergoes a
chemical change.
- Process that changes materials by
altering the chemical composition
of its components
3
Hinge Point Questions:
How are weathering and erosion different?
Weathering is the process of breaking down rock and
minerals while erosion is the process of moving sediments
from one location to another.
What are some signs of weathering?
Does the material look like it is going to crumble? Does it
have cracks? Is the color changing?
Mechanical Weathering
Physically breaking or wearing down the parent
material into smaller pieces without changing them
chemically.
A PHYSICAL CHANGE
Parent material can disintegrate, but not change is
composition.
TYPES
Freezing and Thawing
Abrasion
Living Organisms
Exfoliation
Freezing and Thawing: Mechanical Weathering
• Water gets into cracks in rock, the water freezes, it expands and the
outward force widens the cracks.
• The water thaws, the cracks can refill and continue the process.
• Over time, this process acts as a wedge, slowly widening the crack
until it breaks the rock apart.
• Ice Wedging most often occurs in regions of constant “freeze and
thaw” cycles in late winter and early spring.
• Examples: potholes in roads to giant cracks in a slab of rock on a cliff.
Freezing and Thawing: Mechanical Weathering
Abrasion: Mechanical Weathering
• Occurs when rock is gradually scraped and sanded down by
particles carried by water, wind, or ice.
• Sediment carried by flowing water can collide and gradually
grind away rocks in a streambed.
• Sediment can be carried by wind or slow moving glaciers
• Overtime, this sediment can carve and shape the land.
GLACIERS
RUNNING WATER
WIND
Exfoliation: Mechanical Weathering
Occurs when outer layers of rocks flake off caused by
changes in pressure or temperature due to the
expansion or contraction of rocks.
When rock expands after a massive
weight is removed.
• Glaciers recede and rock no longer has
compression.
Rocks in daily temperatures with
large changes undergo exfoliation.
• Hot during the day and cold at night,
daily cycle of expansion and
contraction.
Living Organisms: Mechanical Weathering
When living organisms cause rock to weather and break apart.
• Tree roots grow into cracks of rocks, widening the cracks as the tree
grows.
• Animals burrow (moles, gophers, and insects) and can loosen and
break apart rocks in the soil.
Living organisms played a critical role in revitalizing the Mt. St. Helens
region after the 1980 eruption.
• Gophers that survived the eruption deep underground helped to
mix the ash, soil, and rock, distributing nutrients in the soil evenly
allowing the conditions to be suitable for life.
Hinge Point Questions:
Identify and describe 3 of the 4 examples of
mechanical weathering.
• Ice Wedging – Freeze and thaw of water in cracks.
• Abrasion – Carve and polish rock by wind, water, and ice.
• Exfoliation – Outer layer of rock flakes off by compression
and/or heating differences.
• Living Organisms – Trees grow in cracks/Animals burrow
mixing soil
How can heating and cooling cause weathering?
As the rock is heated it expands, then contracts as
temperatures drop. This expansion and contraction can
cause the outer layer of rocks to flake off.
Chemical Weathering
Occurs when chemical interactions change the
chemical structure or composition of minerals within a
rock, breaking down the rock.
A CHEMICAL CHANGE
TYPES
Oxidation
Living Organisms
Hydrolysis
Carbonation
Oxidation: Chemical Weathering
Commonly known as rusting…occurs when rocks and minerals
are exposed to oxygen, reacting with certain minerals and
changing their chemical composition.
• Accelerated in environments of high moisture and
temperature.
• Rocks containing iron (Fe) oxidizes when exposed to oxygen
producing iron oxide (Fe2O3).
• This causes the rock to weaken and crumble easily.
Hydrolysis: Chemical Weathering
• Occurs when minerals react with ions, such as
hydrogen (H+) from water.
• When it rains acidic water comes into contact
with silicate minerals to form new minerals.
• This causes larger rocks to break into smaller
pieces or to change into silts or clays.
June 27, 2013
14
Carbonation: Chemical Weathering
• Occurs when carbon dioxide (CO2) dissolves in water (H2O), forming
carbonic acid (H2CO3).
• This process occurs naturally in the atmosphere creating acid rain.
• When acid rain falls on and seeps into the cracks of rocks rich in
minerals that react with acids (calcite - CaCO3), it will break down
the rock.
• The effects of acid rain can be seen in many old buildings, statutes
and tombstones that were constructed using limestone and marble,
which are both rich in calcite.
June 27, 2013
15
Living Organisms: Chemical Weathering
When living organisms secrete a weak acid that
dissolves some of the minerals in the rock.
The dissolved minerals provide the organism
with some of the nutrients that help them grow.
Hinge Point Questions:
Describe some evidence of chemical weathering.
• In chemical weathering, the might experience a change in
color of the rock, a substance growing on the rock, or a
new substance being created.
How is acid rain produced.
Atmospheric carbon dioxide mix with water vapor to
produce a weak acid when it rains.
Rates of Erosion
• Environmental conditions constantly bombard rocks
at different intensities, affecting the rates at which
they weather.
• Weathering is a gradual process that occurs over
hundreds, thousands, and even millions of years.
• There are 5 factors that affect the rate of weathering.
Type of Rock
Surface Area
Human
Activity
Exposure
Climate
Rates of Erosion
Type of Rock
Surface Area
• Each type of rock
(igneous, sedimentary,
and metamorphic) have
a unique set of physical
properties.
• The greater the surface
area exposed to an
erosional agent, the
faster it will weather.
• Igneous rock like granite
will weather slower than
softer rock like
sandstone, and
sandstone will wear
away slower than
limestone in a wet
environment.
• Overtime, the amount
of the rocks surface area
will increase, increasing
the rate of weathering.
• This process compounds
over time eventually
breaking down the rock.
Exposure
• Uncovered rocks will
generally weather faster
than covered rocks (soil,
water, or ice).
• Example: A large
boulder that is mostly
buried may not
weather as quick as a
small rock that is lying
on top of the ground.
Rates of Erosion
Climate
•
Climate is a combination of temperature,
humidity, rainfall, and sunlight at a given
region on Earth.
•
Different combination of these factors will
affect the rate at which rock weathers.
•
Hot and Humid conditions accelerate
oxidation of rocks containing iron.
•
Large variations in temperature can cause
exfoliation.
•
Regions of heavy precipitation will weather
sedimentary rock (sandstone) faster than
dryer climates.
Human Activity
• The biggest influence on
weathering rates. (ie: clearing
lands – construction)
• What might take millions of years
to naturally break down might take
a only days due to human activity.
EXAMPLES:
• Landscaping/agricultural practices
• Mining
• Clearing forest/trees
• Industrialization also affects
weathering rates - greenhouse