Transcript Weathering - MWMS HW Wiki

Weathering
Weathering
• Weathering is simply the breaking down of
rocks and other materials at Earth’s
surface.
• This may be done by mechanical OR
chemical means.
Types of Weathering
• Mechanical Weathering (physical)
– Takes place when rock is split or broken into
smaller pieces of the same material without
changing its chemical makeup.
Example: Breaking of a rock cliff into boulders
and pebbles
Weathering
Mechanical
• Common weathering processes:
– Ice wedging
– Temperature changes
– Root action
– Wind
– Running Water
1. Ice Wedging (Frost Action)
– Water takes up about 10% more space when
it freezes.
– This expansion puts great pressure on the
walls of a container.
– Water held in the cracks of rocks wedges the
rock apart when it freezes.
– Often occurs in places where temperatures
vary from below the freezing point of water to
above the freezing point.
Where does Ice Wedging Occur?
• Occurs mostly in porous rocks and rocks
with cracks in them
– Bare mountaintops are especially subject to
ice wedging.
Where do I see ice wedging?
• Ice Wedging causes:
– Vast fields of large, sharp-cornered boulders
– Potholes on streets and highways
2. Temperature Changes
(Exfoliation)
• Heat makes things expand, or become
larger.
• Cooling makes things contract, or become
smaller.
How does temperature change
break rocks?
• During the day, heat can cause the
outside of a rock to expand
• At night, the outside of the rock cools and
contracts
 This constant expanding and contracting
can weaken the rock and cause it to break
Repeated heating and cooling
may cause pieces to flake or
peel off.
3. Root Action
• Plant roots in search of minerals and water can grow into
cracks in rocks.
• The pressure of the growing root can make rocks break
apart.
• Example: Cracks in sidewalks
Tree roots pushing apart rock
4. Abrasion
• When rocks or sediments rub against
another rock.
• Could be caused by wind or running water.
5. Wind
6. Running Water
Friction &
Repeated
impact
7. Burrowing Animals
Types of Weathering
• Chemical Weathering (decomposition)
– Takes place when the rock’s minerals are changed
into different substances.
– Usually caused by reactions with oxygen, water, or
acids.
Agents of Chemical Weathering:
Water
1. Hydrolysis
• The chemical reaction of water with other
substances is called hydrolysis.
• When heated or dried a little, minerals that
have undergone hydrolysis may fall apart
• For example:
– Feldspar + H20  Clay
2. Oxidation
• The chemical reaction of oxygen with
other substances is called oxidation.
– Iron-bearing minerals are the ones most
easily attacked by oxygen.
– Examples:
• Magnetite
• Pyrite
• Oxidation of these minerals results in
rusting, or iron oxides  Rocks weaken
and crumble
Rusting
3. Carbonation
• Carbon dioxide dissolves easily in water.
– It forms a weak acid called carbonic acid
– This is the same compound that is in
carbonated drinks.
– This is how acid rain is produced.
What does Carbonation Affect?
• Has the greatest effect on
calcite than any other
mineral.
• This process changes the
mineral calcite  Limestone
and marble are made of
calcite so they are affected
by carbonation
4. Other Acids
• Some green plants can produce weak
acids  MOSSES
• Decaying organisms also produce acids
– These acids can seep into cracks and break
the rock apart.
Factors that affect the rate of
weathering
1. Climate
• The amount of water in the air and the
temperature of an area are part of the
CLIMATE.
• The more water there is in the air, the
faster weathering will occur.
• Weathering occurs fastest in hot, wet
climates; slowest in hot, dry climates
This is a monument called
Cleopatra’s Needle. It was
carved in Egypt around
1450 B.C. The sides are
carved with hieroglyphs, the
writing of ancient Egypt.
What is the climate like in
Egypt?
In 1800, the monument
was moved to New York
City. Almost immediately,
the
hieroglyphs began to fade.
In only a few years, the
Egyptian writing became
indistinct!
What is the climate like
in NYC?
What happened to Cleopatra’s
Needle?
•
•
•
•
Cleopatra’s Needle was carved from granite, a hard tough, crystalline rock.
Although it is tough, granite is changed by the atmosphere.
Some of the minerals that make up granite change to clay.
Chips and flakes of minerals break away from the granite surface.
2. Surface Area
• Most weathering occurs on exposed
surfaces of rocks and minerals.
• The more surface area a rock has, the
more quickly it will weather.
3. Rock Composition
• Some minerals that hold rocks together
weather more quickly than others.
• Quartz resists weathering.
• Feldspar weathers easily.
• Think about chemical weathering:
– Rocks with calcite, such as limestone, will
weather easily
– Rocks with iron will weather faster than those
without.
Limestone Statue
Oxidation of Copper
4. Chemical Reaction
• Carbonic acid dissolves calcite, a mineral
found in marble and limestone.
• Acid rain  Pollution speeds up the
process of weathering
– Factories and cars release carbon dioxide
and other gases, such as sulfur and nitrogen.
– These gases contribute to acid rain.
Hollows out great caverns in
limestone bedrock.