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MINERALS MAKE UP MOST ROCKS
DIFFERENT MINERALS HAVE DIFFERENT
PROPERTIES.
ROCKS ARE BROKEN DOWN TO FORM SEDIMENTS.
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HOW MECHANICAL WEATHERING BREAKS DOWN
ROCKS.
HOW CHEMICAL WEATHERING CHANGES ROCKS
WHAT FACTORS EFFECT THE RATE AT WHICH
WEATHERING OCCURS.
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WEATHERING IS THE PROCESS BY WHICH NATURAL
FORCES BREAK DOWN ROCKS INTO SMALLER AND
SMALLER FRAGMENTS.
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CONDITIONS IN THE ENVIRONMENT CAUSE
WEATHERING
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ROCKS BREAK DOWN INTO SMALLER PIECES AND
SEDIMENTS.
THESE SEDIMENTS CAN FORM ROCK OR SOIL
TWO TYPES OF WEATHERING
 MECHANICAL
 CHEMICAL
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Breaks rock apart without changing their chemical
composition.
This is a mechanical process
Rocks are broken into different shapes and smaller
pieces
Types of Mechanical Weathering
1.
Ice Wedging / Frost Action
2.
Pressure Release & Exfoliation
3.
Plant Root Growth /*Organic Activity
4.
Abrasion
5.
*Gravity
6.
*Temperature
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The repeated freezing and melting of water in
the crevices and cracks of rock.
Water turns to ice at freezing temperatures and
expands. When water in the pores or cracks of
rock freezes, this causes the rock to crack and
break over time.
*This is what breaks up the pavement in roads
and the cement in sidewalks. This action causes
winter potholes.
*This action is one factor that wears down
sharp mountain peaks into rounded hills.
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Rock under the earth is under great pressure.
This pressure pushes up against the rock, and the
pressure pushes the rock itself up.
Therefore, the pressure inside the rock is high
(near the bottom of the rock), the pressure at the
surface is low (near the surface of the rock).
Therefore the pressure from below is released at
the top of the rock.
This causes the rock to crack.
Exfoliation: The breaking of rock into sheets or
layers (like an onion skin).
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Plants and animals can cause mechanical
weathering.
Plants: Trees, bushes and other plant roots grow
into cracks of rocks where they find water and
nutrients (plant food). The roots grow and wedge
rocks apart. This is called *root-pry. Plant roots
can even break apart very large rocks.
Organic Activity
Animals: animals dig into rocky areas to find
shelter or food. This digging slowly scrapes away
at the rock causing it to weather.
Abrasion
 Water can wear down rocks on riverbeds, along shorelines, and
during tides.
 This process of wearing down by the rubbing of one object
against another (friction) is called abrasion.
Gravity
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Gravity pulls loosened rock down a mountain cliff.
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Landslide: A large movement of rocks and soil.
Temperature
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Rocks can be broken by changes in temperature.
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As the outside of a rock heats up it expands while the cool inside
of the rock remains the same. At night the rock cools and
contracts. This expanding and contracting causes particles on the
surface to crack and flake off.
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Exfoliation: Breaking of the rock in curved sheets or slabs.
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WATER AIR AND OTHER SUBSTANCES REACT WITH
THE MINERALS IN ROCK.
THIS CAUSES THE CHEMICAL COMPOSITION OF THE
ROCK TO CHANGE. SOME ROCKS DISSOLVE, OTHERS
ARE CHANGED INTO DIFFERENT MINERALS.
Types of Chemical Weathering
1.
Dissolving
2.
Rusting
1.
2.
3.
4.
Water
Carbonic Acid
Plant Acid
Sulfuric Acid
Water
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Some minerals dissolve in ordinary water. Examples: salt and
halite
*Carbonic Acid
Carbon dioxide (pollution from cars, fossil fuels, and decaying
organic matter) dissolves in water forming a weak acid called
carbonic acid.
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When Carbonic acid reacts chemically with another substance it is
called *carbonation.
water + carbon dioxide = carbonic acid
A. *Acid Rain
This acid is formed when carbon
dioxide in the air dissolves in
rain.
B. Pollutants increase the
amount of carbon dioxide in
the air.
C. Carbonic acid reacts with
calcite the main mineral in
limestone.
D. Granite: Carbonic acid reacts
with the granite mineral
feldspar to form the mineral
in clay called Kalolinite.
*Plant Acids
 Some plants give off weak acids that breakdown rock.
Examples: Lichen and moss
*Sulfuric Acid
 This acid is very strong. It is formed when pollutants from
coal burning and other combustion emit sulfur oxides into
the air. These compounds mix with rainwater.
*Rusting
Many common minerals contain iron. When these iron minerals
dissolve in water, oxygen in the air combines with the iron
to form iron oxides or rust.
*Oxidation
 Oxygen chemically combines with another substance.
Example: Iron in rocks combine with air to form rust.
Weathering occurs at different rates depending on the following factors.
*Surface Area: The more of a rocks surface that is exposed to the air, the faster
it will break down.
*Rock Composition:
A. Different rocks break down at different rates.
B. Rocks that resist weathering are called *Stable Rock
*Climate:
A. The amount of water and heat influence weathering. The climate effects
weathering of rock.
B. Example: Granite is stable in cool dry climates, but weathers quickly in
tropical climates
C. Heat causes weathering to happen more quickly.
*Pollution: The amount of pollutants effect weathering.
*Vegetation: The amount and kinds of vegetation effect weathering.
*Time: The length of time a rock is exposed to the Earth’s surface affects the
amount of weathering.