Download 2.1 Rocks and Weathering

Notes 2.1 Rocks and Weathering
Weathering & Erosion
Weathering and erosion continuously work together to wear down and carry away
rocks at Earth’s surface.
Weathering is the process in which rocks and other substances are broken down on
Earth’s surface. Water, ice, heat, and cold all help in the process.
Examples: Repeated freezing and thawing can crack rocks apart into smaller
pieces. Water can dissolve minerals that hold rocks together.
As rocks are broken into smaller pieces, erosion can carry the pieces away.
Erosion is the removal of rock particles by wind, water, ice, or gravity.
Geologists know that weathering and erosion have shaped Earth’s surface over
millions of years. Geologist make inferences based on the principle of
uniformitarianism, which states that the same processes that occur today operated
in the past.
Mechanical Weathering
Mechanical weathering is the process that occurs when a rock is physically broken
down into smaller pieces, over very long periods of time.
The causes of mechanical weathering include the following…
1) Release of Pressure
Pressure on a rock decreases when erosion removes material from the
surface of the rock, causing the outside of the rock to crack and flake off.
2) Freezing and Thawing
During ice wedging, water freezes in a crack in a rock. As the water freezes
it expands and causes the crack to widen and deepen. When the ice melts,
the water seeps deeper into the cracks. Eventually after repeated freezing
and thawing, the cracks expand until pieces of the rock break off.
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3) Animal Actions
Burrowing animals loosen and break apart rocks in the soil.
4) Plant Growth
Roots of trees and other plants grow in the cracks of rocks. As the roots
grow, they force the cracks further apart, eventually breaking the rocks into
smaller pieces.
5) Abrasion
Abrasion refers to the grinding away of rock by rock particles by wind,
water, ice, or gravity.
Chemical Weathering
Chemical weathering is the process that breaks down rock through chemical
changes.
The causes of mechanical weathering include the following…
1) Water
Water is the most important cause of chemical weathering. Water is able to
dissolve rock or other substances in the rock.
2) Oxygen
Oxygen is able to weather iron. Iron combines with oxygen in the presence
of water in a process called oxidation, which forms rust. Rocks that contain
iron will form rust, which makes the rock soft and crumbly. and gives it a
red or brown color.
3) Carbon Dioxide
When the gas, carbon dioxide dissolves in water, it forms a weak acid called
carbonic acid. Carbonic acid easily weathers rocks like marble & limestone.
4) Living Organisms
Plant roots produce weak acids that slowly dissolve rocks around the roots.
Lichens, plant-like organisms that grow on rocks, also produce weak acids
that chemical weather rock.
5) Acid Rain
Sulfur, carbon and nitrogen compounds produced by the burning of fossil
fuels can pollute the air. When these compounds react with water vapor in
clouds, they form acids. These acids mix with raindrops and fall as acid
rain, which causes rapid chemical weathering.
Rate of Weathering
The most important factors that determine the rate of weathering include the type
of rock and the climate.
Type of Rock
The minerals that a rock contains determines how fast it weathers. Rocks
that contain minerals that dissolve easily in water will weather faster than
rocks that contain minerals that do not dissolve easily in water.
Some rocks will dissolve more quickly because they are more permeable.
Permeable is a term used to describe materials that are full of tiny, connected
air spaces that allow water to seep through it. Permeable rocks weather
rapidly, since the water flowing through them dissolves and removes broken
down materials.
Climate
Climate refers to the long-term weather conditions in an area. Both
mechanical and chemical weathering occur faster in wet climates. Cool
climates provide for the freezing and thawing of water that causes
mechanical weathering. Since chemical reactions are faster at higher
temperatures, chemical weathering occurs more quickly in a climate that is
both hot and wet.