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• Abrasion• Wind • Water • Gravity

• Plants• Animals

Chemical Weathering• The process by which

rocks break down as a result of chemical reactions.• Agents:

• Water• Weak acids• Air

ICE

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

• 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• 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

CausesVolcanoesAir 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