WeatheringEarth Science – Ms. Bray

Weathering and Soil Formation

Objectives

Define mechanical and chemical weathering.

Discuss agents of weathering.

Give examples of each type of weathering.

Two Important Types of Weathering

Chemical weathering

Rock changes composition and becomes a different type of rock.

Mechanical weathering

Breaks rock into smaller pieces by using physical processes

Chemical WeatheringRocks are decomposed, dissolved or

loosened by chemical processes to form residual materials

Chemical reactions break down the bonds holding the rocks together, causing them to fall apart, forming smaller and smaller pieces.

Chemical WeatheringThe most common types of

chemical weathering are: Oxidation HydrolysisCarbonation

OxidationOxidation takes place when

oxygen combines with other elements in rocks to form new types of rock.

These new substances are usually much softer, and thus easier for other forces to break apart.

OxidationIn this image you

can see rust starting to form on the left side of the rock.

This is due to oxidation, which is when the iron in rocks mixes with oxygen and creates rust.

HydrolysisHydrolysis occurs

when water combines with the substances in rocks to form new types of substances, which are softer than the original rock types.HYDRO means

WATERSo, just think:

water and other elements are breaking down the rock.

In this photo, you can see how waterflowed continuously and over time it created a hole in this rock.

Carbonation

Carbonation takes place when carbon dioxide reacts with certain types of rocks forming a solution, that can easily be carried away by water.

Carbonation

White carbonate veins where carbon dioxide has been transformed into rock show in a dark deposit.

Let’s review all of that real quick…

What is chemical weathering?Chemical reactions break down the bonds holding

the rocks together, causing them to fall apart

OxidationOxygen mixes with rock to make a new type of rock

HydrolysisWater breaks down rock

CarbonationCarbon dioxide reacts and breaks down rock

Mechanical Weathering(This is also called Physical Weathering!)

Breaks rock into smaller pieces. These smaller pieces are just like the

bigger rock, just smaller.

That means, the rock has changed physically without changing its composition.

The smaller pieces have the same minerals, in just the same proportions as the original rock.

Hang on a sec…This is the MAIN difference between

chemical and mechanical weathering!

In chemical weathering, the composition of the rock changes and a NEW type of rock is formed.

In mechanical weathering, the rock is broken down, but the type of rock is NEVER changed.

Ok, we can talk more about mechanical weathering now!

There are two types of mechanical weathering:Ice wedgingAbrasion

Ice WedgingIce wedging is

mechanical weathering where rocks are broken by water expanding as it is frozen.

When water flows into a crack and freezes it expands and breaks the rock.

In this picture, millions of years ago there was a glacier coveringthis rock. Water got into it, froze, expanded, and cracked the rock.

Ice Wedging or Frost Wedging(They mean the same thing )

Another example of frost wedging:

AbrasionRocks break into pieces by bumping into or

rubbing against each other.

Gravity causes abrasion as a rock tumbles down a mountainside or cliff.

Moving water causes abrasion as particles in the water collide and bump against one another.

Strong winds carrying pieces of sand can sandblast surfaces.

Ice in glaciers carries many bits and pieces of rock. Rocks embedded at the bottom of the glacier scrape against the rocks below.

Abrasion: What does it look like?Abrasion makes rocks with sharp or jagged edges

smooth and round.Like when you find pebbles at the beach!

This one is abrasion from water causing the rocks to rub against and bump into each other.

This one is abrasion from a glacier running over the rock and smoothing it out over time.

What now?You need some practice!

Go to the VOISE Earth Science homework page and click on the weathering review activity.