Weathering Earth 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 Weathering Rocks 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 Weathering The most common types of chemical weathering are: Oxidation Hydrolysis Carbonation

Oxidation Oxidation 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.

Oxidation In 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.

Hydrolysis Hydrolysis 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 WATER So, just think: water and other elements are breaking down the rock. In this photo, you can see how water flowed 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 Oxidation Oxygen mixes with rock to make a new type of rock Hydrolysis Water breaks down rock Carbonation Carbon 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 wedging Abrasion

Ice Wedging Ice 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 covering this 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:

Abrasion Rocks 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.