Weathering

Weathering Weathering is simply the breaking down of rocks and other materials at Earth’s surface. This may be done by mechanical OR chemical means.

Types of Weathering Mechanical Weathering (physical) Takes place when rock is split or broken into smaller pieces of the same material without changing its chemical makeup. Example: Breaking of a rock cliff into boulders and pebbles

Weathering Mechanical Common weathering processes: Ice wedging Temperature changes Root action Wind Running Water

1. Ice Wedging (Frost Action) Water takes up about 10% more space when it freezes. This expansion puts great pressure on the walls of a container. Water held in the cracks of rocks wedges the rock apart when it freezes. Often occurs in places where temperatures vary from below the freezing point of water to above the freezing point.

Where does Ice Wedging Occur? Occurs mostly in porous rocks and rocks with cracks in them Bare mountaintops are especially subject to ice wedging.

Where do I see ice wedging? Ice Wedging causes: Vast fields of large, sharp-cornered boulders Potholes on streets and highways

2. Temperature Changes (Exfoliation) Heat makes things expand, or become larger. Cooling makes things contract, or become smaller.

How does temperature change break rocks? During the day, heat can cause the outside of a rock to expand At night, the outside of the rock cools and contracts  This constant expanding and contracting can weaken the rock and cause it to break

Repeated heating and cooling may cause pieces to flake or peel off.

3. Root Action Plant roots in search of minerals and water can grow into cracks in rocks. The pressure of the growing root can make rocks break apart. Example: Cracks in sidewalks

Tree roots pushing apart rock

4. Abrasion When rocks or sediments rub against another rock. Could be caused by wind or running water.

5. Wind

6. Running Water Friction & Repeated impact

7. Burrowing Animals

Types of Weathering Chemical Weathering (decomposition) Takes place when the rock’s minerals are changed into different substances. Usually caused by reactions with oxygen, water, or acids.

Agents of Chemical Weathering: Water

1. Hydrolysis The chemical reaction of water with other substances is called hydrolysis. When heated or dried a little, minerals that have undergone hydrolysis may fall apart For example: Feldspar + H20  Clay

2. Oxidation The chemical reaction of oxygen with other substances is called oxidation. Iron-bearing minerals are the ones most easily attacked by oxygen. Examples: Magnetite Pyrite Oxidation of these minerals results in rusting, or iron oxides  Rocks weaken and crumble

Rusting

3. Carbonation Carbon dioxide dissolves easily in water. It forms a weak acid called carbonic acid This is the same compound that is in carbonated drinks. This is how acid rain is produced.

What does Carbonation Affect? Has the greatest effect on calcite than any other mineral. This process changes the mineral calcite  Limestone and marble are made of calcite so they are affected by carbonation

4. Other Acids Some green plants can produce weak acids  MOSSES Decaying organisms also produce acids These acids can seep into cracks and break the rock apart.

Factors that affect the rate of weathering

1. Climate The amount of water in the air and the temperature of an area are part of the CLIMATE. The more water there is in the air, the faster weathering will occur. Weathering occurs fastest in hot, wet climates; slowest in hot, dry climates

This is a monument called Cleopatra’s Needle This is a monument called Cleopatra’s Needle. It was carved in Egypt around 1450 B.C. The sides are carved with hieroglyphs, the writing of ancient Egypt. What is the climate like in Egypt?

In 1800, the monument was moved to New York City In 1800, the monument was moved to New York City. Almost immediately, the hieroglyphs began to fade. In only a few years, the Egyptian writing became indistinct! What is the climate like in NYC?

What happened to Cleopatra’s Needle? Cleopatra’s Needle was carved from granite, a hard tough, crystalline rock. Although it is tough, granite is changed by the atmosphere. Some of the minerals that make up granite change to clay. Chips and flakes of minerals break away from the granite surface.

2. Surface Area Most weathering occurs on exposed surfaces of rocks and minerals. The more surface area a rock has, the more quickly it will weather.

3. Rock Composition Some minerals that hold rocks together weather more quickly than others. Quartz resists weathering. Feldspar weathers easily. Think about chemical weathering: Rocks with calcite, such as limestone, will weather easily Rocks with iron will weather faster than those without.

Limestone Statue

Oxidation of Copper

4. Chemical Reaction Carbonic acid dissolves calcite, a mineral found in marble and limestone. Acid rain  Pollution speeds up the process of weathering Factories and cars release carbon dioxide and other gases, such as sulfur and nitrogen. These gases contribute to acid rain.

Hollows out great caverns in limestone bedrock.