Weathering

Weathering and Erosion

Weathering is the chemical or physical breakdown of a rock or mineral due to exposure to the atmosphere.

Erosion is the removal or transport of material by 5 possible agents: running water, glaciers, waves, wind, and gravity.

Humans and animals may also play a part in erosion.

Types of Weathering There are two major types of weathering:

1. Mechanical (physical) – breakdown of rock or mineral. Just breaks it into smaller pieces, composition of the material is NOT changed!!

2. Chemical – changes the composition of material as it breaks down.

Physical (Mechanical) Weathering Some ways physical weathering can occur:

1. Freezing and thawing of moisture in rock openings causing rock disintegration. (Water expands as it freezes, this breaks the rock into pieces. Occurs A LOT in our state – causes most potholes!!!)

Frost Wedging animation

2. Erosion of upper rock causes underlying rock to expand resulting in cracking and peeling of rock in slabs.

Think of onion skin layers and peeling them off!!

3. Organic Activity – plant roots or animals like burrowing worms can cause rock to breakdown.

4. Wetting and Drying - addition of water to mineral structure causes structure to expand.

                                                                                                        

Chemical Weathering There are also several different types of chemical

weathering:

1. Hydrolosis – chemical reaction between water and other substances.

(feldspar weathered into clay by hydrolosis)

2. Carbonic Acid – dissolving of substances by acids in water often leaving insoluble remains.

3. Oxidation – chemical reaction of a substance with oxygen causing a rust or patina to form.

Weathering and Climate Climate plays a very important role in weathering.

The temperature and amount of moisture influences the type of weathering that will occur.

Warm and wet climates produce mostly chemical weathering.

Cool and slightly wet climates produce mostly physical weathering.

                           

Cleopatra’s Needle in Egypt – warm dry climate – little to no weathering occurs

Cleopatra’s Needle was moved to New York and has weathered considerably in only 80 years due to the cold and wet climate.

Weathering and Resistance

The harder the rock or mineral is, the more resistant it will be to weathering.

Any rock with quartz will be quite resistant due to quartz’s hardness. What is quartz’s hardness? Rocks made mostly of calcite, a very soft mineral, will be easily

weathered. What is calcite’s hardness?

Porosity and PermeabilityPorosity: The percentage of pore space

in a material

Permeability: The ability of a material to let liquid (water) pass through it.

Impermeable: When something will not let liquids pass through it (usually water is the liquid).

Permeability If something has Low Porosity, then permeability will be SLOW

If something has High Porosity, then permeability will be FAST