Weathering

1. Weathering Mechanical and Chemical

2. What Caused This?

3. What is Weathering? • First step to forming soil and sedimentary rock. • Breakdown of rock into smaller pieces • The smaller pieces do not move to a new location, but remain next to one another until erosion carries them away. • Mostly occurs at the surface but can also occur below ground • Many types of weathering

4. What is Mechanical Weathering? • Breakdown of rock into smaller pieces without any change in the chemical composition of its minerals. • Sometimes called “physical” weathering • Rock is torn apart by physical force, rather than by chemical breakdown. • Smaller pieces do not move to a new location, but remain next to one another until erosion carries them away.

5. Mechanical - Ice Wedging • Ice Wedging • Water fills joints of rocks and freezes • Water expands 10% when it freezes, pushes rock apart • Repeated freeze and thaw cycles over the years causes rock to break along joint

6. Mechanical - Exfoliation • Exfoliation or unloading • Rock breaks off into leaves or sheets along joints which are parallel to the ground surface • Caused by expansion of rock due to uplift and removal of surface material that covered the rock

7. Mechanical - Thermal • Thermal expansion • Repeated daily heating and cooling of rock • Heat causes expansion; cooling causes contraction • Different minerals expand and contract at different rates causing the rock to split

8. Mechanical - Biotic • Biotic – means life • Weathering caused by living organisms • Plant roots most common cause, act as a wedge widening cracks • Other causes of biotic weathering are digging animals, microscopic plants and animals, algae and fungi.

9. What is Chemical Weathering? • Breakdown of rock into smaller pieces because of change in the chemical composition of its minerals. • Chemical reactions break down the bonds holding the rocks together, causing them to fall apart. • Chemical weathering occurs in all types of rock but smaller rocks are more susceptible because they have a greater amount of surface area. • Chemical weathering is more common in locations with lots of water and heat speeds it up. • Rock reacts with water, gases and solutions (may be acidic); these reactions will add or remove elements from minerals.

10. Chemical - Oxidation • Oxidation - oxygen combines with other elements in rocks to form new types of rock • New substances are usually much softer than original, easier for other forces to break rock apart • Causes a “rusting” of the rock, often causes a color change in the rock

11. Chemical - Hydrolysis • Hydrolysis - water combines with the substances in rocks to form new types of substances • New substances are softer than the original rock types • Most common reaction is the hydrolysis of feldspars producing clay minerals • e.g. feldspar kaolinite

12. Chemical - Carbonation • Carbonation – Carbon dioxide is dissolved in water making carbonic acid • Weak acid is formed when carbon dioxide in the air mixes with rain. This is the same acid found in soft drinks.  • Acid is too weak to harm plants and animals but slowly causes feldspars and limestone to decompose. • Occurs when carbonic acid reacts with a rock to form a new substance

13. Chemical - Biotic • Organisms - can change the chemistry of their environment  • Plants lower the local pH to make it more acidic • Bacteria act as catalysts in various geochemical processes