Mechanical and Chemical Weathering Mechanical and Chemical

What Caused This?

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

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.

Mechanical - 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

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

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

Mechanical – Biotic (Plant & Animal) Biotic – means life Weathering caused by living organisms Plant roots most common cause, act as a wedge and widen cracks Other causes of biotic weathering are digging animals, microscopic plants and animals, algae and fungi.

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 lesser amount of surface area. Chemical weathering is more common in locations with lots of water and heat, because it speeds it up. Rock reacts with water, gases and solutions (may be acidic); these reactions will add or remove elements from minerals.

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

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

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

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