The Process of Weathering Rocks

Weathering The breaking down of rock into smaller pieces that remain next to each other. Weathering forms sediments. There are two types of weathering.

Two Types of Weathering Mechanical weathering is the physical breaking down of rock without any change in the chemical composition of the rock. 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 Weathering: Ice Wedging Water enters the cracks in rocks, and then freezes and expands about 10%. The ice strains the walls around the cracks and causes cracks to deepen and widen. When the ice thaws, water can flow further into the rock. Repeated freeze-thaw cycles weaken the rocks. Over time, the rock breaks up along the cracks into angular pieces.

Ice Wedging

Mechanical Weathering : Exfoliation Rocks formed deep in the Earth are under a huge amount of pressure. When erosion removes the overlying rock, the once buried rocks are exposed and the pressure on them is released. The outer rock parts tend to expand. The expansion sets up stresses which cause fractures to form parallel to the rock surface. Over time, sheets of rock break away from the exposed rocks along the fractures.

Exfoliation

Mechanical Weathering: Thermal Thermal weathering results from expansion or contraction of rock, caused by extreme changes in temperature. Common in deserts, where it is hot in the day and cold at night; different minerals expand and contract at different rates causing the rock to split The outer layers peel off into thin sheets.

Thermal

Mechanical Weathering : Biotic (force) Biotic weathering is caused by living organisms. Most often, plant roots are the cause. They act as a wedge, widening and extending the cracks. Digging animals can also cause weathering.

Biotic (physical force)

Two Types of Weathering 2. Chemical weathering is the breaking down of rock into smaller pieces because of chemical changes within the rock. Rock reacts with water, gases, and solutions (may be acidic); these reactions will add or remove elements from minerals. Chemical reactions break down the bonds holding the rocks together, causing them to fall apart.

Chemical Weathering: 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. A reddish-brown coloration on the surface is called rusting.

Oxidation

Chemical Weathering: Hydrolysis Hydrolysis is the weathering reaction that occurs when water and chemical compounds in a rock meet. This results in the decomposition of the rock surface by forming new compounds. Most common reaction is the hydrolysis of feldspars producing clay (kaolinite)

Hydrolysis

Chemical Weathering: Carbonation Carbonation takes place when carbon dioxide reacts with water or rain, forming a weak carbonic acid. This is the same acid found in soda. The acid is too weak to harm plants and animals, but slowly causes feldspars and limestone to decompose. The substances in rocks form new types of softer substances.

Carbonation

Chemical Weathering: Biotic (acid) Lichens and similar plants live on the surfaces of rocks. Plants lower the local pH to make it more acidic. Their roots give off a chemical that dissolves rocks and minerals.

Chemical Weathering: Biotic (Acid)