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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.
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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 1. Mechanical weathering is the breaking 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, 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.
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.
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
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.
Two Types of Weathering 2. Chemical weathering is the breaking 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.
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)
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.
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.
