Q9. Weathering refers to the combination of processes that break up and corrode solid rock, eventually

Question

transforming it into sediment. Geologists refer to rock that has not undergone weathering as unweathered
or “fresh”. Nature can attack rocks via two types of weathering: physical and chemical.
1) Physical (or mechanical) weathering breaks intact rock into unconnected clasts (grains or
chunks), collectively called debris or detritus. Each size range of clasts has a name. Many
different phenomena contribute to physical weathering. State at least two (2) phenomena and
explain briefly. (4mks)
2) Chemical weathering refers to the many chemical reactions that alter or destroy minerals when
rock comes in contact with water solutions and/or air. There are four (4) most common reactions
involved in chemical weathering. Identify and explain only 2 of the 4 reactions. (4mks)

Answer 1

1) Physical Weathering Phenomena

a) Freeze-Thaw Weathering (Frost Wedging): This process occurs when water seeps into cracks in the rock. When temperatures drop, the water freezes and expands, exerting pressure on the surrounding rock. This repeated freezing and thawing cycles can cause the rock to fracture and break apart into smaller clasts.

b) Exfoliation (Sheeting): This phenomenon involves the peeling away of layers from the outer surface of a rock. It often occurs in igneous rocks due to the release of pressure when overlying materials are removed (such as through erosion). As the pressure decreases, the rock expands, causing outer layers to crack and flake off.

2) Chemical Weathering Reactions

a) Hydrolysis: This reaction involves the chemical breakdown of minerals in rocks when they interact with water. For example, feldspar, a common mineral in granite, can be altered to clay minerals through hydrolysis when water reacts with it. This process alters the mineral’s structure and composition, leading to the formation of softer and more stable materials.

b) Oxidation: Oxidation occurs when minerals, particularly those containing iron, react with oxygen present in water or air. This reaction can lead to the formation of iron oxides, commonly known as rust. Oxidation weakens the structure of the rock and can cause it to break down over time, contributing to the overall weathering process.

Both physical and chemical weathering work together to break down rocks and contribute to the formation of sediment.