1.3.1. Weathering

Physical weathering reduces the size of the parent rock material, without any change in the original composition of the parent rock. Physical or mechanical processes taking place on the earth's surface include the actions of water, frost, temperature changes, wind, and ice. They cause disintegration and the products are mainly coarse soils. The main processes involved are exfoliation, unloading, erosion, freezing, and thawing. The principal cause is climatic change.

Physical weathering is a process where rocks are broken down into smaller pieces without changing their chemical composition. This type of weathering is primarily caused by physical forces like water, wind, and temperature changes. For instance, water can seep into cracks in rocks, freeze, and expand, eventually causing the rocks to break apart. The main processes include:

• Exfoliation: The outer layer of rock peels off like the layers of an onion.
• Unloading: When overlying materials are removed, reducing pressure on rocks, causing them to crack.
• Erosion: The wearing away of rock surfaces by wind or water.
• Freezing and Thawing: Repeated freeze-thaw cycles can lead to rock breakage. These processes create mainly coarse soil particles.

Think of physical weathering like an ice cube in a glass of water. As the temperature drops, the ice expands and cracks the glass, similar to how frozen water expands in rock cracks. Over time, this leads to the rocks breaking down into smaller pieces.

Chemical weathering not only breaks up the material into smaller particles but alters the nature of the original parent rock itself. The main processes responsible are hydration, oxidation, and carbonation. New compounds are formed due to the chemical alterations. Rainwater that comes in contact with the rock surface reacts to form hydrated oxides, carbonates, and sulphates. If there is a volume increase, the disintegration continues. Due to leaching, water-soluble materials are washed away and rocks lose their cementing properties. Chemical weathering occurs in wet and warm conditions and consists of degradation by decomposition and/or alteration. The results of chemical weathering are generally fine soils with altered mineral grains.

Chemical weathering involves the breakdown of rocks and minerals through chemical reactions, which change the minerals' composition. This process breaks down the rocks into smaller particles while also transforming them chemically, resulting in new substances. Key processes include:

• Hydration: Water combines with minerals, leading to transformations.
• Oxidation: Reaction with oxygen that can lead to rusting of minerals.
• Carbonation: Carbon dioxide reacts with water to form carbonic acid, which dissolves some minerals. As water-soluble materials are washed away (a process called leaching), rocks lose their structural integrity and become weaker, leading to the formation of finer soils.

Imagine a piece of marble sculpture left outside in a humid environment. The marble slowly reacts with the carbonic acid in rainwater, leading to fine particles washing away. Over time, the sculpture becomes weathered and altered, similar to how chemicals change the minerals in a rock during chemical weathering.

The effects of weathering and transportation mainly determine the basic nature of the soil (size, shape, composition, and distribution of the particles). The environment into which deposition takes place, and the subsequent geological events that take place there, determine the state of the soil (density, moisture content) and the structure or fabric of the soil (bedding, stratification, occurrence of joints or fissures).

Weathering contributes significantly to soil formation by breaking down rocks into smaller particles. The characteristics of the soil—such as size, shape, and composition—are influenced by the weathering processes and how the materials are subsequently transported to new locations. For example, the deposition environment plays a critical role; a soil formed in a river (with constant water flow) will differ from that formed in a still lake (where sediment settles slowly). Additionally, geological events like floods can compact the soil, affecting its density and moisture content.

Consider how two different types of soil form in distinct environments. If we have sand deposited by a river, it will be coarser and have larger particles due to the force of moving water. In contrast, soil that forms on a lake bottom tends to be fine and clay-like because of the slower settling process. Just like each environment influences the type of soil that forms, different weathering patterns help shape the land we see.

Transportation agencies can be combinations of gravity, flowing water or air, and moving ice. In water or air, the grains become sub-rounded or rounded, and the grain sizes get sorted so as to form poorly-graded deposits. In moving ice, grinding and crushing occur, size distribution becomes wider forming well-graded deposits. In running water, soil can be transported in the form of suspended particles, or by rolling and sliding along the bottom. Coarser particles settle when a decrease in velocity occurs, whereas finer particles are deposited further downstream. In still water, horizontal layers of successive sediments are formed, which may change with time, even seasonally or daily.

Transportation of soil particles is the process by which weathered materials are moved from one location to another. Various forces, such as gravity, flowing water, air, and even glacial movement, contribute to this process. For example:

• In flowing water: Larger particles roll along the bottom while finer particles are suspended. When the water slows, coarser materials settle out first.
• In air: Wind can lift and carry small particles over long distances, often leaving finer soils behind.
• In glaciers: The crushing action grinds rocks into finer materials, creating a mix of particle sizes. Each method of transportation affects the sorting and distribution of soil particles in unique ways.

Picture how a river carries different sizes of rocks. As it flows faster, it can transport larger boulders, but when it slows down (like in a river bend), those boulders drop out. Meanwhile, tiny pebbles or sand can still be carried for miles. Similarly, the wind can pick up fine dust particles and transport them to far-off locations, just like some plants rely on wind to spread their seeds.

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