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Which Causes of Weathering Cannot Be Classified as Mechanical?

Weathering is a process by which rock is broken down into smaller pieces. Not all of these processes are mechanical--that is to say, physical--in nature. Some rely on chemical reactions to degrade the integrity of rock's structure. In other cases, biological processes cause a change in exposed rocks. Generally rock will experience many forms of weathering at some point in its existence.

Oxidation
- This form of chemical weathering occurs when oxygen in the air or water reacts with metallic minerals like iron. The oxygen causes a reddish-brown "rust" to develop and leech into the surrounding environment. Basaltic rocks may produce an orange colored byproduct as a result of oxidation. The dark red soils found in tropical climates are typically the result of high levels of oxidation.
Hydrolysis
- Hydrolysis is the process by which hydroxide and hydrogen ions in water disrupt the normal atomic structures of certain rocks. The hydroxide and hydrogen ions force mineral ions out of the rock. These are carried away, while what's left behind is an entirely new substance. Clay is often the result of the hydrolysis reaction, as feldspar formed deep within the Earth is exposed to the effects of surface water.
Acidic Effects
- Carbon dioxide present in water can react with other chemicals to produce a weak form of acid. Known as carbonic acid, this substance is particularly effective at dissolving certain kinds of rock. Limestone is one example, as carbonic acid from rainwater or groundwater can dissolve the calcium in the rock. The limestone surface becomes porous and pitted. This weakened surface is highly vulnerable to further weathering processes.
Root Pressure
- Plant root structures are remarkably strong and persistent. Tree roots can grow into existing cracks in a rock face and widen the gaps further. Some very hardy plants need almost no topsoil, getting their nutrients entirely from the rock surface. The cracking and splitting caused by roots provide greater surface area on which other forms of weathering can act. As plants produce waste material or decompose, the chemicals released accelerate destruction of the original bedrock.
Biological Weathering
- Certain microorganisms make themselves perfectly at home on the surface or inside of rocks. Algae can grow on rock and create moist, warm patches where normal weathering processes act faster. The waste products of microorganisms can slowly eat away at rock surfaces. Ancient stone buildings are often the target of these biological assaults. Larger organisms inadvertently crush and transport rocks from one location to another. The life functions of plants and animals contribute chemicals to the ecosystem that react with soil and rocks.

Nature