10 Major Difference Between Physical And Chemical Weathering

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Rocks can break for a variety of reasons, including stress along natural zones of weakness and due to biological and chemical activity. Weathering can be described as a natural process whereby rock, soil, some plants and human-made objects are broken down or degraded having been in contact with the Earth’s atmosphere, water/rain, wind, biological organisms and/or chemicals, it is a process that wears rock into soil over time, produces caves and sinkholes and build land mass as eroded particles accumulated as sediments. Weathering is distinguished from erosion by the fact that the latter usually includes the transportation of disintegrated rock and soil away from the site of degradation.

Weathering is an important part of the rock cycle, breaking down large rocks so that they turn into soil or reform into different rocks. There are two important classification of weathering, that is:

  • Physical/mechanical weathering
  • Chemical weathering

What Is Physical Weathering?

Physical weathering is a term used in science that refers to the geological process of rocks breaking apart without changing their chemical composition. Over time, movements of the earth and environment can break apart rock formations, causing physical weathering.  Physical weathering is affected by a number of things in the environment including soil and minerals, animals, pressure, changes in temperature, water and ice.

After a rock has weathered, erosion occurs, transporting bits and pieces away. Eventually, a deposition process deposits the rock particles in a new place.

Examples/Types  of Physical Weathering

  • Roots of plants growing into cracks may put pressure on the surrounding rock, eventually breaking adjacent rocks apart as the roots grow. 
  • When water sinks into cracks in a rock and temperature drops low enough, the water freezes into ice. The ice expands and forms wedges in the rock that can split the rock into smaller fragments.
  • When water in a river or stream moves quickly, it can lift up rocks from the bottom of that body of water. When the rocks drop back down they hit into other rocks, and tiny pieces of the rocks can break apart.
  • Animals that burrow underground such as moles, gophes or even ants can cause physical weathering by loosening and breaking apart rocks.
  • Forest and range fires can cause rocks that are located along the surface of the ground to weather.
  • Block disintegration occurs when rocks split along the lines of weakness. This normally happen as a result of repeated cycles of freezing and thawing, in climates where there is a lot of moisture and temperatures goes up and down a lot around the freezing point.

What You Need To Know About Physical Weathering

  1. Physical weathering also referred to as mechanical weathering, is the process that breaks rocks apart without changing their chemical composition.
  2. The physical breakdown results from the exposure to the atmosphere and environmental temperature changes, moisture, biological activity etc.
  3. Stability of the rock decreases.
  4. Does not affect the molecular structure of rock and soil, the rock just cracks and crumbles into smaller pieces.
  5. Type of physical weathering include exfoliation,  freeze, Abrasion, thermal expansion

What Is Chemical Weathering?

Chemical weathering can be described as weathering that result in the alteration of the molecular structure of rocks and soil. All rocks are made up of minerals, whose crystal structures are made up of elements. The elements in these minerals can react with substances in their immediate environment such as water or oxygen and change the chemical makeup of the minerals. In some cases, this chemical change will cause the minerals in the rock to be structurally weaker and more vulnerable for disintegration.

Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near surface environment. New or secondary minerals develop from the original mineral rock. Chemical weathering is enhanced by geological agents such as presence of water and oxygen, as well as by such biological agents as acids produced by microbial and plant-root metabolism.

A number of different processes can result in chemical weathering processes are hydrolysis, oxidation, reduction, hydration, carbonation and dissolution.

The process of chemical weathering tends to:

  • Increase bulk creating stress within rocks
  • Lower the density of minerals
  • Decrease particle size resulting in increased surface area
  • Creates more mobile materials
  • Create more stable minerals.

Examples/Types of chemical weathering

  • When it rains, water seeps down into the ground and comes in contact with granite rocks. The feldspar crystals within the granite react with the water and are chemically altered to form clay minerals, which weaken the rock. This can simply be referred to as hydrolysis.
  • Rocks can get rusty if they contain iron. When iron reacts with oxygen, it forms iron oxide which is not very strong. So when a rock gets oxidized, it is weakened and crumbles easily, allowing the rock to break down. This can be referred to as oxidation.
  • Dissolved carbon dioxide in rainwater or in most air forms carbonic acid this acid react with minerals in rocks resulting in weathering. This process is referred to as carbonation.
  • Limestone and rocks high salt dissolve when exposed to water. The water carries away the ions. This can be referred to as dissolution.

What You Need To Know About Chemical Weathering

  1. Chemical weathering is a process by which rocks are broken down by chemical reactions.
  2. Chemical weathering requires a flow of water and carbon dioxide through the layer of soil.
  3. Stability of the rock is decreased.
  4. Results in the alteration of the molecular structure of rock and soil.
  5. Types of chemical weathering include hydrolysis, hydration, carbonation, oxidation and acidification.

Difference Between Physical And Chemical Weathering In Tabular Form

BASIS OF COMPARISON PHYSICAL WEATHERING CHEMICAL WEATHERING
Description Physical weathering also referred to as mechanical weathering, is the process that breaks rocks apart without changing their chemical composition.   Chemical weathering is a process by which rocks are broken down by chemical reactions.  
Factors Influencing The Process The physical breakdown results from the exposure to the atmosphere and environmental temperature changes, moisture, biological activity etc.   Chemical weathering requires a flow of water, oxygen and carbon dioxide through the layer of soil.  
Rock Stability Stability of the rock decreases.   Stability of the rock is decreased.  
Effect On Rock’s Molecular Structure Does not affect the molecular structure of rock and soil, the rock just cracks and crumbles into smaller pieces.     Results in the alteration of the molecular structure of rock and soil.  
Rock Breakdown Rock is simply broken down into small fragments. Rock is broken down into small fragments; some are turned into clay while other rocks change their color completely.
Types Type of physical weathering include exfoliation,  freeze, Abrasion, thermal expansion   Types of chemical weathering include hydrolysis, hydration, carbonation, oxidation and acidification.  
Speed of Occurrence Physical weathering is a gradual process, though faster than chemical weathering. Chemical weathering entails transformation and modification of mineral components and thus, it the entire process is slower when compared to physical weathering.
Minerals Minerals inside the rock do not dissolve completely. Some minerals such as calcite may dissolve completely.
Prevalence Particularly active in cold climates where frosts shattering dominates. It is also prevalent in desert climates. Most intense in hot and wet climates.
Formation of New Minerals Does not result in the formation of new minerals. Chemical reactions between rocks and water results in the formation of new minerals and release of dissolved substances.