Weathering is the breakdown of rocks and minerals on Earth’s surface, shaping landscapes over geologic time. It is categorized into physical weathering (mechanical disintegration) and chemical weathering (altering rock composition). Both processes require energy and a medium, typically water, to drive reactions and fracturing. Cold and dry environments, such as polar regions or high-altitude deserts, lack these necessary conditions, causing landscapes to change very slowly.
How Low Temperatures Slow Chemical Reactions
The speed of any chemical reaction, including the chemical breakdown of rock minerals, is directly governed by temperature. Chemical reactions require molecules to collide with enough kinetic energy to overcome the activation energy barrier. In cold environments, molecules within the minerals and surrounding water move slowly and possess less kinetic energy.
This reduction in molecular motion means that successful collisions needed to break mineral bonds happen far less frequently. Reaction kinetics show that decreasing temperature significantly slows the rate of geochemical processes. For example, the rate of a chemical reaction can roughly double for every \(10^\circ \text{C}\) increase in temperature. This illustrates the retarding effect of consistently low temperatures on chemical weathering, causing the alteration of minerals like feldspar to proceed at a sluggish pace.
The Necessity of Liquid Water for Chemical Breakdown
The lack of moisture in cold, dry places is equally important in slowing chemical weathering. Water is often a direct chemical reactant, not just a medium for transporting dissolved ions. For instance, hydrolysis involves water molecules reacting with minerals, such as silicates, to form new, stable compounds like clay minerals.
Without sufficient liquid water, this chemical alteration cannot take place, effectively halting the reaction. Dissolution, where soluble minerals are dissolved by water, also requires a liquid solvent. Even acidic solutions, which accelerate dissolution, require liquid water to form, such as when atmospheric carbon dioxide dissolves into rainwater to create carbonic acid. In extremely dry conditions, the solvent is unavailable. In perpetually cold conditions, water is locked up as inert ice, preventing it from participating in these essential chemical transformations.
Restrictions on Physical Weathering Processes
Physical weathering, the mechanical fracturing of rock, is severely limited in cold, dry environments. The most effective form of physical breakdown in cold climates is frost wedging, which relies on the expansion of water when it freezes. This process requires liquid water to first infiltrate existing cracks and pores within the rock structure.
In dry cold environments, there is often not enough liquid water available to seep into rock fractures, preventing the cycle from starting. If water is present in extremely cold polar regions, it remains permanently frozen deep within the rock. Frost wedging requires a cyclical process of freezing and thawing to exert pressure, meaning the temperature must repeatedly cross the \(0^\circ \text{C}\) threshold. When temperatures are perpetually below freezing, the ice remains stable and non-cyclical, preventing the repeated expansion and contraction needed to fracture the rock.