Weathering is the natural process through which rocks, soils, and minerals break down at the Earth’s surface. This process is distinct from erosion, which involves the transport of broken material away from its original location. Physical weathering, often referred to as mechanical weathering, focuses on the disintegration of rock into smaller fragments without altering its chemical composition. The resulting rock pieces remain chemically identical to the parent material. Forces driving this breakdown include temperature changes, pressure variations, and the actions of water, wind, and organisms.
The Mechanism of Mechanical Stress
Physical weathering begins with the application of force that creates mechanical stress within the rock structure. This stress must surpass the rock’s inherent strength to initiate fracturing. Rocks possess natural weaknesses, such as microscopic cracks or joints, which act as starting points for disintegration. These forces then widen existing fissures or create new fractures, leading to the physical separation of the rock mass.
Breaking a large rock into smaller fragments dramatically increases the total surface area. This greater exposed area prepares the material for subsequent chemical weathering processes. Although physical force does not change the rock’s chemistry, it exposes fresh mineral surfaces to water and atmospheric gases, facilitating chemical reactions.
Breakdown Caused by Temperature and Pressure Release
One driver of physical weathering in climates with fluctuating temperatures is frost wedging. This process occurs when water seeps into pre-existing cracks or joints within a rock mass. When the temperature drops below freezing, the water changes state into ice, expanding its volume by about nine percent. This volumetric increase exerts pressure on the surrounding rock walls, forcing the crack to widen slightly.
The repeated cycle of freezing and thawing, common in mountainous or high-latitude regions, stresses the rock, leading to the progressive splintering and eventual detachment of fragments. This action is responsible for the formation of sharp, angular rock debris fields known as talus slopes at the base of steep cliffs.
In contrast to freeze-thaw cycles, a geological process called exfoliation is driven by the release of confining pressure. Rocks that form deep underground, such as intrusive igneous rocks like granite, are subjected to pressure from the overlying rock material.
Exfoliation and Pressure Release
As erosion and uplift remove this overburden, the rock mass beneath experiences unloading. The rock expands upward in response to this pressure release, causing fractures to develop parallel to the exposed surface. These fractures allow large, concentric sheets of rock to peel away, much like the layers of an onion, resulting in characteristic dome-shaped formations.
Thermal Expansion
Thermal expansion is a less dominant form of weathering, but it contributes to rock breakdown, particularly in arid desert environments where temperatures swing drastically between day and night. Rocks are composed of various minerals, and each mineral type expands and contracts at a slightly different rate when heated and cooled. The differential movement between adjacent mineral grains creates internal stresses within the rock structure. Over long periods, this repeated internal stress can cause the rock’s outer layer to flake off or contribute to the formation of micro-cracks.
Breakdown Caused by Friction, Salts, and Organisms
Physical weathering is also accomplished through the mechanical friction of moving materials, a process known as abrasion. This involves the scraping, grinding, and wearing away of rock surfaces due to the collision with other solid particles. Abrasion is evident in riverbeds where sediment-laden water tumbles and polishes rocks into smooth, rounded pebbles. Strong winds carrying sand grains can also abrade exposed rock faces in deserts, scouring and pitting the surfaces over time.
Glacial ice acts as an abrasive agent, carrying embedded rock fragments that grind against the bedrock beneath, leaving behind smoothed and striated surfaces.
Salt Wedging
Another expansive force, similar to frost wedging, is salt crystal growth, or salt wedging, which is common in coastal and arid regions. Water containing dissolved salts seeps into the small pores and fissures of the rock. As the water evaporates, it leaves behind salt crystals, often composed of halite or gypsum. These crystals slowly grow in size, exerting pressure on the surrounding rock grains as they expand within the confined spaces.
The outward force of the growing salt crystals eventually forces the rock apart, a process that can create honeycomb-like pits and hollows on exposed rock faces.
Biological Activity
Biological activity plays a direct role in physical weathering through a mechanism called root wedging. Plant roots, especially those of large trees, grow into existing cracks in the rock. As the roots thicken and expand, they act as natural wedges, applying pressure to the rock walls and forcing the fractures to widen. The growth of roots can split apart boulders and even crack concrete sidewalks and foundations.
Burrowing animals, such as rodents and worms, also contribute to physical weathering by disturbing and moving soil and rock fragments, which exposes new material to other weathering agents.