Physical weathering is a geological process involving the mechanical breakdown of rocks into smaller fragments without changing their chemical composition. Also known as mechanical weathering, this process relies on the application of force to disintegrate rock material while preserving the original mineral makeup. The reduction in rock size significantly increases the total surface area exposed to the environment. This disintegration prepares the material for subsequent transport by agents of erosion, such as water, wind, ice, and gravity.
Breakdown Through Freeze-Thaw Cycles
The repeated freezing and thawing of water, known as frost wedging or frost shattering, is a powerful agent of physical weathering in colder climates. This mechanism requires water to seep into existing cracks, joints, and pores within the rock mass, combined with temperature fluctuations around the freezing point.
When the temperature drops below zero degrees Celsius, the liquid water transforms into ice. Water expands by approximately 9% when it freezes, exerting immense outward pressure on the confining walls of the crack.
This pressure can widen cracks, potentially exceeding 30,000 pounds per square inch. When temperatures rise, the ice melts, allowing water to penetrate deeper into the enlarged fissure.
The relentless cycle progressively weakens the rock structure, leading to the eventual splitting and fragmentation of the rock mass. The broken fragments often accumulate at the base of slopes, forming piles of angular debris known as scree or talus.
Breakdown Through Pressure Release
Pressure release weathering, also termed unloading, affects rocks formed under tremendous confining pressure deep beneath the Earth’s surface, such as granite or certain metamorphic rocks. Over geological timescales, the overlying material is gradually removed through erosion and uplift.
The removal of this overburden reduces the confining pressure, allowing the underlying rock mass to expand or rebound. This expansion typically occurs perpendicular to the exposed surface.
This elastic rebound causes the outer layers of the rock to fracture in sheets parallel to the surface. This process, known as sheeting or exfoliation, results in the rock peeling away in thin, curved slabs, similar to the layers of an onion.
The continuous removal of these outer layers creates distinctive, smooth, rounded landforms called exfoliation domes. Examples include Stone Mountain in Georgia and Half Dome in Yosemite National Park, California.
Breakdown Through Salt Crystal Growth
The mechanical breakdown of rock caused by the growth of salt crystals is called haloclasty or salt wedging. This weathering is effective in arid climates with high evaporation rates and along coastal zones where saline water is available.
The process begins when saltwater penetrates pores and fractures within the rock structure. As the water evaporates, it leaves behind dissolved mineral salts that crystallize within the internal spaces.
The growth of these crystals, such as sodium sulfate and magnesium sulfate, exerts powerful internal stress. This crystallization pressure forces the rock grains apart, leading to disaggregation or granular disintegration.
Some salt crystals can expand significantly in volume, generating internal pressure that exceeds the rock’s tensile strength. This expansion causes the rock to flake or spall, often forming small, cave-like features known as tafoni or honeycomb weathering.
Breakdown Through Friction and Impact
Abrasion is the physical grinding, scraping, or wearing away of rock surfaces by the friction of other moving particles. This form of mechanical weathering is driven by kinetic forces that move sediment against stationary rock, typically smoothing and rounding the edges of the fragments involved.
In river systems, water carries sediment and rocks, causing them to collide and scrape against the streambed, which reduces their size and shapes them into smooth pebbles. Wind acts as an abrasive agent in deserts, carrying sand particles that “sandblast” exposed rock surfaces.
Glacial ice also drives abrasion, as rocks embedded in the bottom of the moving glacier scrape against the underlying bedrock. This action grinds and polishes the bedrock, creating characteristic striations.
Gravity causes abrasion when rockfalls or landslides occur. The material fractures and breaks through impact as pieces tumble down a slope.