Mechanical weathering, also known as physical weathering, is the process by which rocks and minerals are broken down into smaller fragments without undergoing any change in their chemical composition. This disintegration involves the application of physical forces that fracture and separate the rock material. Unlike chemical weathering, which alters the rock’s internal structure, mechanical weathering is purely a physical breakdown. The various mechanisms of this process act to increase the rock’s surface area, making it more susceptible to further physical and chemical changes.
The Force of Ice and Extreme Temperature Changes
Frost wedging is a powerful and geographically widespread form of mechanical weathering that relies on the unique properties of water. The process begins when liquid water seeps into cracks and fissures within a rock mass. When the temperature drops below freezing, the water transitions to ice, expanding its volume by approximately 9%. This volumetric increase exerts immense outward pressure on the rock walls, potentially reaching 14 megapascals.
Repeated cycles of freezing and thawing, common in mountainous or mid-latitude regions, intensify this pressure, progressively widening the cracks until rock fragments are dislodged. The resulting angular, broken rock debris often accumulates at the base of cliffs to form piles known as talus slopes.
Temperature fluctuations can also cause mechanical breakdown through thermal expansion, or insolation weathering. This occurs when rocks are repeatedly heated by the sun and cooled at night, especially in arid desert environments with large diurnal temperature swings. Different minerals within a rock, such as those in granite, possess varying thermal expansion coefficients, meaning they expand and contract at different rates.
This differential movement creates internal stresses and microscopic fractures, especially in the outer layers. Over time, this repeated stress leads to the peeling away of surface layers in a process called granular disintegration or thermal exfoliation. The cumulative stress from these continuous expansion and contraction cycles systematically weakens the rock structure.
Stress Caused by Unloading and Crystal Formation
Pressure release, or unloading, causes fracturing in deeply formed rocks when they are exposed at the Earth’s surface. Intrusive igneous rocks, like granite, form under tremendous confining pressure from overlying material. When erosion removes this overburden, the rock expands upward and outward. This release of pressure creates extensional stresses, causing sheet-like fractures to form parallel to the exposed surface. These fractures are known as exfoliation or sheeting, responsible for the characteristic rounded, dome-like formations seen in large granite batholiths.
Another internal force that fractures rock is salt wedging, or haloclasty, which is common in coastal and desert settings. This process begins when saline water permeates into small pores and cracks within the rock structure. As the water evaporates, it leaves behind dissolved salts, which crystallize and begin to grow. The growing salt crystals, particularly sodium and magnesium salts, exert pressure on the surrounding rock, forcing the mineral grains apart.
This internal pressure expands the fissures, causing the rock to weaken and crumble. It often results in a pitted or honeycomb-like texture called tafoni.
Physical Wear and Living Organisms
Abrasion involves the physical grinding and scraping of rock fragments against each other or against a solid rock surface. This action is driven by agents of erosion, including water, wind, and glacial ice. Moving water carries sediment that collides with the channel bed and banks, smoothing and rounding the rock fragments. Wind acts as an abrasive agent in arid regions, blasting sand particles against exposed rock faces. Glacial abrasion occurs as ice sheets drag embedded debris across the underlying bedrock, creating grooves and polishing the surface.
Living organisms also contribute to mechanical weathering through biomechanical action. The most recognizable form is root wedging, where plant roots seek out moisture and nutrients by growing into existing cracks and fissures. As the roots increase in diameter, they exert an outward force strong enough to widen the rock fractures.
Burrowing animals, such as rodents and earthworms, physically move and loosen soil and rock fragments as they dig. This exposes fresh surfaces to other weathering agents.