Weathering is the natural process that breaks down rocks and minerals on Earth’s surface into smaller fragments or altered compounds. This action occurs right where the rock is located, distinguishing it from erosion, which involves the transport of broken materials. The two primary categories of this geological breakdown are chemical weathering and physical weathering. This article focuses specifically on the mechanical actions that cause physical weathering and provides detailed examples of these forces at work.
What Defines Physical Weathering
Physical weathering, also known as mechanical weathering, is the disintegration of rock material through the application of a physical force. This process breaks a large rock mass into smaller pieces without changing the chemical composition of the minerals within it. A granite boulder, for example, remains granite after physical weathering, but is reduced to smaller grains. The application of mechanical stress, such as pressure or temperature change, is required for this type of breakdown. This fracturing is significant because reducing the size of the rock increases its total surface area, which then allows other weathering processes to act more effectively.
Breakdown Caused by Ice and Water
One of the most widely recognized examples of physical weathering is frost wedging. This occurs in climates where temperatures frequently cycle above and below the freezing point of water. Liquid water seeps into pre-existing cracks and fissures within the rock mass. When the temperature drops below freezing, the water turns to ice and expands its volume by approximately 9%. This volumetric expansion exerts a substantial outward pressure on the surrounding rock walls. With repeated freeze-thaw cycles, this pressure gradually widens the crack until a piece of the rock breaks off entirely, creating piles of angular rock fragments called talus slopes at the base of cliffs.
Water itself, even in its liquid form, can contribute to mechanical breakdown through hydraulic action. This occurs when powerful waves or fast-moving river water crash against a rock face and force air and water into small cracks. The intense, sudden compression of this trapped air and water exerts significant pressure on the rock walls, contributing to the fracturing of the material. This force is particularly noticeable along rocky coastlines where the constant impact of high-energy waves works to widen existing weaknesses in the stone.
Breakdown Caused by Pressure and Temperature
Physical weathering is also driven by changes in the load surrounding a rock mass, creating internal stresses. Pressure release, or unloading, occurs when deeply buried igneous rock masses are exposed at the surface by the erosion of overlying material. The removal of this confining pressure allows the rock to expand outward, causing fractures that are roughly parallel to the exposed surface. This expansion results in large sheets or slabs of rock peeling away from the main body, a process known as exfoliation or sheeting.
Temperature fluctuations can also induce stress through thermal expansion and contraction. Rocks are composed of various minerals, each with a different thermal expansion coefficient, meaning they expand and contract at different rates when heated or cooled. In environments with large daily temperature swings, such as deserts, the rock’s exterior heats and expands faster than the interior during the day. This differential movement creates internal stress and micro-fractures, eventually leading to the flaking or granular disintegration of the rock surface.
Salt Crystal Growth (Haloclasty)
A related mechanism, common in arid and coastal settings, is salt crystal growth, or haloclasty. Water containing dissolved salts seeps into pores and cracks in the rock structure. As the water evaporates, the salts crystallize and grow, exerting an outward pressure on the surrounding rock similar to ice expansion. Certain salts, such as sodium sulfate, can also swell significantly through hydration, generating enough force to dislodge mineral grains and cause the rock to crumble.