Cracks That Form in Rocks Due to Expansion

Joints

• Joints are fracture lines in rocks that develop when rock expands through extremes of heat or cold or when pressure is abruptly released from within the rock. When joints develop, these cracks open more areas of rock to the weathering action by either wedging the joint further apart or continually abrading the exposed areas of the rock. Rocks exposed to weathering processes continuously break down into smaller and smaller pieces--a process which can take either hundreds or thousands of years.

Temperature Change

• Rocks form from different minerals that combine to form a solid, rigid structure. These minerals may mix unevenly in a rock, with different minerals reacting to temperature changes in differing ways. Some rocks, when exposed throughout the day to highs and lows in temperature, will crack over time because the minerals inside expanded in the daytime heat, and contracted once the sun set. Rock may also crack when exposed to extreme temperatures from fire or volcanic activity. Fluctuating temperatures are one aspect of physical weathering and one of the major causes of cracks in rocks.

Pressure Effects

• Rocks may contain gases trapped within them, or a slab of rock may be located on top of a chamber of either a liquid or gas. Shifts in the pressure these substances exert on the surrounding rock structure can compromise the alignment of the rock's mineral structure or weaken the bonds between minerals, causing cracks to form in the rock's structure. This can happen when an underground water or oil supply is drained, or if a mine collapses beneath the ground. Pressure changes are another example of how physical weathering can develop joints in rocks.

Joint Expansion

• Weathering can also expand existing joints in rocks, accelerating the rock's decomposition. In another process of physical weathering, water expands as it cools to form ice. When ice forms in a rock joint, it can exert enough force to widen cracks that already exist in rocks. In biological weathering, plant roots grow into rock joints to widen cracks further as the roots gain size over the course of their life.