Weathering, erosion and deposition are slow processes that gradually alter the Earth’s surface over thousands or millions of years. They are caused by multiple natural forces including airflow, rainwater run-off, sunrays, frost or pressure which all have an influence.
Weathering refers to the physical and chemical breakdown of rocks and minerals over time. Erosion involves movement of these degraded particles while deposition involves depositing or depositing this debris back onto its original source.
Physical weathering breaks apart rocks and minerals into smaller components that are easier to transport and erode, such as sand, silt, gravel and rock fragments known as lithic clasts. Physical weathering may occur through processes such as heating/cooling/frost wedging/impact from glaciers/animals etc.
Rocks often break through natural zones of weakness such as bedding planes in sedimentary rocks, exfoliation on metamorphic rocks or joints in massive igneous rocks to unload themselves from overlying layers, which then allows both physical and chemical weathering processes to widen cracks within them.
Water can contribute to physical weathering through freezing and thawing cycles, chemical weathering that dissolves rocks, or moving rock particles through erosion. Fast-moving waters tend to erode more quickly than slow waters, producing landscape features like meandering rivers and Oxbow lakes more quickly than slow waters do. Living organisms or poor solubility salt deposits within rock pores and cracks may further aid erosion forces.
Chemical weathering involves changes to the mineral’s chemical composition, unlike physical weathering which breaks rocks apart into smaller pieces. Chemical weathering can also alter rock shapes by rounding or porifying it more, changing its overall form as a result.
Minerals that form deep within the Earth may undergo chemical weathering processes that transform them into more stable forms that are suitable for use at Earth’s surface. Limestone can be dissolving over time by carbonic acid seeping into the ground from rain or spring water seeping through, leading to sinkholes, caves, and cliffs being formed as a result.
Chemical weathering is a critical process in the formation of soils, with tiny bits of weathered rocks mixing with plant remains, fungi, bacteria, and other material to form fertile soil. Exposure time affects how susceptible rocks are to weathering; those exposed for longer are more vulnerable; for example lava flows that quickly cover themselves are less likely to experience weathering than flows left exposed for extended periods.
Biological weathering occurs when living or once-living organisms help break down rocks and soil through natural means, such as animal burrowing in rocks or breaking them apart with their claws, plants growing into cracks in rocks to widen existing fractures further, eventually cracking open the rock itself, etc.
Other organisms can help speed the chemical process of weathering by emitting chemicals into the atmosphere that turn to acids in the atmosphere, eventually returning as rain on Earth with acidity that damages plants and makes breaking down rocks more difficult.
Chemical weathering includes various forms, such as oxidation, carbonation and hydration. Rocks containing iron can be easily eroded when exposed to water due to oxidation – turning it into rust – while limestone can dissolve due to carbonation when rainwater reacts with it, producing caves or sinkholes similar to China’s Stone Forest.
Weathered rocks become softer and smaller with age, while erosion occurs when their particles are carried away by rivers or winds – these natural processes have left our planet with landscapes like the Grand Canyon sculpted from stone by erosion. No single rock has ever been strong enough to resist them all!
Human activities also influence the speed and type of weathering. Air pollution from coal-fired power plants produces acids which attack rocks, leading to chemical weathering; and pave over land alters soil’s ability to absorb rainfall, increasing erosion.
Damming rivers is an effective way of combatting erosion by stopping sediment from moving downstream, slowing its rate of deposit accumulation over time and possibly even helping prevent floods altogether. The lesson also features labs and worksheets for student use during learning.