Weathering is the process of breaking rocks apart using chemicals and mechanical forces, while erosion refers to their gradual removal from their original positions through gravity, water flow or wind currents.
Freezing and thawing erosion causes cracks to widen in rock surfaces, while ice wedging refers to when wedges of ice grow to fill cracks between rocks.
Weathering
At any point in time since Earth began existing, weathering and erosion have played a crucial role in shaping its landscape. Weathering breaks apart rocks into smaller pieces which can then be transported via erosion before being deposited somewhere else through deposition. Water, wind, ice, plants and gravity all play their parts to cause weathering to take place.
Water is essential in chemical weathering as its reactions with rocks soften them, making them easier for other forces to break apart. Such weathering tends to occur more in warm, humid environments as their conditions facilitate weathering reactions that lead to its progression.
Mechanical weathering refers to processes like expansion and contraction caused by heat from sunlight or freezing water enlarging rock surfaces and cracking them, or by erosion from waves pounding coastlines or glaciers (see Chapter 14). Geologists measure erosion by looking at how well-rounded sediment grains are; more rounded particles indicate further transportation and more vigorous erosion.
Erosion
Erosion is the process of shifting soil, rocks, mud or other materials from one location to the next through wind, river water (including seawater and riverine streams), glacial ice (glaciers) or living organisms.
Erosion is caused by liquid water: raindrop impact, runoff from steep slopes and floodwaters are all key agents of erosion. Erosion may happen as part of one event such as flash flooding or over longer time periods such as riverine systems.
Different forms of erosion produce distinctive geological features. Hoodoos in Bryce Canyon and Goblin Valley state parks of Utah were formed from rocks weathered differently, wherein more resistant rocks stood up while less resistant ones weathered away into fine materials like sand or even fine pebbles, leaving behind their mark as they weathered away over time. Sandstone arches in Arches National Park followed this same pattern. Aeolian erosion — wind-driven erosion — transports dust, sand and small pebbles from place to place, similar to how sandstone arches would.
Deposition
Deposition is an unintentional but unavoidable side-effect of weathering and erosion, occurring when fragments of Earth are carried by wind, flowing water or the sea and then deposited somewhere new – for instance sand grains picked up by winds become dunes; minerals dissolved in water that have then been deposited onto plant roots or marine organisms form sediment deposits.
Most forms of erosion require water for their progress and are affected by both amount and temperature of moisture. Steep mountain slopes for instance increase mechanical weathering because more surface area is exposed to weathering and erosion, while moisture also accelerates chemical weathering by softening rocks, making them more likely to break apart physically; hence why we find so much chemical weathering occurring in coastal environments and at higher temperatures.
Impacts
Weathering and erosion continuously shape Earth’s rocks into ever-evolving landscapes. Their rates depend on numerous factors, including climate and topography; local rock types may also influence erosion rates in certain regions.
Erosion is a mechanical process driven by water, gravity (see Chapter 10), wind or ice (see Chapter 12 or 14) or both and often moves sediment to new locations. Liquid water acts as the main agent in this process of erosion.
Sediments are rocks and organic materials deposited as the result of erosion, from large blocks to microscopic particles.
Some sediments are produced through physical weathering while others result from chemical weathering; for example, boulders scattered on the ground are probably produced through physical weathering while beach beaches often exhibit large clumps of sand and pebbles due to chemical weathering processes. Plants may also contribute to this weathering by growing through cracks in rocks.