2.4 Weathering, Erosion, and Deposition

Weathering, erosion, and deposition shape Earth's surface. These processes break down rocks, move sediments, and build new landforms. They're part of a continuous cycle that creates diverse landscapes like canyons, deltas, and beaches.

Physical and chemical weathering break rocks into smaller pieces. Erosion then moves these fragments to new locations. Finally, deposition settles the sediments, forming new features. Together, these processes constantly reshape our planet's surface.

Weathering, Erosion, and Deposition

The Continuous Cycle of Earth's Surface Modification

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• Weathering is the process by which rocks are broken down into smaller pieces through physical and chemical means at or near the Earth's surface
• Erosion is the process by which weathered rock and soil are transported from one location to another by agents such as water, wind, ice, and gravity
• Deposition is the process by which eroded sediments are dropped or settled out of the transporting medium and accumulate in a new location
• Together, weathering, erosion, and deposition are responsible for the formation and continuous modification of landforms and landscapes on Earth's surface
• These processes work in a continuous cycle, with weathering breaking down rocks, erosion transporting the weathered material, and deposition building up new landforms (sedimentary rocks)

The Roles of Weathering, Erosion, and Deposition in Shaping Earth's Surface

• Weathering breaks down rocks into smaller fragments and particles, providing the raw materials for erosion and deposition
• Erosion removes and transports weathered rock fragments and soil particles from their original location, reshaping the landscape by creating features such as valleys, canyons, and plains
• Deposition builds up new landforms and sedimentary layers by settling out transported sediments in locations such as river deltas, beaches, and alluvial fans
• Over time, the cumulative effects of weathering, erosion, and deposition can significantly alter Earth's surface, creating diverse landscapes and geological features (Grand Canyon, Nile Delta)

Physical vs Chemical Weathering

Physical Weathering Processes

• Physical weathering, also known as mechanical weathering, is the breakdown of rocks into smaller pieces without changing their chemical composition
• Examples of physical weathering processes include:
  • Frost wedging: water freezing and expanding in rock cracks, causing the rock to split apart
  • Exfoliation: rock layers peeling off due to pressure release, often seen in granite outcrops (Half Dome in Yosemite)
  • Abrasion: rocks colliding and grinding against each other, particularly in high-energy environments like rivers and beaches
  • Thermal expansion and contraction: repeated heating and cooling of rocks causing them to crack and break apart (desert environments)
• Biological weathering, a type of physical weathering, occurs when living organisms contribute to the breakdown of rocks through processes such as root growth and burrowing (tree roots splitting sidewalks)

Chemical Weathering Processes

• Chemical weathering is the breakdown of rocks through chemical reactions that alter the mineral composition of the rock
• Examples of chemical weathering processes include:
  • Oxidation: reaction with oxygen, often resulting in the formation of iron oxides (rust) in iron-rich rocks
  • Hydrolysis: reaction with water, causing minerals to dissolve or transform into clay minerals (feldspar to clay)
  • Carbonation: reaction with carbonic acid (formed when water and atmospheric carbon dioxide combine), leading to the dissolution of carbonate rocks (limestone caves)
  • Acidification: reaction with acids, such as those produced by lichens or acid rain, accelerating the breakdown of rocks (marble monuments)
• Chemical weathering is often more prevalent in warm, humid climates where water and chemical reactions are more abundant

Sediment Transport and Deposition

Erosional Processes and Agents

• Erosion and deposition are driven by various agents, including water, wind, ice, and gravity, which transport sediments from one location to another
• Water erosion occurs through processes such as:
  • Sheet erosion: uniform removal of surface material by overland flow (hillslopes after heavy rain)
  • Rill erosion: small channels formed by concentrated surface runoff (agricultural fields)
  • Gully erosion: larger channels formed by further concentration of runoff (badlands topography)
  • Stream and river erosion: downcutting and lateral erosion by flowing water (meandering rivers, oxbow lakes)
• Wind erosion occurs when strong winds pick up and transport loose sediments, particularly in arid environments with little vegetation cover (sand dunes, loess deposits)
• Glacial erosion occurs when glaciers move across the landscape, plucking and abrading rocks and sediments (U-shaped valleys, cirques)
• Mass wasting, a type of gravity-driven erosion, involves the downslope movement of rock, soil, and debris through processes such as rockfalls, landslides, and creep (Yosemite rockfalls, coastal cliffs)

Depositional Environments and Features

• Deposition occurs when the energy of the transporting medium decreases, causing sediments to settle out and accumulate
• Examples of depositional features include:
  • Deltas: formed by the accumulation of river sediments at the mouth of a river (Mississippi River Delta)
  • Alluvial fans: fan-shaped deposits formed where steep mountain streams enter a flat plain (Death Valley)
  • Floodplains: flat areas adjacent to rivers that are built up by the deposition of sediments during floods (Nile River floodplain)
  • Beaches and barrier islands: formed by the deposition of sand and sediments along coastlines (Miami Beach)
  • Sand dunes: formed by the accumulation of wind-transported sand in arid environments (Sahara Desert)
  • Glacial moraines: ridges of debris deposited along the edges or at the terminus of a glacier (Kettle Lakes in the Midwest)

Factors Affecting Weathering Rates

Climatic Influences on Weathering

• Climate plays a significant role in weathering rates, with higher temperatures and greater moisture generally accelerating chemical weathering processes
• Warm, humid climates promote chemical weathering by providing abundant water and heat for chemical reactions (tropical rainforests)
• Cold climates with frequent freeze-thaw cycles enhance physical weathering through frost wedging (alpine environments)
• Arid climates tend to have slower chemical weathering rates due to limited water availability but may experience more physical weathering due to temperature fluctuations and wind (deserts)

Biological and Geological Factors Influencing Weathering

• The presence of vegetation can slow erosion rates by stabilizing soil and reducing the impact of raindrops and surface runoff (root systems holding soil in place)
• Rock type and structure influence weathering and erosion rates:
  • Softer, more porous rocks (sandstone) generally weather and erode more quickly than harder, more resistant rocks (granite)
  • Rocks with many fractures or joints are more susceptible to physical weathering (fractured bedrock)
• Topography affects erosion and deposition rates:
  • Steeper slopes generally experience faster erosion due to increased runoff velocity and mass wasting (mountain slopes)
  • Gentler slopes or depressions favor deposition by reducing the energy of the transporting medium (floodplains, lakes)

Anthropogenic Influences on Weathering and Erosion

• Human activities can significantly increase erosion rates by removing protective vegetation cover and altering natural drainage patterns:
  • Deforestation exposes soils to increased erosion by wind and water (clear-cutting for agriculture or logging)
  • Agriculture practices such as tilling and overgrazing can reduce soil stability and increase erosion (Dust Bowl of the 1930s)
  • Urbanization and the construction of impervious surfaces (roads, buildings) increase surface runoff and erosion (urban stream syndrome)
  • Mining and quarrying activities can expose large areas of bare rock and soil to weathering and erosion (strip mining, mountaintop removal)
• Human-induced climate change may alter precipitation patterns and intensities, affecting weathering and erosion rates (increased frequency of extreme weather events)

Other Factors Influencing Weathering and Erosion

• The intensity and frequency of precipitation events can affect erosion rates, with high-intensity storms generally causing more erosion than gentler, more prolonged rainfall (flash floods)
• The size and shape of sediment particles influence their susceptibility to erosion and the distance they can be transported before being deposited:
  • Smaller, lighter particles (clay, silt) are more easily eroded and transported than larger, heavier particles (sand, gravel)
  • Angular particles are more resistant to erosion than rounded particles due to their interlocking nature (talus slopes)
• Time is a crucial factor in weathering and erosion, as these processes occur gradually over long periods (millions of years for significant landscape changes)