7

• Weathering


  The process that decomposes rocks and converts them to loose gravel, sand, clay, and soil. It involves little or no movement of the decomposed rocks and minerals.
• Types of weathering


  • Mechanical weathering
  • Chemical weathering
• Mechanical weathering


  Also called physical weathering, it reduces solid rock to small fragments but does not alter the chemical composition of rocks and minerals.
• Pressure-release fracturing


  When a granitic pluton solidifies from magma at depth, the overlying rock and sediment may erode, decreasing the pressure and causing the rock to expand and fracture.
• Plutons


  • Rock bodies that cool beneath the surface, including batholiths, sills, dikes, and laccoliths.
• Frost wedging


  When water accumulates in a crack and then freezes, the ice expands and wedges the rock apart.
• Abrasion


  The mechanical wearing of rocks by friction and impact, such as rocks, grains of sand, and silt colliding with one another when carried by currents or waves.
• Organic activity


  When soil collects in a crack in bedrock, a seed may sprout and the roots work their way into the crack, expanding and widening it.
• Thermal expansion and contraction

  Rocks at the surface expand when heated and contract when cooled, and the forces generated may fracture the rock.
• Chemical weathering


  Occurs when air and water chemically react with rock to alter its composition and mineral content.
• Dissolution


  Water dissolves minerals by pulling atoms away from the crystal structure when outside attractions are stronger than the bonds within the crystal.
• Hydrolysis


  Water reacts with one mineral to form a new mineral that has water as part of its crystal structure, such as feldspar weathering to form clay.
• Oxidation


  Elements like iron react with atmospheric oxygen, causing minerals to decompose.
• Mechanical and chemical weathering acting together

  After mechanical processes fracture a rock, water and air seep into the cracks to initiate chemical weathering.
• Salt cracking


  When salty water evaporates, the growing salt crystals exert forces that loosen mineral grains and widen cracks in the rock.
• Exfoliation


  A process in which plates or shells of weathered material split away from granite, possibly due to hydrolysis-expansion.
• Erosion


  The removal of weathered rocks by rain, running water, wind, glaciers, or gravity, transporting the material to a new location.
• Weathering decomposes bedrock, and plants add organic material to the regolith to create soil, but soil does not accumulate indefinitely due to erosion.
• Turk & Thompson, 2012: 'From the breaking waves brings the salt to the rock'
• Figure 7. Sea Eroding a Cliff
• SH1632 07 Handout 1
• Erosion


  The removal of weathered rocks that occurs when rain, running water, wind, glaciers, or gravity transports the material to a new location
• Weathering


  Decomposes bedrock, and plants add organic material to the regolith to create soil at Earth's surface
• Soil does not accumulate and thicken throughout geologic time
• Interactions with flowing water, wind, and glaciers erode soil as it forms
• Weathered material simply slides downhill under the influence of gravity
• All forms of erosion combine to remove soil about as fast as it forms
• Soil is usually only a few meters thick or less in most parts of the world
• Mass Wasting


  The downslope movement of Earth material, primarily caused by gravity
• Landslide


  A general term for mass wasting and for the landforms created by mass wasting
• Gravity


  • Acts constantly on all slopes, the strength of a rock and soil usually hold the slope in place
• Steep slopes


  • Are especially vulnerable, and landslide scars are common in the mountains
• Angle of repose


  The maximum slope or steepness at which loose materials remains stable
• Rocks commonly tumble from cliff to collect at the base as angular blocks of talus
• Talus


  The angular blocks interlock and jam together, typically has a steep angle of repose, up to 45o
• Sand grains


  Do not interlock and have a lower angle of repose, about 30o to 35o
• Water


  Small amounts bind sand grains together, excess water lubricates the sand and adds weight to a slope
• Vegetation


  Roots hold soil together and plants absorb water, a vegetated slope is more stable than a similar bare one
• Landslides are common in deserts and regions with intermittent rainfall
• Earthquakes and Volcanoes


  Can cause landslides by shaking an unstable slope or melting snow and ice