Weathering

• Weathering is the decomposition and disintegration of rocks and minerals at the Earth's surface by mechanical and chemical processes.
  Mechanical weathering is the physical disintegration of rock into smaller pieces. (e.g. plant roots may grow in cracks in building stones, enlarging the cracks and eventually toppling the walls.) The smaller fragements are identical to the original rock. Mechanical weathering includes six processes: frost wedging, salt cracking, abrasion, biological activity, pressure release fracturing, and thermal expansion and contraction.
  Frost Wedging - a process when freezing water in cracks pushes a rock apart. Results in a form of mechanical weathering. Rocks that have undergone frost wedging often fall from cliffs collecting in talus slopes.
  Salt Cracking - Rocks may be cracked by growing salt crystals. Salt water sometimes seeps between mineral grains in a rock, and when the water evaporates, the dissolves salts crystalize. Growing crystals widen cracks and push grains apart.
  Abrasion - the mechanical wearing and grinding of rock surfaces by friction and impact. Produces rocks with a smooth, rounded appearance.
  Biological Activity - occurs when rocks are pushed apart by growing plants.
  Pressure Release Fracturing - Tectonic forces push granite up to form a mountaing range. As overlying rock erodes, pressure on granite decreases. The rock expands, cracking in the process.
  Exfoliation - a process in which large concentric plates split off the rock. This process commonly fractures granite.
  Thermal Expansion and Contraction - A rock expands when it is heated and contracts as it cools.If a rock heats rapidly, the surface expands faster than the interior and objects may fracture.
  Chemical weathering occurs when oxygen, water, acids, and bases attack rocks chemically. A chemically weathered rock contains different minerals and has a different chemical composition from the original rock.
  Weathering by Solution - Acids and bases increase the rates at which most minerals dissolve. Common minerals dissolve in water.