Weathering / Soils

Weathering  The process by which natural forces break down rocks  Erosion: The break up and transport of earth materials

Mechanical Weathering  The breaking up of rocks by physical forces  Ice Wedging: water freezing, expanding and splitting rock  Pressure release: (exfoliation) Inside pressure causes rock to expand when overlying burdens are removed  Plant root growth: plant roots expand cracks  Abrasion: wearing down by friction (moving water rubbing rocks against each other)

Ice Wedging Root growth ExfoliationAbrasion

Chemical Weathering  Breakdown of rock by chemical reactions  Dissolving: minerals dissolving in water or weak acids  Rusting: Iron in rocks combining with oxygen to produce rust (iron oxide)

Soil  A mixture of weathered rock particles, organic matter, water and air  Humus: decayed plant and animal material

Soil Horizons  A horizon: upper most layer (topsoil) containing the most humus. Darkest layer  B horizon: little humus, brown to reddish brown, contains clay that washes down from A horizon  C horizon: deepest layer, largest least- weathered rock particles, light yellowish brown

Soil Properties  Texture: size of particles in soil  Sand: largest, visible, feels gritty  Silt: smaller – microscopic, feels silky  Clay: smallest – microscopic, feels sticky when wet

Properties continued Color: comes from iron compounds and humus humus – black, iron – reddish original rock- yellows or tans bright colored soils drain well Pore space: the spaces between particles Chemistry: pH (acidity) determines how well nutrients dissolve