Weathering / Soils

Weathering The process by which natural forces break down rocks

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)

Root growth Ice Wedging Exfoliation Abrasion

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)

Sediment: Solid materials (rock fragments, minerals, organic remains) that are carried by water or air that settle on the bottom of a water body or ground.

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

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

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

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