Rocks and Weathering Chapter 8.1

Weathering- the breaking of rock into smaller fragments Mechanical weathering – breaks rock apart without changing the chemical composition Examples: Rocks colliding with other rocks in rivers Tree roots lifting rock Ice wedging caused by freezing and thawing

The composition of the rock is changed Chemical weathering – reaction of air, water and/or other substances with the minerals in rock The composition of the rock is changed

Water is the main cause of chemical weathering It dissolves away some minerals leaving a different composition behind Compounds in air combine with water and react with minerals upon contact Some metals react with oxygen and water (oxidation)

Acids cause chemical weathering Water and carbon dioxide form carbonic acids which dissolve calcite and create caves Plant roots and decaying plants create acids that dissolve minerals making rock weak

Climate effects the speed of weathering - Chemical weathering is most rapid in warm, moist areas. - Mechanical weathering is most rapid where freezing and thawing occur.

Chemical or Mechanical? Colonies of lichens growing on the surface of rocks. Chemical

Chemical or Mechanical? The wedging of tree roots along natural joints in granite outcrops Mechanical

Chemical or Mechanical? Solution of limestone by acidic groundwater Chemical

Chemical or Mechanical? The oxidation of minerals which contain iron Chemical

Chemical or Mechanical? Animal burrows dug in rock which let in water and air Mechanical

Chemical or Mechanical? Repeated freezing and thawing of water cracks rock or joints Mechanical

Chemical or Mechanical? The action of water, salt and air on car fenders and panels

Chemical or Mechanical? The action of water, salt and air on car fenders and panels Chemical

Chemical or Mechanical? The sun fading paint on houses and cars Chemical

Chemical or Mechanical? Formation of potholes in streets during severe winters Mechanical

Chemical or Mechanical? Lifted sections of sidewalk along tree-lined streets Mechanical

How Soil Forms Section 8.2

Soil Profile Soil – a mixture of weathered rock and organic matter - can take hundreds of years to form - varies in thickness Humus – decaying plants and animals that mix with weathered rock Soil Profile - shows the layers of soil and weathered rock

O-Horizon Litter layer- leaves A-Horizon Top Soil – dark colored, sediment, decayed material, insects, worms B-Horizon Lighter color, little or no humus, contains minerals from leaching C-Horizon Some material leached from B-horizon, partly weathered rock, no humus Solid rock below soil profile