Weathering The process by which rock is broken down into smaller pieces May be physical or mechanical (they mean the same thing) May be chemical May be biological

Physical or Mechanical Agents (things that cause weathering): Ice (changes in temperature) Wind Water Gravity Plants Animals

Physical or Mechanical - Abrasion Abrasion – the grinding or rubbing and wearing away of rock (think sandpaper) Wind Blows sand and silt against rocks Water Carries sand, pebbles over rock In fast-moving streams and rivers, pieces of rock that are being moved by the flow rub against one another and against other pieces resting on the river bed Other rocks Collide or tumble against each other Gravity carries rocks downhill (think about landslides or rock falls) Ice & glaciers pieces of rock that are frozen into the ice are dragged along beneath the glacier Plants Roots grow in cracks. As the plant grows, the root becomes larger. The pressure of a growing root can be great. The pressure makes cracks in the rocks larger, and, as roots grow, the rocks break apart. Animals

Physical or Mechanical - Abrasion Plants (also biological weathering) tree roots grow into fractures in a rock and break the rock apart Animals can dig or burrow on rocks

Physical or Mechanical Agents Ice Frost action – alternate freezing and thawing (cold-hot-cold-hot-cold-hot) Rock expands and contracts over and over until rocks cracks and breaks Ice wedging – Water gets into cracks in rocks during warm weather Freezes during cold weather, expands and cracks rocks

Chemical Weathering The process that breaks down rock through chemical changes The decomposition of rocks due to chemical reactions occurring between the minerals in rocks and the environment Creates holes or soft spots in rock, so the rock breaks apart more easily

curved plates of rock are stripped from rock below. This results in exfoliation domes or dome-like hills and rounded boulders

Chemical Weathering Agents (things that cause chemical weathering) Water The main agent of chemical weathering Many minerals in rock react with water Oxygen When oxygen combines with some other substances, a new substance is formed (think about rust) Carbon dioxide Carbon dioxide in the air or soil can combine with water Produces a weak acid, called carbonic acid, that can dissolve rock. Plants Produce weak acids that dissolve the rock (biological weathering) Lichens (combinations of fungi and algae) which live on rocks, secrete acides that break down the rock Animals

Relationship Between Mechanical & Chemical Weathering Chemical and mechanical weathering often go hand in hand; mechanical weathering breaks rock into pieces, this exposes more surface area to chemical weathering.