Weathering and Soil

Weathering Effects – surface processes break down rock into small pieces called sediment.

Mechanical Weathering Physical processes break rocks into fragments with the same chemical makeup and characteristics as the original rock. Plant roots and burrowing animals cause mechanical weathering Ice wedging – water enters cracks and freezes and expands, breaking rocks apart. Small pieces of rock have more surface area than larger pieces of rock and weather faster.

Chemical Weathering Chemical reactions dissolve minerals in rocks or change them into different minerals. Carbonic acid, formed from carbon dioxide gas and water, and plant acids can react with minerals to weather rock. Oxidation – chemical process that occurs when iron is exposed to oxygen in the air.

Effects of Climate – pattern of weather that occurs over time. Mechanical weathering is more rapid than chemical weathering in cold climates. Chemical weathering is more rapid than mechanical weathering in warm, wet climates. Rock type can affect rates of weathering.

Formation of soil Can take thousands of years Soil is a mixture of weathered rock, decayed organic matter, mineral fragments, water, and air. Formation is influenced by climate, slope, types of vegetation, and length of time that rock has been weathering.

Soil Profile Make up of different layers of soil 1. Horizon A – top soil layer May be covered with organic litter that may turn into humus Fertile layer with more humus and less rock and mineral particles than other soil horizons 2. Horizon B – middle soil layer Contains less humus and is lighter in color than A horizon Minerals travel from A horizon to B horizon in a process called leaching

Horizon C Bottom soil layer Has very little organic matter and is not strongly affected by leaching Contains rock – the parent material of the soil Glaciers can deposit soil that did not form from the bedrock beneath it.

Soil Types Differ in different places Different regions have different climates that affect soil development Parent rock affects soil formation and type of vegetation that grows in a region Time affects soil development because the longer the weathering has occurred, the less the soil resembles the parent rock. Soil on steep slopes develops poorly

Soil Erosion Soil erosion or loss is important because plants do not grow as well when topsoil is lost.

Causes and effects of soil erosion Many human activities disturb the natural balance between soil production and soil erosion. Agricultural cultivation – Increased farming removes the plant cover, leaving soils open to wind and water erosion Forest harvesting – removes forest vegetation which increases erosion and particularly damages tropical rain forest soil Overgrazing results when animals graze until almost all ground cover disappears. Urban construction clears land of vegetation and removes soil

Links Enchanted learning Animation of soil layers http://courses.soil.ncsu.edu/resources/soil_classification_genesis/soil_formation/soil_transform.swf Animation