Ch. 5- Weathering and Erosion Mrs. Giblin Earth Science

Weathering – “breaking of rocks”. 2 Types of Weathering: Mechanical Weathering – physical forces break rocks without changing its chemical composition. Causes: 1. Frost Wedging- When water gets into a crack and freezes. -Freezing causes water to expand by 9%! -Causes rocks to split apart.

Unloading – The pressure over an igneous rock formation gets released. exfoliation – outer layers of rocks pop off. Biological Activity – Tree roots growing into rocks. Burrowing animals. Humans-driving/walking over surfaces.

Chemical Weathering – Transformation of rock into 1 or more compounds. Water – water absorbs gases (oxygen, CO2) and creates “chemicals”. -CO2 + H2O = Carbonic acid (acid rain) -spheroidal weathering – water running over rocks wears them into spheres.

Rates of Weathering: Rock Characteristics: Climate: Mineral composition Cracks Climate: Temperature Moisture Causes frost wedging! MORE chemical weathering occurs when the climate is warm and moist!

Regolith- layer of rock and mineral fragments Regolith- layer of rock and mineral fragments. Soil- Part of the regolith that supports plant growth. 4 Components of Soil: -mineral matter – broken down rocks. -organic matter – “humus” – decaying plants and animals. -water -air } Both are found in pore spaces between particles. Soil Texture – Proportions of different particle sizes. -The USGS has categorized soils by texture. Soil Structure – How easily soils can be cultivated and eroded. -How easily water & nutrients move through the soil.

Soil Formation: Parent Material – source of mineral matter. Soil formed on bedrock-residual soil. Soil formed on unconsolidated sediments – transported soil. Transported soil forms quickly. Regolith – layer of rock & mineral fragments. Time – the longer the soil has been forming, the thicker it becomes. Climate – greatest factor of formation. Variations in temperature and precipitation affect soil type. Affects what plants and organisms live there. Organisms – Add to organic matter in the soil. Decompose dead material. Bacteria release nitrogen. Burrowing organisms mix soil. Slope – Steep slopes have more erosion and less water content. Flat areas have less erosion and less drainage. Direction of slope-facing sun?

C Horizon – partially weathered parent material. Horizons: Layers of soil classified by differences in composition, texture, structure, and color. Soil Profile: A vertical section of the horizons. A Horizon – topsoil Made mostly of organic matter. Full of insects, fungi, and microorganisms. B Horizon – subsoil Contains clay washed through the A horizon by water. Clay that accumulates forms “hardpan”. Lower limit for plant roots and burrowing animals. C Horizon – partially weathered parent material.

Soil Types: Pedalfer – forms in temperate areas that receive > 63 cm. of rain/year. Usually brown-red in color. Pedocals – forms in drier areas with grasses and brush. Usually light grey-brown. Laterite – forms in hot, wet tropical areas. Usually orange-red.

Soil Erosion: Water, wind, other agents. Sheet erosion – thin sheets of H2O move the particles. Human activities – farming, construction, and logging. Soil Deposition: Sediments deposited in rivers may block entrances for ships. Contamination of sediments causes contamination of streams. Controlling Erosion: Rows of trees – windbreaks Terracing hillsides Rotating crops Cover crops

Mass Movements: Transfer of rock and soil down-slope due to gravity. Triggers of Mass Movements: Water – Soil becomes saturated causing the particles to slide past each other easily. Oversteepened slope – when the angle of a slope is too great to keep the soil stable. Caused by construction of roads or streams undercutting valley wall. Removal of Vegetation – Roots help to hold soil together. Earthquakes – Aftershocks can cause large amounts of soil to dislodge.

Types of Mass Movements: Rockfalls – rocks fall freely through the air. Slides – Block of material moves suddenly along a flat, inclined surface. Rock slide – (includes pieces of bedrock) Land slide Slumps – movement of material along a curved surface. Flows – mass movements containing a lot of H2O. Mudflows Earthflows Creep – movement of only a few mm or cm per year. Usually caused by freezing and thawing.