Chapter 12: Weathering

Chapter 12.1

Weathering – The breakup of rock due to exposure to process that occur on Earth’s surface.

Mechanical Weathering - (disintegration) the breakdown of rock into smaller pieces (no change in composition) Frost wedging occurs when water gets into cracks of the rock and freezes when freezing water occupies 10% more space. Bare mountaintops are very susceptible forming large sharp cornered boulder fields below Pavement potholes are also caused by frost wedging

Abrasion – caused by water wind and ice scraping rocks, sand and pebbles against one another. Plants and animals Roots grow and split rock Animals make holes Upward Expansion Exfoliation – the release of pressure from overlying rocks causes rocks underneath to expand upward and crack.

Chemical weathering – (decomposition) Takes place when the rock’s minerals are chemically changed into something different. Usually caused by water or water vapor

Hydrolysis – reaction of water with other substances Usually increased in effect due to an acid in the water Carbonic acid – rainwater with CO2 has the greatest effect on calcite (limestone) forming underground caverns. Acid rain – sulfur dioxide, nitrogen and carbon dioxide released by industries react with water in the air.

Oxidation - oxygen with other substances Oxygen and iron will form a red iron oxide called hematite

Rates of Weathering The more the rock is broken down the faster it weathers due to more surface area exposed to water. Composition of the rock Sandstone with a silica (quartz) cement is more durable than one with a calcite cement Climate – a warm wet environment is more conductive to both chemical and mechanical weathering

Chapter 12.2

Soil – is made of loose weathered rock material with organic matter (dead stuff)

How soil forms Parent material – what the soil came from Residual – from the under laying rock Transported - brought in by rivers glaciers or wind and deposited in the area. Soil Profile – how the soil is layered in 3 distinct zones A horizon – usually dark to light gray (topsoil) B horizon – often red to brown (subsoil) C horizon – partially weathered bedrock

Soil Composition – most important factor is climate p.266 in book. Tropical forest soil – has a very deep soil profile because of the warm wet region it forms in.

Chapter 12.3

Mass Movement (The downward transportation of weathered material by gravity) Erosion (the removal and transport of material by natural agents)

Types of Mass Movements Loose rock material continually moving downhill after it has been weathered. May form a talus pile at the base of a steep hill or mountain a talus pile can have a 40° slope. Landslide - movement of bedrock and soil down slope. Creep – a very slow down slope movement – fence post or trees may move over time. Slump – Blocks of land fall straight down. Earth flow – Mass of weathered material and water move down slope. Mudflow – rapid movement of water with large amounts of suspended clay and silt

Chapter 12.4

Soil as a resource Less than 25% of earth’s land can be used for crop growing.

Soil fertility – the ability of soil to grow plants Determined by the minerals, water and organic matter in the soil Soil Depletion Caused by crop plants using up the soils nutrients and not being replaced because the organic material from the plants is not replaced Fertilizers are used to replenish nutrients, but cause serious environmental damages. Salinization – When desert soils are artificially watered, the water leaves behind minerals that accumulate over years and the soils can no longer sustain crops.

Erosion and Soil Conservation Types of Soil Conservation Windbreaks – trees planted along fields to break up the wind Constructing terraces – to slow the water running downhill Contour forming – Plow with the contour lines to reduce the speed of downhill water. Strip farming – Alternate bare ground crops (corn) with ground covering crops (alfalfa).