Weathering, Soil, and Erosion

Chapter 5

Weathering Def: the breakup of rock due to exposure to

processes that occur at Earth’s surface 2 types:

Mechanical- the breakdown of rock that takes place when a rock is broken into smaller pieces of the same material without changing its composition

Chemical- the breakdown or decomposition of rock that takes place when minerals are changed into different substances

Mechanical Weathering

ProcessesFrost wedging

Def: a process in which water freezes in the cracks of a rock and wedges it apart

Mostly in porous rocks and in rocks with many cracks

In places where there are frequent freezes and thaws

Ice Heaving- creates potholes; ice lifts pavement up and it collapses when it thaws

Examples of Frost Wedging and Ice Heaving

Mechanical Weathering cont…

AbrasionDef: the wearing away of

rock material by grinding action

Water, wind and ice are capable of moving rocks.

Sand is a product of abrasion.

Mechanical Weathering cont…

Plants and AnimalsGrowth contributes

Roots wedged into tiny pores and crevices

Burrowing contributesDigging holes into soilBringing rock fragments to

surface

Mechanical Weathering cont…

Upward ExpansionExfoliation- the peeling of surface layers from exposed

bedrockRock is uplifted by tectonic forces (ex: granite)Overlying rock is worn away which reduces pressureUpward expansion causes the granite to break along curved

jointsLarge sheets of loosened rock break away

Upward Expansion Examples

Chemical Weathering

WaterHydrolysis- the chemical

weathering by reaction of

water with other substancesAcids: seeps into the ground

and reacts chemically with

many common mineralsCalcite

Dissolves completely Underground caverns

Chemical Weathering

cont… Acid Rain

Def: rainwater that contains unusually high amounts of acids that can be traced back to pollutants

Increases the rate of chemical weathering

Chemical Weathering cont… Oxidation

Def: chemical reaction of oxygen with other substances

Effective with minerals containing iron

Formation of rust, or iron oxides

                    

Rates of Weathering Weathering is usually a slow process Factors that affect the rate of weathering:

Surface ExposureMore surface exposed, weathers more quickly

Composition of RockVarious rocks are affected differently by weathering

processesClimate

Warm/wet climates: both chemical and mechanicalCold/dry climates: mainly mechanical

Soil Def: loose, weathered rock and organic material in

which plants with roots can growFormed by weatheringContains air, water, organic material, mineral matterParent material

Def: material from which a soil is formedResidual soil- soil whose parent material is the bedrock

belowTransported soil- soils formed from transported materials

Deposits left by winds, rivers, and glaciers

Soil Profile Def: cross section of earth exposed by the digging Soil horizons- three distinct zones of mature soil

A-horizon- topsoilDarker, contains humus (organic material), sandy

B-horizon- subsoilRed or brown, iron oxides, clay from topsoil, calcium and

magnesium

C-horizonWeathered parent material, rock fragments

Soil Composition 3 noticeable parts: sand, silt, and clay

Affects soil’s ability to hold water and air Factors that affect composition:

TimeParent materialPlants and animalsTopographyClimate- MOST IMPORTANT!

12.3 Mass Movements and Erosion

Mass movements- the downward transportation of weathered materialsGravity causes materials to fall, slide, or move

at slow speeds to lower levelsErosion- the removal and transport of

materials by natural agents such as wind and running water

Mass Movements Talus- rock fragments that have

been weathered from a cliff and pulled down by gravity

Landslide- movement of a mass of bedrock or loose soil and rock down the slope of a hill, mountain, or cliff Steep slopes Regions near volcanoes and in

earthquake-prone regions

Mass Movements Creep- slow, imperceptible

movement of soil down a slope Causes fence posts, poles and other

objects fixed in soil to lean downhill

Slump- a block of land tilts and moves downhill along a surface that curves into the slope Tends to occur because bottom of

slope can no longer support top of slope

Mass Movements Earthflows- the downslope movement

of a mass of earth materials that have been saturated with water Slower and less fluid than a mudflow;

velocity affected by amount of water present, the composition of the soil, and the steepness of the slope

Mudflows- the downslope movement of water that contains large amounts of suspended clay and silt Rapid movement; capable of moving

rocks, boulders, trees, and houses; lahars- volcanic mudflows

Erosion Topography- uplifting and weathering/erosion are

happening at same timeMore uplifting- rugged and sharp (Himalayas)More erosion- smooth and rounded (Appalachians)

Climate- in humid areas, water is primary agent and causes rounded topography

Composition of rock- some types are more resistant than other rock typesVolcanic neck- the solidified lava filling the center vent

of an extinct volcano (picture pg 270)