• This thing, all things devours.This thing, all things devours.• Birds, beasts, trees, flowers.Birds, beasts, trees, flowers.• Gnaws iron, bites steel.Gnaws iron, bites steel.• Grinds hard stones to meal.Grinds hard stones to meal.• Slays kings, ruins towns, Slays kings, ruins towns, • and beats high mountains down.and beats high mountains down.

• J. R. R. Tolkien: The Hobbit

Destructional ProcessesDestructional Processes

Glacially-modified topography associated with the Prairie Coteau of SD, ND, MN.  During the Wisconsinan, the glacier divided into two southward-flowing lobes. Paraphrased from here.

U-shaped Valley, Main Trunk, Hanging ValleyGlacial Erosion

http://www.ndsu.nodak.edu/instruct/schwert/anaglyph/

Eastern New Mexico,

northwestern Texas.  Included

is the area of White Sands

National Monument and

Guadalupe Mountains

National Park

Destructional Landforms

are shaped by Weathering and Erosion

WeatheringWeathering

Carlsbad Caverns

White Sands

Guadalupe M

ountains

Weathering and SoilsWeathering and Soils

• Mechanical Weathering

• Chemical Weathering

• Soils

Prof. A.H.Thompson, Dirty Devil River, 1872Thompson Construction of Topographic Maps Flipbbok

Powell and Thompson Exploration of the Colorado RiverUSGS geologists studying weathering in the Mancos Shale

Almon Harris ThompsonAlmon Harris Thompson

Frost Frost WedgingWedging

1. Mechanical weathering

Evidence of Evidence of Frost Frost Wedging in Wedging in Wheeler Wheeler Park, Park, NevadaNevada

Source: Tom Bean/DRK Photo

Salt Weathering

Unloading = Unloading = ExfoliationExfoliation

Thermal Expansion and Contraction

Source: Tom Bean

SpallingFrostFire

Tree Roots Growing in Rock Fractures

Animal Burrows

Source: Runk/Schoenberger/Grant Heilman

Mechanical Weathering, Mechanical Weathering, Cubes/Surface AreaCubes/Surface Area

2. Chemical Weathering2. Chemical Weathering• Major processes of chemical weathering

• Dissolution– Soluble ions contained in underground water

• Oxidation– Chemical reaction where compound loses electrons– Important in breaking down mafic minerals– Rust colored weathering of Basalt

• Hydrolysis H+ or OH- ions– Reaction of any substance with water– Water’s ions replace different ions in mineral– Feldspars, most abundant crust minerals, become fine clay particles.

Easily transported.

Role of Water

DissolutionDissolution Weathered and Weathered and

Unweathered Unweathered Limestone Limestone BouldersBoulders

Source: Ramesh Venkatakrishnan

CaCO3 +2HCl => H2CO3 + Ca++ + 2Cl-

H2CO3 => H2O + CO2

1414

Hydrolysis – Feldspar to ClayHydrolysis – Feldspar to Clay

Mechanical fracture due to chemical weathering

Feldspars become

Vegetation and Soil DevelopmentVegetation and Soil DevelopmentNeed the extra humic acid to

dissolve limestone

Oxidation of BasaltOxidation of Basalt

Rust (Iron Oxide) forms

Bowens Reaction Series and Weathering

Mg2SiO4 + 4CO2 + 4H2O => 2Mg+ = 4HCO3- + H4SiO4 (aq)

2KAlSi3O8 + 2H2CO3 + 9H2O => Al2Si2O5(OH)4 + 4 H4SiO4 (aq) + 2K+ + 2HCO3 -

An idealized soil profile

3. Soils

Remember the different horizons

O

A

E

B

C

Organic

Activity

Exited

Back

Crushed Rock

pedalfer pedocal laterite

tropicsShortgrass Evergreen forests

Note no E or C in some

Rainfall seasonal

"Laterite"

Soil FormationSoil FormationFactorsFactors

• Climate: Climate: – Temperature Temperature

• chemical weathering faster hot climateschemical weathering faster hot climates• Rates of frost wedging increased by fluctuations of temperature around freezingRates of frost wedging increased by fluctuations of temperature around freezing• Thermal expansion & contraction extreme in hot cloudless areasThermal expansion & contraction extreme in hot cloudless areas

– Rainfall Rainfall • increases chemical weatheringincreases chemical weathering

• Parent Rock characteristics Parent Rock characteristics – Limestone easily dissolved, Basalt easily oxidized, Granite easily hydrolyzed, Limestone easily dissolved, Basalt easily oxidized, Granite easily hydrolyzed,

Sandstone resistant to soil formationSandstone resistant to soil formation

• TimeTime• SlopeSlope• OrganismsOrganisms

Equator to Poles Climate Factors

Equatorialand tropicalrain forests

SavannahsLow-latitudedeserts andsemi-deserts

Grasslands(steppes)

Temperate regionsand mixedboreal forests

Arcticandtundraregions

EquatorA

nnua

l pre

cipi

tatio

nIn

crea

sing

dep

thof

wea

the

ring

1800mm

600mm

40ºC

30ºC

20ºC

10ºC

Precipitation

Temperature

Evaporation

Bedrock ator very nearsurface

Soil Soil

Bedrock

Deeply weatheredbedrock(~40 - 50 meters deep)

Tem

per

atur

e

30 degrees Latitude

Climate

Chemical weatheringby dissolution Limestone

Weathering-resistant sandstone yields little soil

SoilSoil Soil

Feldspar-richgranite

Iron-richbasalt Chemical

weatheringby oxidation

Chemicalweatheringby hydrolysis

Factors related to bedrock composition "Parent Rock"

Sandstone in humid climates develops very poor soil on

which the dominant natural vegetation is woodland as in this

forest in PA

From Drury, Ch. 4

Parent Rock Parent Rock ResistantResistant

Limestone 1.

Prolonged chemical weathering of carbonates in humid climates leaves a thin residual veneer of clay-rich soil, which is often quite fertile, as in Kentucky

However, the extremely good internal drainage may preclude all but the largest surface streams.  The often low-lying surface is generally pock-marked by many depressions over sink holes.

From Drury, Ch. 4

Parent Rock Parent Rock SolubleSoluble

Limestone 2. In recently glaciated areas, such as northern England, soil development has not been possible over limestones despite very humid conditions. 

Vegetation only occupies those surfaces veneered with glacial till.

TimeTime

Slope and Soil ThicknessSlope and Soil Thickness

SlopeSlope

• Massanutten Mt., Va., a syncline

• Ridges sandstone• Valleys limestone

and shale are less resistant

• The north and south forks of the Shenandoah River flank Massanutten Mountain. Note the meanders on floodplains of thick, fertile soils

http://www2.jpl.nasa.gov/srtm/virginia.html

Soil thickness depends on many factors, including parent rock weather-ability, and slope.