Weathering and Weathering and SoilSoilGeology 101Geology 101

Ted BrewsterTed Brewster

WeatheringWeathering The act of breaking down existing The act of breaking down existing

rockrock Can be done by Can be done by mechanicalmechanical or or

chemicalchemical processes processes Both methods serve to create Both methods serve to create

sediment from existing rock typessediment from existing rock types

Mechanical Mechanical WeatheringWeathering

Physical Disintegration of the rockPhysical Disintegration of the rock The rock breaks into smaller piecesThe rock breaks into smaller pieces More surface area exposed to More surface area exposed to

chemical weathering processeschemical weathering processes The rock changes physically but The rock changes physically but

not chemicallynot chemically ExamplesExamples

Frost wedging, roots, pressure Frost wedging, roots, pressure release, and abrasionrelease, and abrasion

Change in Surface AreaChange in Surface Area

Frost WedgingFrost Wedging

Water fills into small cracks within the Water fills into small cracks within the rockrock

When water freezes into ice it expandsWhen water freezes into ice it expands This small expansion slowly makes the This small expansion slowly makes the

crack larger, so more water can fill incrack larger, so more water can fill in Over time and successive melting and Over time and successive melting and

freezing cycles the cracks open up freezing cycles the cracks open up enough to split the rock into piecesenough to split the rock into pieces

Frost WedgingFrost Wedging

Frost WedgingFrost Wedging

Roots – Biological Roots – Biological WeatheringWeathering

The act of plant roots breaking rocks The act of plant roots breaking rocks apartapart

The roots of the plant find cracks and The roots of the plant find cracks and slowly grow downward into the crackslowly grow downward into the crack

Over time the effect of the root Over time the effect of the root growth breaks the rock into piecesgrowth breaks the rock into pieces

Tree in Tree in LimestonLimeston

ee

Tree RootsTree Roots

Pressure ReleasePressure Release

Most rocks are formed at depth and Most rocks are formed at depth and then are moved to the surfacethen are moved to the surface

The pressure at depth is high, as the The pressure at depth is high, as the rocks move to the surface they rocks move to the surface they expandexpand

This is similar to compressing a This is similar to compressing a sponge, then letting expand in watersponge, then letting expand in water

The expansion causes the mineral The expansion causes the mineral grains to break away from each othergrains to break away from each other

Pressure Release - Pressure Release - SheetingSheeting

Spheroidal WeatheringSpheroidal Weathering

AbrasionAbrasion

Removing small pieces of rock over Removing small pieces of rock over time by wind or watertime by wind or water

Like using sandpaper on the rockLike using sandpaper on the rock Slowly the rock is just worn awaySlowly the rock is just worn away Causes the rounding of angular Causes the rounding of angular

piecespieces

Abrasion – Death Valley, Abrasion – Death Valley, CACA

Differential WeatheringDifferential Weathering

Layers of rock are not of the same Layers of rock are not of the same resistance to weatheringresistance to weathering

Some layers are left intact while Some layers are left intact while others are weathered away almost others are weathered away almost completelycompletely

Most prominent in layers that have Most prominent in layers that have been tilted onto their sidesbeen tilted onto their sides

DifferentiaDifferential l

WeatherinWeathering - g - DikeDike

Sorting and RoundingSorting and Rounding As pieces of sediment move away As pieces of sediment move away

from their parent rock they changefrom their parent rock they change Sorting – the ratio of different sizes Sorting – the ratio of different sizes

of material within the rockof material within the rock More uniform size indicates farther More uniform size indicates farther

from sourcefrom source Rounding – the shape of the edges Rounding – the shape of the edges

of the sediment piecesof the sediment pieces More rounding indicates farther from More rounding indicates farther from

sourcesource

SortingSorting

RoundingRounding

Chemical WeatheringChemical Weathering Rock decomposition that actually Rock decomposition that actually

dissolves the rock minerals and dissolves the rock minerals and forms new onesforms new ones

Most minerals form at depth with Most minerals form at depth with little waterlittle water

Surface has abundant water, so the Surface has abundant water, so the reaction can form new mineralsreaction can form new minerals

Some minerals are soluble in water Some minerals are soluble in water and dissolveand dissolve Limestone, halite, gypsum, etcLimestone, halite, gypsum, etc

Weathering of GraniteWeathering of Granite

Chemical WeatheringChemical Weathering

Slate vs LimestoneSlate vs Limestone

Acid WeatheringAcid Weathering

Many carbonate minerals are Many carbonate minerals are susceptible to acidssusceptible to acids

If the water around them contains an If the water around them contains an acid, then the rock is more likely to acid, then the rock is more likely to dissolve quicklydissolve quickly

We see this in cities with acid rainWe see this in cities with acid rain Buildings, Statues, Tombstones, etcBuildings, Statues, Tombstones, etc

Cleopatra’s NeedleCleopatra’s Needle

Salt Weathering - GreeceSalt Weathering - Greece

ClimateClimate

Climate plays a big role in Climate plays a big role in determining the type of weathering determining the type of weathering most commonmost common

Desert – mechanical abrasionDesert – mechanical abrasion Tropical (warm and wet) – chemicalTropical (warm and wet) – chemical Arctic – mechanical frost wedgingArctic – mechanical frost wedging

Climate and WeatheringClimate and Weathering

SoilsSoils

Weathered, unconsolidated Weathered, unconsolidated materials on top of bedrockmaterials on top of bedrock

Capable of supporting plant growthCapable of supporting plant growth Other material is called regolithOther material is called regolith

Soils form over long periods of timeSoils form over long periods of time Exact time depends on the climate Exact time depends on the climate

conditionsconditions Soils have layers called Soils have layers called horizonshorizons

Soil HorizonsSoil Horizons

O horizonO horizon Dark colored layer just below the Dark colored layer just below the

vegetationvegetation Contains decomposed plant material called Contains decomposed plant material called

humus that forms organic acidshumus that forms organic acids A horizonA horizon

Zone of leaching formed by percolation of Zone of leaching formed by percolation of rain water through the O horizonrain water through the O horizon

Wet climates have larger A horizons than Wet climates have larger A horizons than desertsdeserts

Soil HorizonsSoil Horizons

B horizonB horizon Accumulation of leaching products from Accumulation of leaching products from

horizon Ahorizon A Tends to be clay rich and stained red by Tends to be clay rich and stained red by

hematite and limonitehematite and limonite C horizonC horizon

Incompletely weathered bedrockIncompletely weathered bedrock Transitional layer between bedrock and Transitional layer between bedrock and

evolving soil aboveevolving soil above

Soil Soil Profile Profile

by by ClimateClimate

Soil Horizons - HawaiiSoil Horizons - Hawaii

Soil Horizons in AZSoil Horizons in AZ

Sedimentary StructuresSedimentary Structures

Ancient structures that can Ancient structures that can determine the depositional determine the depositional environment of the rockenvironment of the rock

Graded BedsGraded Beds

Graded Beds – sorted layers of sediment

Ripple MarksRipple Marks

•Ripple marks – shallow ripples that indicate a current direction

Cross BeddingCross Bedding

Cross bedding – angled sediment layers that indicate either wind or water flow direction

Fossil RootsFossil Roots

Fossil plant roots – filled in holes made by roots

Mud CracksMud Cracks

•Mud cracks – filled in cracks from muddy and dry environments

Dinosaur FootprintDinosaur Footprint

•Fossil footprints, tracks, or burrows – any impression in the sediment made by living creatures that was then filled in