Weathering must happen before erosion can take place!

Which comes first

Erosion or weathering?

WEATHERINGWEATHERING

Definition: The physical and chemical Definition: The physical and chemical processes that break down rock on processes that break down rock on earth’s surface.earth’s surface.

2 Types of Weathering2 Types of Weathering

PHYSICAL/ MECHANICAL WEATHERINGPHYSICAL/ MECHANICAL WEATHERING – – rocks are broken rocks are broken apart increasing the surface area.apart increasing the surface area.

ROCKROCK BEFORE WEATHERING = BEFORE WEATHERING =

ROCK AFTER WEATHERING (just ROCK AFTER WEATHERING (just smaller)smaller)

CHEMICAL WEATHERINGCHEMICAL WEATHERING – – Chemical reactions CHANGE Chemical reactions CHANGE minerals. minerals.

ROCK BEFORE WEATHERING ROCK BEFORE WEATHERING ≠ ROCK ≠ ROCK AFTER WEATHERINGAFTER WEATHERING

PHYSICAL WEATHERINGPHYSICAL WEATHERING

INCREASES SURFACE AREA

MORE CHEMICAL WEATHERING CAN OCCUR

There are four main sources of power There are four main sources of power for mechanical weathering.for mechanical weathering.

Gravity, Water, Wind, & WavesGravity, Water, Wind, & Waves

Of these, water appears to be the Of these, water appears to be the leader in changing the surface.leader in changing the surface.

PHYSICAL WEATHERINGPHYSICAL WEATHERING MOST IMPORTANT IN OUR CLIMATEMOST IMPORTANT IN OUR CLIMATE

FROST ACTION – FROST WEDGINGFROST ACTION – FROST WEDGING

ALTERNATE FREEZE AND THAW

Water expands when frozen

ROCKS ARE CRACKED BY WATER FREEZING

EXAMPLES: POT HOLES AND FROST HEAVES

Spheroidal WeatheringSpheroidal Weathering

Fluids enter rock along joints or

fractures and the edges become

rounded.

PHYSICAL WEATHERING – PHYSICAL WEATHERING – ROOT ACTIONROOT ACTION

PLANT ROOTS UPLIFT AND FRACTURE PLANT ROOTS UPLIFT AND FRACTURE ROCKROCK

Root PenetrationRoot Penetration

Powerful plant Powerful plant roots grow into roots grow into rock cracks and rock cracks and cause fractures.cause fractures.

PHYSICAL WEATHERING - PHYSICAL WEATHERING - ABRASIONABRASION

ROCK PARTICLES GRIND AGAINST ONE ROCK PARTICLES GRIND AGAINST ONE ANOTHER ANOTHER

WATER WIND

AbrasionAbrasion

Both wind and Both wind and water can water can

cause abrasioncause abrasion as rock as rock

fragments fragments bounce off bounce off each other.each other.

AbrasionAbrasion

This “moon This “moon looking” picture looking” picture of an Antarctic of an Antarctic

rock, shows rock, shows weathering weathering

from the wind from the wind blown sands.blown sands.

PHYSICAL WEATHERING PHYSICAL WEATHERING EXFOLIATION – PEELING AWAY EXFOLIATION – PEELING AWAY

OF ROCKOF ROCK Unloading – due to reduced pressure at Unloading – due to reduced pressure at

earth’s surface rocks will expand but they earth’s surface rocks will expand but they will crack because they are brittlewill crack because they are brittle

Fluctuating Temperatures will cause rocks Fluctuating Temperatures will cause rocks to contract and expand causing cracks to contract and expand causing cracks Examples: deserts and mountainsExamples: deserts and mountains

PHYSICAL WEATHERING - PHYSICAL WEATHERING - EXFOLIATIONEXFOLIATION

Unloading and ExfoliationUnloading and Exfoliation of Igneous Rocks of Igneous Rocks

Dome ExfoliationDome Exfoliation

Exfoliation domes form as the slabs of rock slip or slide off the bedrock. E.g. Yosemite

Glacial WeatheringGlacial Weathering

Glaciers and avalanches can Glaciers and avalanches can cause weathering as ice and cause weathering as ice and

rock interact.rock interact.

Other Agents of WeatheringOther Agents of Weathering

Things likeThings like flash flash floods, mud floods, mud

slides, slides, landslides, and landslides, and other forms of other forms of mass wasting mass wasting

can cause can cause weathering. weathering.

Chemical

Weathering

EXAMPLES OF CHEMICAL EXAMPLES OF CHEMICAL WEATHERINGWEATHERING

OXIDATION OCCURSOXIDATION OCCURS when free oxygen when free oxygen combines chemically with metallic combines chemically with metallic elements (usually iron) elements (usually iron)

AKA AKA RUSTRUST

Chemical WeatheringChemical Weathering Oxygen dissolved Oxygen dissolved

in water will oxidize in water will oxidize some materials like some materials like

nails and rocks!nails and rocks!

(Rust)(Rust) Carbon dioxide Carbon dioxide dissolved in water dissolved in water forms a weak acid forms a weak acid

called carbonic called carbonic acid, which helps acid, which helps

dissolve rock dissolve rock material.material.

Chemical Weathering of Statues, Bath, UK

CARBONATIONCARBONATIONWater containing carbonic acid dissolves Water containing carbonic acid dissolves

minerals (all rain water is slightly acidic)minerals (all rain water is slightly acidic)

Most strongly affected are calcite Most strongly affected are calcite minerals:minerals:

limestone and marblelimestone and marble

CARBONATIONCARBONATION

Stalagmites and stalactitesStalagmites and stalactites

CARBONATION – SINK HOLESCARBONATION – SINK HOLES

Karst Topography Karst Topography

forms caves, caverns,forms caves, caverns,

and sinkholesand sinkholes

CAVES AND SINK HOLES

How Granite WeathersHow Granite WeathersChemical weathering – the alteration of the internal structure of minerals by chemical reactions

Potassium Feldspar + carbonic acid + water

Clay mineral + Potassium bicarbonate + Silica

CHEMICAL WEATHERING CHEMICAL WEATHERING HYDRATIONHYDRATION

OCCURS WHEN OCCURS WHEN WaterWater combines with combines with minerals – most often in granite (mica and minerals – most often in granite (mica and feldspars) to form feldspars) to form CLAYCLAY

Chemicalweathering

CLIMATE CONTROLS WEATHERINGCLIMATE CONTROLS WEATHERING

PHYSICAL WEATHERING:PHYSICAL WEATHERING:

COLD AND MOIST COLD AND MOIST

ALTERNATE FREEZE / THAWALTERNATE FREEZE / THAW

CHEMICAL WEATHERING:CHEMICAL WEATHERING:

WARM AND MOISTWARM AND MOISTIN BOTH CASES – WATER IS THE PRIMARY IN BOTH CASES – WATER IS THE PRIMARY INGREDIENT THAT PROMOTES WEATHERINGINGREDIENT THAT PROMOTES WEATHERING

WHAT KIND OF CLIMATE WHAT KIND OF CLIMATE SUPPORTS WEATHERING?SUPPORTS WEATHERING?

COLD →→→→→→→→→→ HOT

HIGH

PPT

↑

↑

↑

↑

↑

↑

↑

LOW

PPT

Differential Weathering due to Differential Weathering due to Climate and CompositionClimate and Composition

• Masses of rock do not weather uniformly due Masses of rock do not weather uniformly due to regional factors and to regional factors and compositioncomposition..

• Results in many unusual and spectacular rock Results in many unusual and spectacular rock formations and landformsformations and landforms

Differential WeatheringDifferential WeatheringRocks weather at different rates because of different

mineral makeup, degree of jointing, and exposure to the elements.

More resistant rock protrudes as ridges and pinnacles.

Bryce National Park, Utah

More resistant More resistant rock on toprock on top

Rates of WeatheringRates of Weathering

Granite versus Marble

Factors influencing weathering:

-Mineral makeup

WHICH LAYER IS MORE WHICH LAYER IS MORE RESISTANT TO WEATHERING?RESISTANT TO WEATHERING?

LEAST RESISTANT?LEAST RESISTANT?

A

BC

D

YOU MIGHT ALSO SEE IT LIKE YOU MIGHT ALSO SEE IT LIKE THIS.THIS.

WHICH DIAGRAM IS AN ARID WHICH DIAGRAM IS AN ARID CLIMATE? HUMID CLIMATE?CLIMATE? HUMID CLIMATE?

A B

Humid – more rounded

Arid – more angular

Weathering ReviewWeathering Review

ChemicalChemical Rocks are broken Rocks are broken

down through down through chemical changes chemical changes

Common in warm, Common in warm, humid climateshumid climates

Example: Limestone Example: Limestone highly affected by highly affected by acidsacids

MechanicalMechanical Rocks are broken Rocks are broken

down through down through physical changesphysical changes

Common in harsher Common in harsher climatesclimates

Example: water Example: water freezing in cracks freezing in cracks expands and breaks expands and breaks rock apartrock apart

WEATHERING SUMMARYWEATHERING SUMMARY

PHYSICAL WEATHERING – PHYSICAL WEATHERING – INCREASES SURFACE AREA - INCREASES SURFACE AREA - COLD/HUMID CLIMATECOLD/HUMID CLIMATE

CHEMICAL WEATHERING – CHEMICAL WEATHERING – WARM/HUMID CLIMATEWARM/HUMID CLIMATE

IN BOTH CASES, WATER IS THE IN BOTH CASES, WATER IS THE PRIMARY INGREDIENT PRIMARY INGREDIENT

THE END PRODUCTS OF THE END PRODUCTS OF WEATHERINGWEATHERING

CALLED CALLED SEDIMENTS OR SOILSEDIMENTS OR SOIL

SOIL: a combination of sediment, rock SOIL: a combination of sediment, rock minerals, and minerals, and humushumus (organic material (organic material from biologic activity)from biologic activity)