Weathering

Soils

Erosion

Mass Wasting

Weathering:the physical and chemicalbreakdown of rock – exposed to wind, water, ice, and living organisms

Regolith:loose or broken rock layers resulting from weathering Regolith covers most of the earth. Soil:the upper-most layer of regolith that supports life

Do “things” last for ever ? Consider :

new concrete

a 1955 Chevrolet

Rocks can be broken down or chemically altered on theearth’s surface.

Mechanical Weathering: Rocks are physically broken down by varioussurface processes into smaller parts withoutchanging physical properties.

Chemical Weathering:

Rocks are altered from one form to a completely new form with a different set of physical properties.

I weathering

I will get an A on my exams and quizzes

Discuss with a friend:

1.Describe the difference between mechanical and chemical weathering.2. Give two examples of MW and CW you have observed.

Mechanical Weathering – the breaking down of rocks

How do we expose “more surface” area for chemical weathering to do its job ?

Let’s show some surface

Less surface area More surface area

Increased mechanical weathering accelerates chemical weathering.

Rock

Types of Mechanical Weathering ProcessesFrost Wedging: repeated cycles of freezing and thawing

rockrock

Water Freezes (expands)pieces break off(watch for falling rock)

Dominantly takes place in freezing / thawing areas

Unloading – reduction of pressure from overlying rocks -- take the pressure off me.

overlying rocksSheet like structures

Sheet like structures

granite pluton

eroded and uplifted

Water

granite pluton

Frost Wedging – rocks are broken intoplates and split apart.

Exfoliation Dome – granite is “sheeting” from loss of pressure due to unloading processes.

Types of Mechanical Weathering Processes

Biological Activity – breaking down rocks through • animal burrowing• humans• plant roots

Animal burrowing

Plant root weathering

Human intervention

I mechanical weathering

I will get an A on my exams and quizzes

Discuss with a friend:1.Why is MW important?

2. Briefly describe how frost wedging, unloading and biological activity contributes to mechanical weathering

Chemical Weathering Rocks are chemically altered producing a new

compound

Why would a perfectly good rock want to change it’s appearance? STABILITY! Rocks require stability and achieve

equilibrium with it’s surroundings

• Most rocks are not chemically stable at the earth’s surface

• Minerals (compositions) change to become stable at surface conditions

What common agent can begin the process of chemical alteration?????• The Universal Solvent – WATER (H2O)

How would we speed up the action of water? Rain + ?????What in the atmosphere is added to water to create a weak acid?

H2O (rain) + CO2 (atmosphere) = H2CO3 (carbonic Acid)

Let’s weather some granite with the natural acid H2CO3 Granite – the most abundant continental rock weathering the potassium feldspar (orthoclase)

2KAlSi2O8 + 2(H+ + HCO3) + H2O Al2Si2O5(OH)4 + 2K + 2HCO3 + 4SiO2

orthoclase carbonic acid water kaolinite clay

in solution

potassiu

m

bicarb

onate

silica

Granite has been chemically weatheredand broken down intoindividual grains.

Reduced to Clay

I Chemical weathering

I will get an A on my exams and quizzes.

Discuss with a friend:1.What is the universal solvent?2. How is carbonic acid naturally Produced?3. How would you describe the “last” or most reduced material ----- the end product of chemical weathering?

Soil – product of weathering containing solids, liquids, and gas.

Loam – dominant soil fraction

SAND

CLAY

SILT

Humus – partially decayed organic material

What’s in your soil?

CO2/Air – fills the soil voids

Water – wets the soil and carries high levels of dissolved substances

Soil Profiles:• horizons: a layer of soil with

distinctive physical and chemical properties.

• sequence of soil horizons from the surface to the underlying bedrock

O

A

E

B

C

O- Organic matter (humus)A- Dark layer mixed with mineral and organic matterE- light colored layer produced from removal of soluble material “leaching”B- accumulation of clay C- weathered parent rock

Factors that influence soil formation:• Parent Material:• residual regolith – soil forms emplace or “insitu”

Soil forms andweathers emplacefrom parent material.

• transported regolith – regolith that is moved by rivers, wind, and ice

transported riverdeposits

transportedregolith on Mars

Factors that influence soil formation:• Climate – soil profiles differ around the world which is directly influenced by climate arid soils vs. tropical soils• Living organisms – a HUGE influence on soils Plants make organic matter. (humus) – animals mix the soil and recycle air and water.

• Topography – the contour or “slope” of the land

• Time – the longer the time the more mature a a soil becomes --- 1000’s of years

Mass Wasting

Mass WastingWhat do these pictures havecommon?

What is Mass Wasting ?

• down-slope movement of rock and soil under the direct influence of GRAVITY

What force acts alone – to createmovements of the earth?

The role of Mass wasting– What’s it goodfor?

• Responsible for the evolution of landforms – features of the earth

• weathering weakens rock – does not create significant landforms

• need mass wasting to impact weathering and shape significant earth features

Example:

River Valleyfrom weathering

River Valleywidened by mass wasting

I mass wasting

I will get an A on my exams and quizzes.

Discuss with a friend:

1. What is mass wasting ?2. What single “force” acts alone to produce mass wasting processes?3. What is the sole purpose of mass wasting?

So, what “triggers” a masswasting episode allowing gravity tomove the mass downward?

• The role of water• Slopes to steep• removal of vegetation• Earthquakes

The Role of Water• Water saturates surface materials.• Particles loose their cohesion (can’t stick together).

• Particles slide past one another and slip downward under gravity forces.

Slopes to Steep• steeper slopes exceed the “angle of repose” and material moves downward to stabilize or reach equilibrium.

Angle of repose: • the angle at which particles are overtaken by

gravity and move downward• angle depends on size, shape, and moisture content

of accumulated particles

Dry Sand Wet Sand

Removal of Vegetation• where plants are lacking – mass wasting attacking

• root systems bind soil• where soil is removed frequent down slope movements occur

The Role of Earthquakes• Areas that have not moved in “years”, are activated by seismic waves.

Mass slide produced by an earthquakedammed a river forming a lake.

I Mass Wasting

I will get an A on my exams and quizzes

Discuss with a friend:1.Describe the 4 triggers that enable mass wasting process.2. What the angle of repose?3. What is the most dominant “trigger” causing the onset of mass wasting?

How do geologists classify variousmass wasting processes?

• Classification is based on

MaterialUnconsolidated: debris, mud, earth

Bedrock: rock

Motion

Fall: freefall of material

Slide: material moves cohesively along well defined surface

Flow: material moves down-slope as a viscous fluid

I Mass Wasting.

I will get an A on my exams and quizzes

Discuss with a friend:

1.What two criteria do geologist use to classify mass wasting movements?

Classificationof

Mass Wasting Processes

SLUMP • downward movement of a sliding mass, rock or unconsolidated material that moves as a single unit along a curved surface

• moves very slow• over-steepened slopes (T)• water saturated (T)

Rockslide• blocks of bedrock break loose and slide down a slope

Debris slide• unconsolidated material break loose and slide down a slope

Debris flow

Rockslide

Over steepness (T)

Debris Flow• rapid mass movement containing soil and regolith with large amounts of waterWater saturation (T)

Lahars• debris flows composed largely of volcanic material from the flanks

• flows down existing stream valleys• extremely destructive events’• water saturation (T)

Lahars burieda village

Earth Flow• form on hillsides in humid areas with abundant precipitation

• water saturated – hill may break away under gravity and weight of soil

• water saturated (T)• rich in clays and silt• slow moving flows

Soil Creep• the gradual downhill movement of soil and regolith.

• soil moves extremely slow• soil disturbed by rain, roots • water saturation (T)