Chapter 8Chapter 8

Weathering, Weathering, Soils and Soils and

Mass MovementMass Movement

BFRB Pages 157 - 161BFRB Pages 157 - 161

Weathering and Weathering and ErosionErosion

• WeatheringWeathering - - DefDef. - the break down . - the break down of rocks that have been exposed to of rocks that have been exposed to the atmosphere. the atmosphere.

• Once the rocks are broken down, the Once the rocks are broken down, the pieces are transported from one pieces are transported from one place to another. This process is place to another. This process is calledcalled erosionerosion. .

• Erosion is caused by Erosion is caused by windwind, , moving moving waterwater (streams, waves, ocean (streams, waves, ocean currents), currents), iceice (glaciers), and by (glaciers), and by gravitygravity. .

• VIF - Most erosion that takes place on VIF - Most erosion that takes place on Earth is caused by moving waterEarth is caused by moving water

What is W.E.D.?What is W.E.D.? • WWeatheringeathering - - the breakdown of the breakdown of

rock into smaller pieces due to rock into smaller pieces due to physical or chemical changes. physical or chemical changes.

• Once the rocks are broken down, Once the rocks are broken down, the pieces are small enough to be the pieces are small enough to be transported from one place to transported from one place to another. This process is calledanother. This process is called EErosionrosion. .

• Since the weathered rocks have Since the weathered rocks have been moved, eventually they are been moved, eventually they are going to have to be dropped. This going to have to be dropped. This process is called process is called DDepositioneposition..

Remember the Rock Cycle Diagram? Page 6 of the “Handy

Dandy” ESRT’s…

2 Types of 2 Types of WeatheringWeathering

Chemical Chemical weatheringweathering – rocks break down as their minerals change in chemical composition (they become different substances).

    Physical Physical weatheringweathering – rocks are broken down into smaller pieces without changing their chemical composition (what they’re made of).AKA - mechanical weathering

Agents of Agents of Physical Physical

WeatheringWeathering

Types of Physical Types of Physical WeatheringWeathering

• #1 - Frost action (aka Ice Wedging)#1 - Frost action (aka Ice Wedging)– water enters small cracks in the water enters small cracks in the

rock. rock. – when water freezes, it when water freezes, it expandsexpands and and

forces the crack to open more. forces the crack to open more. – the ice melts back into liquid water the ice melts back into liquid water

and fills the crack again.and fills the crack again.– the process repeats over and over the process repeats over and over

again until the rock breaks apart.again until the rock breaks apart.

Mt. Brewster – looks solid, right?Mt. Brewster – looks solid, right?

Close up shows jagged rocks from Close up shows jagged rocks from FROST wedging!FROST wedging!

This boulder has been This boulder has been split apart by frost split apart by frost

action!!!action!!!

Types of Physical Types of Physical WeatheringWeathering

• #2 - Abrasion#2 - Abrasion – It is the physical It is the physical

wearing down of rocks wearing down of rocks as they rub or bounce as they rub or bounce against each other. This against each other. This process is most process is most common in windy areas, common in windy areas, under glaciers, or in under glaciers, or in stream channels.stream channels.

– It can also happen It can also happen during rock slides during rock slides (gravity)(gravity)

Rocks that have Rocks that have undergone different undergone different

kinds of abrasion look kinds of abrasion look very differently!very differently!

Rock weathered by a rockfall Rock weathered by a stream

Types of Physical Types of Physical WeatheringWeathering

• #3 Plant Roots#3 Plant RootsThe roots of trees often wedge in between cracks in rocks and force apart rocks even further as they grow!

Types of Physical Types of Physical WeatheringWeathering

• #4 Animals Animals (worms, groundhogs, rabbits, etc.) burrow into the ground exposing more rock surfaces to the agents of weathering.

Types of Physical Types of Physical WeatheringWeathering

• #5 Wetting and drying#5 Wetting and drying – Breaks up rocks that Breaks up rocks that

are made from clay. are made from clay. – When they are wet When they are wet

they expand, and they expand, and they shrink as they they shrink as they dry. dry.

– As this repeats over As this repeats over and over, the clay and over, the clay becomes weak and becomes weak and cracks (think of all cracks (think of all the projects you the projects you have made out of have made out of clay…they all crack clay…they all crack and fall apart)!and fall apart)!

Types of Physical Types of Physical WeatheringWeathering

• # 6 - Exfoliation# 6 - Exfoliation – soil and rock is soil and rock is

removed (glaciers removed (glaciers or uplifting), or uplifting), exposing rock exposing rock found deep found deep underground. underground.

– This releases the This releases the pressure causing pressure causing the the surfacesurface of the of the rock to expand rock to expand and eventually and eventually crack and flake off.crack and flake off.

Horizontal Exfoliation

Horizontal Exfoliation

Vertical Exfoliation

Agents of Agents of Chemical Chemical

WeatheringWeathering

Agents of Chemical Agents of Chemical WeatheringWeathering

• #1 - #1 - Hydrolysis Hydrolysis – water

(hydro) reacts with minerals such as feldspar and hornblende to form clay which weakens the rock and breaks it apart..

Link to visualization

#2 - Plant Acids #2 - Plant Acids - Lichen & Moss- Lichen & Moss

These plants live and grow on rocks and eventually break them apart by the weak acids that they secrete!

Agents of Chemical Agents of Chemical WeatheringWeathering

Lichen – light green/looks like bread mold

Moss – dark green

Agents of Chemical Agents of Chemical WeatheringWeathering

• #3 - Oxidation #3 - Oxidation – oxygen reacts with

some minerals, especially those containing iron to form rust (called iron oxide).

– The rusty spots weaken the rock and it breaks apart.

– Water is not needed for oxidation to occur, but it does speed up the process!

Chemical Chemical weathering of weathering of basalt - an iron basalt - an iron

rich (mafic) rich (mafic) igneous rock!igneous rock!

You see, it all You see, it all comes comes

together!!!together!!!

Oxidative Oxidative weathering of weathering of

mineral deposits mineral deposits (new deposits are (new deposits are

white/yellow, white/yellow, weathered deposits weathered deposits are reddish-brown)are reddish-brown)

Agents of Chemical Agents of Chemical WeatheringWeathering

• #4 - Acids in water#4 - Acids in water – Carbon dioxide, nitrogen Carbon dioxide, nitrogen

dioxide, and sulfur dioxide, and sulfur dioxide dissolve in water dioxide dissolve in water and create carbonic and create carbonic acid, nitric acid, and acid, nitric acid, and sulfuric acid. sulfuric acid.

– This acidic water can This acidic water can cause rocks to dissolve, cause rocks to dissolve, especially those especially those containing calcite like containing calcite like limestone and marble!limestone and marble!

– Acids may be found in Acids may be found in ground water (forms ground water (forms caves) or in rain watercaves) or in rain water (acid rain).(acid rain).

““Gnarled Rock” – a formation of limestone Gnarled Rock” – a formation of limestone chemically weathered by acid rainchemically weathered by acid rain

Pitted limestone from Pitted limestone from rainfallrainfall

• Carbonic acidCarbonic acid in groundwater in groundwater causes caves to form.causes caves to form.– When the acidic When the acidic

groundwater comes in groundwater comes in contact with limestone, the contact with limestone, the limestone dissolves and limestone dissolves and caves and caverns are caves and caverns are formed.formed.

– Howe Caverns in NY and Howe Caverns in NY and Carlsbad Caverns in NM are Carlsbad Caverns in NM are examples of these beautiful examples of these beautiful geologic formationsgeologic formations

Carlsbad Caverns – New Carlsbad Caverns – New

MexicoMexico

StalactitesStalactites

StalagmitesStalagmites

Rates of Rates of WeatheringWeathering

Factors that affect rates of Factors that affect rates of weathering:weathering:

• Rock’s resistance to weathering (HARDNESS)

• Amount of surface area• Climate:

– Chemical weathering occurs faster in warm, wet climates

– Mechanical weathering occurs faster in cold or dry climates

The central area of rock was The central area of rock was less less resistantresistant to weathering…thus the to weathering…thus the

“arch” was formed! This is “arch” was formed! This is differential weathering.differential weathering.

Devil’s Tower, Wyoming - An igneous intrusion Devil’s Tower, Wyoming - An igneous intrusion (volcanic neck – the underneath of the volcano) (volcanic neck – the underneath of the volcano) surrounded by surrounded by less resistantless resistant sedimentary rock layers. sedimentary rock layers. Erosion has exposed this monolith! This is an Erosion has exposed this monolith! This is an example of differential weathering!!!example of differential weathering!!!

You may recognize it if you’ve ever seen “Close You may recognize it if you’ve ever seen “Close Encounters of the Third Kind”Encounters of the Third Kind”

Surface AreaSurface Area

• Greater surface area increases the rate of weathering

• Surface area is the amount of rock surfaces exposed to the atmosphere

• Weathering creates more surface area

http://courses.soil.ncsu.edu/resources/physics/texture/soilgeo.swf

Dry Climate = Mechanical Dry Climate = Mechanical Weathering!Weathering!

Devil’s Marbles, Australia – Mechanical weathering from wind and sand!

Wet Climate = Chemical Wet Climate = Chemical Weathering!Weathering!

(water speeds up chemical (water speeds up chemical reactions)reactions)

Water runoff seeps into the soil at the

base of granite rock faces. Over time,

water and permanently moist soil

conditions act together to chemically weather away granite

minerals such as feldspar and mica!

Alternating Cold & Warm Alternating Cold & Warm Climate = Mechanical Climate = Mechanical

WeatheringWeathering(via frost action)(via frost action)

Warm & Moist Climate = Warm & Moist Climate = Chemical WeatheringChemical Weathering

(heat & moisture speed up chemical (heat & moisture speed up chemical reactions)reactions)

Parthenon – Athens, Parthenon – Athens, GreeceGreece

Dry Climate = Mechanical Dry Climate = Mechanical WeatheringWeathering

(via wind blown sediment)(via wind blown sediment)

SoilSoilFormationFormation

Soil has 4 components:Soil has 4 components:• Weathered rockWeathered rock• Organic material (humus)Organic material (humus)• AirAir• WaterWater

What is soil made of?

There are 2 types of soil….There are 2 types of soil….PARENT MATERIALPARENT MATERIAL is the rock that the soil is the rock that the soil

forms fromforms fromWhen the parent material is the underlying When the parent material is the underlying

bedrock of the area, the soil is called bedrock of the area, the soil is called RESIDUAL SOILRESIDUAL SOIL (residual = what is left behind (residual = what is left behind when the bedrock weathers – it still “resides” when the bedrock weathers – it still “resides” or lives with the parent material)or lives with the parent material)

If the soil was formed in one place and carried If the soil was formed in one place and carried (transported) elsewhere (by wind, water, (transported) elsewhere (by wind, water, glaciers, etc…) then the soil is called glaciers, etc…) then the soil is called TRANSPORTED SOILTRANSPORTED SOIL

VIF - MUCH OF THE SOIL IN NYS IS TRANSPORTED VIF - MUCH OF THE SOIL IN NYS IS TRANSPORTED SOIL LEFT BEHIND BY GLACIERS!SOIL LEFT BEHIND BY GLACIERS!

Cross section of Cross section of soil layerssoil layers

Each layer is called Each layer is called a HORIZONa HORIZON

Horizon O/AHorizon O/A = = Topsoil (mostly Topsoil (mostly humus, some humus, some weathered rock)weathered rock)

Horizon BHorizon B = Mostly = Mostly weathered rock, weathered rock, some organic some organic material (mineral material (mineral rich zone)rich zone)

Horizon CHorizon C = Broken = Broken BedrockBedrock

BEDROCKBEDROCK

HORIZON AHORIZON A

HORIZON BHORIZON B

HORIZON CHORIZON C

MASS MOVEMENTSMASS MOVEMENTS

MOVEMENTS OF SOIL, MOVEMENTS OF SOIL, ROCK, AND LOOSE ROCK, AND LOOSE

MATERIALS CAUSED BY MATERIALS CAUSED BY THE FORCE OF GRAVITYTHE FORCE OF GRAVITY

CREEPCREEP((Slow Mass Slow Mass Movement)Movement)

• Very slow downward movement of soil– Only noticeable by

the results of trees, fence posts, telephone poles, etc… that are tilted downhill

LANDSLIDELANDSLIDE(Rapid Mass (Rapid Mass Movement)Movement)

• Sudden movement of soil downhill Sudden movement of soil downhill (usually a very steep slope)(usually a very steep slope)

– AVALANCHEAVALANCHE – landslide made of – landslide made of Ice & Snow as well as SoilIce & Snow as well as Soil

– SLUMPSLUMP – small areas of land – small areas of land moving downhill - especially along moving downhill - especially along roadways that cut into the side of roadways that cut into the side of hillshills

MUDFLOMUDFLOWW

or LAHARor LAHAR(Rapid Mass (Rapid Mass Movement)Movement)

• Very fast & dangerous movement of water saturated soil (MUD)

Even though rapid mass movement is more noticeable than slow mass movement,

overall, more material is transported via creep than all others combined – because it

is occurring in many more places and all the time!!!!