Geology 12 Geology 12

PresentsPresents

Sedimentary Rocks

Weathering and Erosion

• Weathering: the physical breakdown (disintegration) and chemical alteration (decomposition) of minerals and rocks at of near the Earth’s surface

• Erosion: removal of weathered material (by water, wind, or ice)

• 3 Types of Weathering:• 1. Physical/Mechanical Weathering:

– Physical forces break rocks into smaller pieces without changing chemistry

– 4 kindsa) Frost action/ice wedging: water

seeps into cracks, freezes, expands (9%) and pushes rock apart. Most effective in areas where temperature fluctuates above & below 0’C

animation link

Joints = rock fractures

cliff

Talus = debris accumulation

Ice wedging

3 Types of Weathering3 Types of Weathering

1. Physical 1. Physical (Mechanical) (Mechanical) Weathering:Weathering:– a) Ice Frost a) Ice Frost

WedgingWedging

Ice Frost Ice Frost WedgingWedging

Talus = fallen Talus = fallen rock debrisrock debris

Talus Talus conecone

• b) pressure release: many minerals/rocks are formed under immense pressure. When they are uplifted and exposed through erosion, the pressure is greatly reduced. The rock expands forming layer of fractures (joints)

Sheet joints

• Exfoliation: slabs of rock break along sheet joints and fall off

• Popping/rock bursts: sudden pressure relief in deep mines rock explodes like a grenade injuring/killing miners

beforeafter

Exfoliation dome

B) Pressure ReleaseB) Pressure Release

Exfoliation Dome: ChiefExfoliation Dome: Chief

• c) Thermal expansion & contraction: rock is a poor heat conductor; in deserts where the temperatures rise and fall 30’C/day, the outside of the rock heats-expands and cools-contracts animation

day night

+ dark minerals get hotter than white minerals

C) Thermal Expansion and ContractionC) Thermal Expansion and Contraction

• d) Salt crystal growth: salts in rock crevices grow. Expand prying the rock apart like ice wedging.

• Road salt in roads & bridges, inside of concrete foundation.

D) Salt Crystal GrowthD) Salt Crystal Growth

• 2. Biological Weathering = activities of organisms

• a) animals: burrow, tramp, bring materials to surface for more weathering, help transmit gases and water to depths, etc. Animation

• b) plants: roots wedge rocks apart• Animation

2. Biological Weathering2. Biological Weathering

A) Animals:MolesA) Animals:Moles

2. Biological Weathering2. Biological Weathering

A) Animals:antsA) Animals:ants

2. Biological Weathering2. Biological Weathering

A) A) Animals:clamsAnimals:clams

B:Plants:root wedgingB:Plants:root wedging

B:Plants: B:Plants: root wedgingroot wedging

Lichens

B) PlantsB) PlantsRoot WedgingRoot Wedging

B) PlantsB) PlantsRoot WedgingRoot Wedging

• 3. Chemical Weathering: chemical alteration of rock/parent material

• 3 kinds

• a) Solution: a solid dissolves (halite, calcite) and is usually helped by CO2 in water which forms carbonic acid (also in plant roots)

• Results in hard water, caves & karst topography (sink holes).

3. Chemical Weathering3. Chemical Weathering

A) SolutionA) Solution

Sinkhole

• b) oxidation: reacts with oxygen to form oxides (rust).

• Ferromagnesians (olivine, pyroxene amphibole & biotite) combine with O2 for form hematite or limonite.

• Hi sulfur/sulfide coal acid rain/runoff• Iron sulfides (pyrite FeS2) sulphuric acid + iron oxide

B) OxidationB) Oxidation

Chevy: like a rock all right!

• c) Hydrolysis: cations (metals) in minerals are replaced by H+ from water

• Ex: Potassium (K) Fs + water clay + K + silica

• Ex: Plagioclase Fs + water clay + (Na, Ca) + silica

C) HydrolysisC) Hydrolysis

Kaolinite clay

• Terminology:

• Differential Weathering: rocks that weather at different rates produce uneven surfaces

Resistant cap rock

plateau

Mesa/butteHodo/

pinnacle rock

Differential WeatheringDifferential Weathering

Hard Caprock

• Spheroidal Weathering: reactangular rocks weather into roundish rocks because corners are attacked on 3 sides, edges on 2 sides, but flat planes on 1 side

• Result: rounded (& smaller) rocks

Spheroidal Weathering

Resistance to Weatheringpoor

good

Higher temperature minerals weather most easily (olivine, calcium-rich plagioclase)

Lower temperature minerals weather least (quartz)

That’s all folks!That’s all folks!

Do Ws 5.1Do Ws 5.1

+ look at Unit 1 Review WS+ look at Unit 1 Review WS