Geology 12 Presents Sedimentary Rocks Weathering and Erosion

Geology 12 Presents

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 kinds a) 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

Ice wedging cliff Joints = rock fractures Talus = debris accumulation

3 Types of Weathering u 1. Physical (Mechanical) Weathering: –a) Ice Frost Wedging

Ice Frost Wedging

Talus = fallen rock debris

Talus cone

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

u B) Pressure Release

Exfoliation 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 heatsexpands and cools-contracts animation day night + dark minerals get hotter than white minerals

C) 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 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 Weathering u A) Animals: Moles

2. Biological Weathering u A) Animals: ants

2. Biological Weathering u A) Animals: clams

u B: Plants: root wedging

u B: Plants: root wedging

Lichens

B) Plants Root Wedging

B) Plants Root Wedging

• 3. Chemical Weathering: chemical alteration of rock/parent material • 3 kinds • a) Solution: a solid dissolves (halite, calcite) and is usually helped by CO 2 in water which forms carbonic acid (also in plant roots) • Results in hard water, caves & karst topography (sink holes).

3. Chemical Weathering u A) Solution

Sinkhole

• b) oxidation: reacts with oxygen to form oxides (rust). • Ferromagnesians (olivine, pyroxene amphibole & biotite) combine with O 2 form hematite or limonite. • Hi sulfur/sulfide coal • Iron sulfides (pyrite Fe. S 2) acid rain/runoff sulphuric acid + iron oxide

u B) Oxidation

Chevy: like a rock all right!

• c) Hydrolysis: cations (metals) in minerals are replaced by H+ from water • Ex: Potassium (K) Fs + water • Ex: Plagioclase Fs + water clay + K + silica clay + (Na, Ca) + silica

u C) Hydrolysis

Kaolinite clay

• Terminology: • Differential Weathering: rocks that weather at different rates produce uneven surfaces Resistant cap rock plateau Mesa/butte Hodo/ pinnacle rock

Differential 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 Weathering poor good

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

That’s all folks! u Do Ws 5. 1 + look at Unit 1 Review WS