(5) + (6) Weathering

Question 1

Weathering

Answer 1

• Breaks down rock into loose particles : Creates sediment & soil & dissolved ions in water
• Change phys. & chem. character of rock
• Surficial process
• Leads to development of sediment & sedimentary rock : Major Earth material at surface ; Minor in total Earth crust

Question 2

Two types of weathering

Answer 2

• Mechanical (or) Physical [physical disintegration, no chemical change]
• Chemical (rock decomposes due to exposure to water and gases in atmosphere)

Question 3

Weathering Is progressive

Answer 3

• Weathering along joints (fractures)
• Rounded corners
• Increases towards the surface
• Greater surface area speeds weathering

Question 4

Mechanical Weathering

Answer 4

• Intact rocks break into pieces
• Major types:
• Frost action **
• Pressure release **
• Plant growth (roots in the cracks of rocks)
• Burrowing animals (burrow into the rock)
• Temperature changes, expansion and contraction (fire suddenly put out, hot interior, cool exterior causes them to explode)

Question 5

Frost Action

Answer 5

• Freeze-thaw cycle
• Expanding water when it freezes
• Boulders created by frost wedging

Question 6

Pressure Release

Answer 6

• Most effective of mechanical weathering
• The rock breaks apart along these joints and exfoliates
• Rock breaks into sheets

Question 7

Where will mechanical weathering be most intense?

Answer 7

• Cool, with abundant water (Freeze thaw)

- Steep slopes (Gravity assists)

Question 8

Chemical Weathering

Answer 8

• Also ‘rock decomposition’
• Rocks react with water and air to produce new chemical products (Makes new minerals and ions in solution. Many of these reactions prefer warm, wet conditions)
• Remember equilibrium? (Will break down without it. Mineral stability)

Question 9

Agents of chemical weathering (Oxygen)

Answer 9

• Interaction of water with rocks
• Abundant in atmosphere: chemically active (oxidation)
• If Fe present, yields Fe-oxide and Fe-hydroxide minerals
• If Al present, yield Al-oxide minerals

Question 10

Agents of chemical weathering (Carbonic Acid)

Answer 10

• Most important natural source of acid: carbon dioxide dissolved in water
• Weak acid, but abundant
  Rainwater; water percolating through soil
• Aids dissolution (dissolving minerals)
• Aids hydrolysis (breakdown of minerals from reactions with water)
• Can cause the formation of new minerals

Question 11

Chemical Weathering: Calcite

Answer 11

• Form caves
• Discolour and dissolve marble and limestone statues, monuments
• Where do the ions go?
  • Hard water
  • Seawater is a product of weathering carrying dissolved ions into the ocean

Question 12

Chemical Weathering: Quartz

Answer 12

• Forms smaller particles of quartz +
• Dissolved silica ion
  • Forms common cement in sedimentary rock
• Quartz is rock-forming mineral LEAST susceptible to chem weathering
  • Strength of bonds in crystal lattice

Question 13

Chemical Weathering: Feldspars

Answer 13

• Clay minerals +
• Ions of SiO2, NA, Ca, K
  • Note cations from K-feldspars and plagioclase: Na, Ca, K
  • Effects of carbonic acid
  • Slow, complex reaction

Question 14

Chemical Weathering: Ferromagnesian minerals

Answer 14

• Clay and minerals +
• Ions of SiO2, Mg, Na, Ca, K +
• Fe oxide minerals

Question 15

Yellowish-reddish soils are rich in Fe oxides (examples)

Answer 15

• Limonite, goethite (yellow)

- Hematite (red)

Question 16

After complete weathering what is left?

Answer 16

• Solids: Clays, quartz, iron oxides, aluminum oxides
• Ions : including Ca2+ SiO2, HCO-3
  o Ions become cement for sediment reactions
  o Calcite, silica common elements

Question 17

Chemical weathering and climate

Answer 17

• Most intense where there is abundant liquid water

- Most intense where warm

Question 18

Soil

Answer 18

• Layer of weathered, unconsolidated material on top of bedrock
  o Some definitions: Contains organic matter and can support plant growth
• Role of clay, quartz
  o Clay holds water plant nutrients
  o Quartz: sand grains drainage

Question 19

Mature soil is layered (soil horizons) in…

Answer 19

Appearance and chemical composition

Question 20

Soil Formation

Answer 20

• Takes long time to form (thousands of years)
• Depends on:
  o Climate: (rainfall, temp)
  o Parent rock composition
  o Organisms (type of vegetation, different root systems)
  o Relief (slope, topography)
  o Drainage
  o Time

Question 21

Soil Formation: Climate

Answer 21

• Most important factor determining soil type
• Dry climate
  o Thin soil
  o Little leaching
  o Little humus
  o Upward movement of soil water (Evaporation and Capillary action)
• Wet climate
  o Form thicker soils
  o Strong leaching
  o Downward movement of soil water
  o High Al and Fe oxides
  o Abundant humus

Question 22

Bauxite (Aluminum Oxide)

Answer 22

• Least soluble of all the weathering products
• Source of aluminum
• Formed in tropical weathering conditions
• Laterite soils
• Highly leached
• Only insoluble oxides remain

Question 23

Weathering produces: Sediment

Answer 23

• Loose, solid particles
• Classified by size:
• Gravel (includes boulders and cobbles)
• Pebbles
• Sand
• Silt
• Clay
• (mud = wet silt and clay)

Question 24

Transportation of Sediments: Agents of transportation

Answer 24

• Running water
• Glaciers
• Waves
• Wind

Question 25

Transportation of Sediments: Rounding

Answer 25

• Particles hit and scrape each other during transport
• Gets rid of irregular surfaces
• Rounding vs angular (gradational)

Question 26

Transportation of Sediments: Sorting

Answer 26

• By size
• Poorly sorted well sorted (mixing of sizes uniform) sizes
• Running water – a good sorting agent

Question 27

Deposition of sediment

Answer 27

• Transported materials settle
  o Occurs when agent loses energy and can no longer transport its load
• Chemical or organic sediment accumulates on the sea or swamp floor
  o Dead organisms accumulate
  o Evaporation and precipitation (ex. Salt))
  o Change in temperature, pressure, chemical of solution – can cause precipitation

Question 28

Preservation of sediments

Answer 28

• Not all sediments are preserved as sedimentary rock

o Marine sediments most easily preserved

Question 29

Lithification of sediment

Answer 29

• Process that converts loose sediment into sedimentary rock
• Compaction and cementation (results in clastic sedimentary rock)
  o Pressure
  o Usually buried under more sediment
• OR crystalize minerals directly from solution (chemical sedimentary rock)