VI. Weathering

1
VI. Weathering

• A. Mechanical Weathering
• B. Chemical Weathering
• C. Soil

2
Weathering

• Mechanical Weathering
• Fracturing, crushing and abrasion breaks solid
  rock into rock fragments and mineral clasts
• Chemical Weathering
• Chemical reactions change the crystalline
  structure (e.g., oxidation) and soften solid
  rock.

See Fig 6.2
3
Weathering
Sediments of
Quartz1 Feldspar1 Clay2 Hematite3 Calcite4

• Weathering, a geologic process
• Breaks rocks into mineral grains
• Converts silicates to clay
• Oxidizes iron minerals (i.e., ferromagnesians)
• Dissolves some ions (e.g, calcium) Erosion
  Transports Sediments

4
Weathering and Plate tectonics

• Tectonic forces lift the land
• Compression and uplift at convergent boundaries
• Isostatic uplift due to crustal thickening and
  buoyancy
• Rocks are fractured and exposed to the elements
• High energy streams and glaciers form
• Weathering and Erosion break the rocks down into
  fragments and mineral particles
• Mechanical and chemical breakdown
• Erosion by gravity, wind, water, ice moves
  material downhill

5
A. Mechanical Weathering

• Intrusive igneous rocks
• Pressure release, sheet joints and exfoliation
• Frost wedging
• (crushing and abrasion)

Fig 6.4
6
Mechanical Weathering
See Fig 6.5
7
Sediments from Mechanical Weathering

• Sediments of
• Parent rock
• Mineral particles
• Angular fragments
• (Near source)

Fig 5.5
See Fig 6.4
See Fig 6.3
8
The Hawaiian IslandsA natural experiment in
weathering

• The Hawaiian islands offer a natural experiment
  on weathering.
• What happens to volcanic islands as they age?

• Hotspot volcanoes
• Older to northwest
• Indicating northwesterly movement of pacific
  plate
• 1½ cm/yr

1.4 cm/yr 150 km 1.5 million cm
9
Hawaii is a young, active volcanic island
Relatively little evidence of weathering
10
Weathering and Erosion of Maui

• No pressure release?
• No frost wedging?
• Then what breaks down these volcanic islands?

Haleakala Past and Present
1.3mya
Evidence of strong weathering and erosion
11
Sediments from Chemical Weathering

• Black sands of basalt and dark minerals
• Pyroxene and
• Ca-feldspar
• Chemically weathered to
• Clay,
• Iron oxides, and
• Dissolved ions
• Washed away and deposited in deep sea

Maui, Hawaii, 1.3 my old
12
Chemical Weathering and Erosion

• Chemical Weathering
• Dissolving ? ions
• Oxidation ? iron oxides
• Clay minerals
• Erosion caries sediments to ocean

Ocean Water
Deep Ocean Sediments
Kauai An ancient volcanic island, 4.7 my old
13
Chemical weathering and Erosion

• Waimea Canyon Incised into Kauais ancient
  volcano

14
Another Ancient Volcanic Island

• Subsiding, weathering, eroding
• Moorea Island
• South-central Pacific
• Fig. 18.22

15
Weathering of Granite
Sediments of
Quartz1 Feldspar1 Clay2 Hematite3 Ions4

• 1. Mechanical Weathering
• forms rock and mineral fragments of parent
  rock
• Chemical Weathering
• 2. Converts silicates to clay
• 3. Oxidizes iron in minerals
• (i.e., ferromagnesian minerals)
• 4. Dissolves some ions
• (e.g, Ca, Na, K, Mg)
• except quartz

Biotite
Quartz
Na Feldspar
16
Rates of Weathering

• Stable at High Temperatures
• Quickly Weathered
• Slowly Weathered
• Stable at Atmos. Temperatures

• Converted to clay, oxides, and ions by chemical
  weathering

Mech. Weath.
17
Other factors increasing Rates of Weathering

• Cool mountainous regions accelerate mechanical
  weath.
• Warm moist climates accelerate chem. weath.
• Increased slopes accelerate weathering
• Gravity caries sediments away from slopes
• Exposing fresh surfaces

18
Other factors increasing Rates of Weathering

• Joints or fractures accelerate both mech. and
  chem. weath.

19
Differential Weathering

• John Ford Point Monument
• Also See Fig. 13.7 of the Colorado Plateau
• Figure 3.8 Ship Rock Arizona
• Volcanic Neck
• Fig 5.3

John Ford Point Monument, Arizona.
Ship Rock, New Mexico Volcanic Neck
20
Weathering forms Soils

• Acidic water percolates
• downward and
• A. Chemically weathers
• minerals in soil to form
• Clays
• Iron Oxides
• Dissolved Ions
• (E.) Leach downward and
• B. Accumulate.
• C. Mechanical weathering
• breaks down bedrock

Fig 6.11
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Sediment Products of Weathering
cm2
cm2
Immature Mature
Transport Distance Near source Far from source
History of Weathering Short history of mechanical weathering Long history of mechanical chemical weathering
Products of Weathering i.e., types of sediment Forms rock and mineral fragments Angular at first Rounded with time and transport Converts silicates to clays, iron oxides and dissolved ions which are transported to the ocean by streams leaving stable minerals, eg. qtz. ( fld.)
m2
22
Chemical Weathering and Erosion

• Chemical Weathering
• Dissolving ? ions
• Oxidation ? iron oxides
• Clay minerals
• Erosion caries sediments to ocean

Ocean Water
Deep Ocean Sediments
Kauai An ancient volcanic island, 4.7 my old
23
VII. Sedimentary Rocks

• A. Formation and Definitions
• B. Sediment History
• C. Sedimentary Structures and
• Environments

24
Chemical Sediments

• Lime mud (CaCO3)
• Biochemical Coral and Shells ? Bioclastic and
  Crystalline CaCO3
• (also diatoms, SiO2)
• Inorganic precipitated directly from water
  ?Crystaline CaCO3
• (also NaCl and CaSO4?2H2O)

25
Chemical and Biochemical Sedimentary Rocks
Fore Reef
Back Reef Reef
Sandstone (sand) Shale (clay)
Limestone (Shell frag. lime mud)

• Limestone (CaCO3)
• Bioclastic Fragments of Coral, Algea and Shells
• Inorganic precipitated directly from water
• Dolomite (CaMg)(CO3)2
• Calcium replaced by magnesium when groundwater
  filters through

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Recipe for Sedimentary Rocks

• Sediment
• Clasts mineral particles from weathering of
  parent rock
• Bioclasts Shell and coral fragments (calcite or
  silica)
• Chemical Precipitate (calcite, silica, salts)
• Accumulation and Preservation
• Sedimentation gt Erosion
• Lithification
• Compaction
• Cementation

A. After Deposition
Overburden compacts and reduces pore space
B. Compaction
Cement from dissolved ions fills some pore space
C. Cementation
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Clastic Particle Sizes
Gravel

• Example (mm)
• Bowling Ball 256
• Pool Ball 64
• Buck Shot 2
• Powder 1/16
• 1/256

Boulder Cobble Pebble Sand Silt Clay
Sand
Mud
very small particles are usually clay minerals