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Weathering Processes: Mechanical and Chemical Changes in Rocks, Lecture notes of Geology

Izmir Institute of TechnologyGeology

An in-depth exploration of weathering processes, focusing on mechanical and chemical changes in igneous, sedimentary, and metamorphic rocks. Topics include external processes, mechanical weathering, chemical weathering, rates of weathering, and examples of weathering in various locations. The document also discusses the impact of climate and temperature on weathering.

What you will learn

  • What are the main agents of chemical weathering?
  • How does climate impact the rates of weathering?
  • What are some examples of weathering in different locations?
  • How does mechanical weathering affect rock?
  • What are the external processes involved in weathering?

Typology: Lecture notes

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10/9/2016
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WEATHERING
Weathering proce sses make sediment f rom igneous, sedimentary, and
metamorphic rocks
External (Surficial) Processes
•Weathe ring – the physical bre akdow n
(disintegr ation) and chemical alteration
(de comp osition ) or rock at or near Ear th’s
sur face
•Mass was ting – the transfer of r ock and s oil
dow nslope unde r the influence of gravity
•Erosion – the physical re moval of m aterial
by mobile agents such as w ater, wind, ice,
or gravity
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Download Weathering Processes: Mechanical and Chemical Changes in Rocks and more Lecture notes Geology in PDF only on Docsity!

WEATHERING

Weathering processes make sediment from igneous, sedimentary, and metamorphic rocks

External (Surficial) Processes

  • Weathering – the physical breakdow n

(disintegration) and chem ical alteration

(decom position) or rock at or near Earth’s

surface

  • Mass w asting – the transfer of rock and soil

dow nslope under the influence of gravity

  • Erosion – the physical rem oval of m aterial

by m obile agents such as w ater, w ind, ice,

or gravity

Mechanical (Physical) Weathering

  • Frost wedging – alternate freezing and thawing

of water in fractures and cracks promotes the

disintegration of rocks

  • Unloading – exfoliation of igneous and

metamorphic rocks at the Earth’s surface due

to a reduction in confining pressure

  • Thermal expansion – alternate expansion and

contraction due to heating and cooling

  • Biological activity – disintegration resulting

from plants and animals

Weathering Rates of weathering

  • Climate
    • Temperature and moisture characteristics
    • Chemical weathering
      • Most effective in areas of warm, moist climates – decaying vegetation creates acids that enhance weathering
      • Least effective in polar regions (water is locked up as ice) and arid regions (little water)
    • Mechanical weathering
      • Enhanced where there are frequent freeze-thaw cycles Weathering

Rates of weathering Joints in a rock are a pathway for water – they can enhance mechanical weathering Joint-controlled mechanical weathering of a granite outcrop produces spherical boulders. Role of Physical Weathering

  1. Reduces rock material to smaller fragments that are easier to transport
  2. Increases the exposed surface area of rock, making it more vulnerable to further physical and chemical weathering

Surface Area and Weathering Mechani cal w eatheri ng reduces grai n si ze but i ncreases the surface area around the grai ns. Location of the monument area

Sheet Joints (Exfoliation) Stone Mountain, GA Half Dome, Yosemite, CA

Crystal Growth Wedging Crystal Growth Wedging involves salt crystals growing from salty waters. Shattered rocks are common in cold and alpine environments where repeated freeze- thaw cycles gradually pry rocks apart. Thermal expansion due to the extreme range of temperatures can shatter rocks in desert environments. Repeated swelling and shrinking of minerals with different expansion rates will also shatter rocks.

The results of slake durability tests 0 48 12 16 2024 28 1 2 3 4 5 Num ber of cycles Weight (gr) Rhyolite Marble Chemical Weathering

  • Breaks down rock components and internal structures of minerals
  • Most important agent involved in chemical weathering is water (responsible for transport of ions and molecules involved in chemical processes) Definition: transformation/decomposition of one mineral into another Mineral breakdown
  • carbonate dissolves
  • primary minerals --> secondary minerals (mostly clays) Net loss of elements retained in the soil. Chemical Weathering

Bowen’s

Reaction

Series

Goldrich

Stability

Series

First to Crystallize Last to Crystallize Slow Weathering Fast Weathering Resistance to Weathering Olivine/pyroxene to clay

  • H 2 CO 3 (acid) Feldspars to clay
  • H 2 CO 3 (acid)

Quartz to quartz (!)

  • anything Calcite to ……. nothing
  • anything This photo of Lime Sink was taken on 20 July 1932, over a week after the drawdown, which occurred over the night of 9-10 July. ‘Karst’ landforms develop in areas underlain with limestone

Oxidation

  • Any chemical reaction in which a compound or radical loses electrons.
  • Important in decomposing ferromagnesian minerals 4Fe + 3O 2 = = > 2Fe 2 O 3 Hydrolysis
  • The reaction of any substance with water.
  • Hydrogen ion attacks and replaces other positive ions. 2KAlSi 3 O 8 + 2(H+^ + HCO3-) + H 2 O = = > Al 2 Si 2 O 5 (OH) 4 + 2K+^ + 2HCO3-^ + H 2 O
  • Hydration : attachment of water molecules to crystalline structure of a rock, causing expansion and weakness
  • Hydrolysis : combination of hydrogen and oxygen in water with rock to form new substances Chemical Weathering

Solution: process by which rock is dissolved in water

  • Is strongly influenced by pH and temperature
  • When water becomes saturated, chemicals may precipitate out forming evaporite deposits.
  • Calcium carbonate (calcite, limestone), sodium chloride (salt), and calcium sulfate (gypsum) are particularly vulnerable to solution weathering. Chemical Weathering pH
  • pH is the negative logarithm of the activity of hydrogen in a solution. Goldich's stability series is the inverse of Bowen's reaction series. The most stable mineral, quartz, is also the last to crystalllize in an igneous magma as it cools.

Mechanical Weathering Physical breakup

  • pressure release
  • water: freeze - thaw cycles
  • crystallization of salt in cracks
  • thermal expansion and contraction All this increases the total surface area exposed to weathering processes. Mechanical Weathering

Exfoliation :

Rock breaks apart in layers that are parallel to the earth's surface; as rock is uncovered, it expands (due to the lower confining pressure) resulting in exfoliation. Mechanical Weathering

Biological Weathering Can be both chemical and mechanical in nature.

  • roots split rocks apart
  • roots produce acids that dissolve rocks.
  • tree throw
  • burrowing animals Biological Weathering How i s the rock bei ng w eathered? Biological Weathering What are these?