Weathering and Soil Resources - PowerPoint PPT Presentation

Loading...

PPT – Weathering and Soil Resources PowerPoint presentation | free to download - id: 687cfd-NmM4M



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Weathering and Soil Resources

Description:

Weathering and Soil Resources Nancy A. Van Wagoner Acadia University – PowerPoint PPT presentation

Number of Views:39
Avg rating:3.0/5.0
Slides: 34
Provided by: vanwagoner
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Weathering and Soil Resources


1
Weathering and Soil Resources
  • Nancy A. Van Wagoner

Acadia University
2
Introduction Why Should I Care?
  • Soil is a Critical Resource
  • Worlds farmers must feed an additional 90
    million people every single year
  • At the current rate of population growth
  • Limiting constraint availability of fertile
    land
  • i.e.. good quality soil, and proper soil
    management

3
  • Soil is a Nonrenewable resource on the human time
    scale
  • How long does it take to produce a 10 cm thick
    layer of soil from bedrock?
  • 100 years to 10,000 years!!!
  • What are current rates of soil loss
  • India
  • 4.3 billion tons per year
  • USA
  • 3.9 billion tons per year
  • 1987 National Resources
  • Inventory, USDA SCS

4
  • Processes that contribute to the loss
  • contamination
  • removal of surface vegetation and residue
  • agricultural cultivation
  • forest harvesting
  • rangeland grazing
  • surface mining
  • urbanization (hwy, building construction)
  • degradation

5
  • Crop residue
  • foliage, stubble, straw
  • left on soil by crops
  • before and after harvest
  • Decreases surface runoff
  • absorbs energy of
  • wind
  • rain

6
  • Summary
  • global food security requires
  • understanding of soils
  • proper soil management
  • WWW resource on Soil Erosion
  • http//soils.ecn.purdue.edu/wepphtml/wepp/wepptut
    /jhtml/intro.html

7
Soil Formation and Weathering
  • What is weathering?
  • The decomposition and disintegration of rocks and
    minerals at the Earths surface by mechanical and
    chemical processes
  • converts rock to gravel, sand, clay and soil

8
combine
9
  • What is erosion
  • The removal of weathered rocks and minerals from
    the place where they formed
  • water
  • wind
  • glaciers
  • gravity

10
  • Types of weathering
  • mechanical weathering
  • The physical disintegration of rock into smaller
    pieces each retaining their original
    characteristics
  • Example
  • chemical weathering
  • The decomposition of rocks and minerals as a
    result of chemical reactions (removal and/or
    addition of elements
  • Example

11
Mechanical Weathering Facilitates Chemical
Weathering
  • increases surface area
  • exposes more surfaces to chemical attack

12
Mechanical Weathering
  • Major Mechanisms
  • frost wedging
  • salt cracking
  • abrasion
  • biological activity
  • thermal expansion and contraction
  • pressure release fracturing

13
Mechanical Weathering
  • Frost Wedging (fig. 10.4)
  • When water freezes it expands
  • Example
  • Volume increases by about 9
  • Water migrates into cracks in rocks
  • Ice crystal growth puts tremendous pressure on
    surrounding rock
  • Enough to break rock
  • Most effective in mountainous areas where daily
    freeze/thaw
  • Talus slopes
  • Dangers to hikers

14
Mechanical Weathering
  • Salt Cracking (fig. 10.5)
  • salts crystallize in cracks in rocks
  • puts pressure on surrounding rock
  • important in
  • dry climates (arid regions)
  • ground water is salty, salts precipitate out of
    solution
  • coastal areas
  • salt spray blows into cracks in rocks

15
Mechanical Weathering
  • Abrasion
  • breakup of rock by friction and impact
  • glaciers (fig. 10-8)
  • wind (fig. 10-7)
  • running water (fig. 10-6)
  • waves

16
Mechanical Weathering
  • Biological Activity (fig. 10-9)
  • plants growing in cracks in rocks
  • burrowing animals
  • humans blasting for roads, development,
    exploration, etc..

17
Mechanical Weathering
  • Pressure release fracturing (fig. 10-10)
  • buried rocks are under confining pressure
  • when exposed they expand due to release of
    confining pressure
  • problem for miners (underground)--causes rock
    bursts

erosion surface
exfoliation joints cracks dev. parallel to
erosion surface
18
Chemical Weathering
  • WATER main agent of chemical weathering
  • pure water by itself is relatively inactive, but
  • pH 7
  • with small amounts of dissolved substances it
    becomes highly reactive
  • many of these substances are found in the
    atmosphere
  • and soil

19
Composition of Clean Dry Air
  • 78 Nitrogen
  • 21 Oxygen
  • 1 other
  • inert gases 0.93
  • carbon dioxide CO2
  • methane CH4
  • Hydrogen
  • oxides of Nitrogen
  • carbon monoxide
  • ozone O3

20
Chemical Weathering (Oxidation)
  • reactions with oxygen
  • common, 21 of atm. oxygen
  • example, Iron bearing minerals oxidize to form
    rust
  • 4FeSiO3 2H2O O2 gt 4FeO(OH) 4SiO4

dissolved silica
rain
oxygen from atm.
limonite hydrated Fe-oxide
Fe-pyroxene
21
Chemical Weathering (solution)
  • solution of soluble substances, such as
  • salt in water

22
Chemical Weathering (acids and bases)
  • CO2 dissolved in water, rain or snow, produces
  • Carbonic Acid
  • Remember, pure water is neutral (not acid or
    base)
  • If we increase the number of H ions in water, it
    becomes an acid, pH lt 7
  • If increase the number of Hydroxyl ions (OH-) it
    becomes a base
  • Acids and bases are more corrosive than pure water

23
Chemical Weathering (acids and bases)
  • All natural rain water is acid rain
  • Why
  • as rain drops fall through the atmosphere and
    through soil
  • react with carbon dioxide in the air, and
    produced by decaying organisms in soil
  • to form carbonic acid
  • H2O CO2 H2CO3 HHCO3

24
Carbonic Acid and Limestone
  • carbonic acid reacts with limestone to dissolve
    it
  • draw equation
  • result is dissolved Ca and HCO3-
  • effect on neutralizing acid

25
  • Certain minerals react with acid solutions to
    neutralize them
  • Examples are
  • Calcite (limestone)
  • minerals of mafic igneous rocks
  • Ca-rich feldspar
  • Olivine

26
Carbonic Acid and Silica-rich rock
  • idealized by the reaction with the mineral
    orthoclase, a common mineral found in granite
  • EQUATION (draw on board)
  • What has happened
  • The feldspar is weathered to clay.
  • Ions are released to be soil nutrients.
  • Silica goes into solution.
  • H replaces K in the crystal structure as OH-
    ions hydrolysis
  • this disrupts and expands the crystal structure
  • Al is retained

27
Other important points
  • Only one hydrogen ion is neutralized for each
    mole of feldspar consumed
  • because clay minerals are by-products of
    weathering
  • form at surface conditions
  • very stable at surface conditions
  • comprise a high percentage of the inorganic
    component of soil

28
Other acids are formed by industrial and
automotive emissions
  • The emissions are
  • SO2 and
  • gases of nitrogen (NO2, N2O)
  • draw reactions on board

29
High silica rocks are wide spread
  • Canadian Shield
  • Appalachians
  • New England
  • Nova Scotia
  • Therefore lakes in these geographic settings
    have a poor buffer against the effects of acid
    rain.
  • Soils in these settings also have a poor natural
    buffer and farmers must add lime (CaCO3) to the
    soil.
  • figure 23.2

30
Hydrolysis results in
  • constituent mineral growth, increase in mineral
    volume
  • puts pressure on the framework of the rock
    resulting in
  • gruss
  • spheroidal weathering

31
Gruss is a pile of hydrated minerals
  • form where hydrated minerals fall off and collect
    at the base of a weathering rock

32
Spheroidal weathering also caused by chemical
weathering
  • sequence of events
  • pressure release forms orthogonal joints
  • water percolates through cracks
  • -hydrated minerals disrupt the framework of the
    rock
  • put pressure outward
  • weathering first reacts more intensely at
    corners, producing a rounded shape
  • finally, onion-skin pieces of rock flake off
  • end up with what looks like giant pile of marbles

33
Factors Controlling Rates of Weathering
  • Particle Size
  • Porosity and Permeability
  • Climate
  • optimum environment for chem. weathering
  • optimum environment for mech. weathering
  • Mineral Stability
About PowerShow.com