Chapter 5 Weathering and Soils

1
Weathering and Soils
2
Earths surface processes

• First a definition
• Weathering Physical breakdown and chemical
  alteration of rock at Earths surface under the
  influence of air and moisture

3
Source Tom Bean/DRK Photo
4
Mechanical Weathering

• Mechanical weathering in-place breaking of
  rocks into smaller pieces without transport
• Four types of mechanical weathering
• Crystal growth
• frost wedging is repeated freezing (expansion)
  and thawing (contracting) of water in cracks
  resulting in disintegration
• Salt wedging is growth of salt crystals in
  fractures in rocks, resulting in disintegration

5
Frost Wedging
6
Evidence of Frost Wedging in Wheeler Park, Nevada
Source Tom Bean/DRK Photo
7
Rockfall caused by frost wedging
8
Salt Wedging - Salt crystals grow in fractures in
bedrock. Expansion eventually breaks the rocks
apart into smaller fragments
9
Salt Wedging
10

• Mechanical Weathering (continued)
• 2. Mechanical Exfoliation AKA Unloading
  exfoliation of igneous rocks at Earths surface
  due to erosion of overlying material causing
  reduction in pressure on bedrock
• - Like peeling layers off an onion
• 3. Thermal expansion and Contraction alternate
  expansion and contraction due to heating and
  cooling
• - important in deserts
• 4. Biological activity disintegration resulting
  from plants and animals
• - root wedging, animal burrows

11
Mechanical Exfoliation of a Pluton
AKA Unloading
12
Mechanical Exfoliation in Yosemite National Park
Source Phil Degginger/Earth Scenes
13
Mechanical Exfoliation of granite AKA Unloading
14
Thermal Expansion and Contraction
Source Tom Bean
15
Tree roots growing in rock fractures,plus
animal burrows, expose deep rocks to water
Source Runk/Schoenberger/Grant Heilman
16
Increase in surface area by mechanical
weatheringPromotes further weathering
17
Weathering controlled by jointing patterns
18
Joint-controlled weathering in igneous
rocks
19
Joints in sandstone - Canyonlands Natl Park, Utah
20
Chemical Weathering

• Results in change in minerals in bedrock as well
  as breakdown to smaller and smaller sizes
• Agent in chemical weathering is chemically active
  water
• Water dissolves and transports ions. The
  dissolved ions are carried by rivers to lakes and
  the sea, where they may be precipitated to form
  chemical sedimentary rocks
• Dissolved ions are also be transported to ground
  water, where they may be precipitated to form
  cement that holds minerals in sedimentary rocks
  together.

21
Chemically Active Water

• H2O CO2 H2CO3
• H2CO3 H (HCO3)-1
• H is a very small and chemically active ion
• H is capable of entering structure of unstable
  minerals, where it combines with ions to break
  down the bedrock
• Example - unstable silicate water carbon
  dioxide clay mineral ions in solution

22
Source Ramesh Venkatakrishnan
23
Chemical Weathering

• Major processes of chemical weathering
• Dissolution
• Complete dissolving of soluble bedrock such as
  limestone, dolostone, rock gypsum or rock salt
• Agent is carbonic acid H2O CO2 gt H2CO3
  (carbonic acid)
• H2CO3 gt H HCO3-
• Soluble ions removed in underground water
• Oxidation
• Chemical reaction where ions in mineral lose
  electrons
• Important in breaking down ferromagnesian
  silicates
• Rust colored hematite and limonite form
• Hydrolysis and Carbonation
• Reaction of any substance with chemically active
  water
• Ions from water solution replace different ions
  in mineral
• Feldspars, most abundant minerals in Earths
  crust,
• become fine clay particles, which are easily
  transported.

24
Dissolution Weathered and Unweathered
Limestone Boulders
Source Ramesh Venkatakrishnan
25
Oxidation of Basalt
Rust (Iron Oxide) forms
26
Bowens Reaction Series and Weathering
27
HydrolysisCleopatras Needle, (Egypt)
Granite in a Dry Climate
Source New York Public Library, Locan History
and Genealogy Division
28
HydrolysisCleopatras Needle, (Central Park,
NYC)
Granite in a Wet Climate
Source Runk/Schoenberger/Grant Heilman
29
Hydrolysis may result in granular disintegration
- Mechanical disintegration from chemical
weathering Feldspar altering to clay increases
volume, and crystal growth causes mechanical
disintegration
30
Angular Boulder Decomposes and Rounds as a
consequence of granular disintegration
31
Angular Boulder Decomposes and Rounds
Source Paul McKelvey/Tony Stone Images
32
Products of Chemical Weathering
Calcite (Limestone) No solids
Ca CO3--
33
Controls on Weathering

• Climate
• Warm, moist climates promote chemical weathering
• Weathering of limestone differs in humid and arid
  climates
• Rock Type
• Differential weathering different rocks weather
  at different rates in the same climate
• More resistant rocks that weather more slowly
  will stand higher in the landscape than
  non-resistant rocks
• Different rocks have different compositions and
  textures
• Unstable or soluble minerals will weather faster
  than stable or insoluble minerals in the same
  climate
• Textures that permit water to enter the bedrock
  will promote more rapid weathering
• Slope
• Weathering is more rapid on steep slopes because
  erosion will continually expose fresh rocks for
  breakdown
• Time

34
Differential Weathering in Colorado Front Range
Produces Hogbacks

• Well-cemented quartz sandstone forms the ridge
• Shale (made of clay) forms the valleys

35
Hogbacks in Wyoming - Aerial View

• Most resistant rock stands high to form the ridge

36
Soils
Our food grows in soils. Also very important in
recognizing past climates
37
Soil

• Soil - combination of mineral and organic matter,
  water, and air
• It is that portion of the regolith (weathered
  rock and mineral) that supports the growth of
  plants

38
Components in soil that support plant growth
39
Soil

• Factors controlling soil formation
• Parent material
• parent material is the underlying bedrock -
  composition affects soil types

40
Soil

• Factors controlling soil formation
• Time
• Soils get better developed (Thicker, with greater
  differences between layers)
• with more time
• Climate
• Biggest control on soil formation
• Key factors are temperature and precipitation

41
Soil

• Factors controlling soil formation
• Plants and animals
• Organisms influence soil properties
• Also furnish organic matter to the soil
  (especially plants)
• Slope
• Steep slopes have poorly developed soils (due to
  faster erosion and downslope transport
• Flatter terrain accumulates soil faster

42
Weathering and soil development are controlled by
bedrock composition
Weathering-resistant sandstone yields little soil
Soil
Soil
Soil
Chemical weathering by hydrolysis
Iron-rich basalt
Chemical weathering by ?oxidation
Feldspar-rich granite
Chemical weathering by dissolution
Limestone
43
Variations in soil development due to
topography
Note location of agriculture
44
Soil Profile

• The soil profile
• Soil forming processes operate from the surface
  downward
• Vertical differences are called horizons zones
  or layers of soil

45
An idealized soil profilewith horizons
46
Soil

• The soil profile
• O horizon organic matter
• A horizon organic and mineral matter
• High biological activity (animals live here)
• Together the O and A horizons make up topsoil
• E horizon little organic matter
• Zone of leaching
• B horizon zone of accumulation
• C horizon partly altered parent material

Mnemonic Only Active Educators Become Champions
47
Remember the different horizons
O A E B C
Organic Animal Activity Souble minerals
Removed Soluble minerals Deposited Crushed Rock
48
Vegetation and Soil Development
1. Plants use Hydrolysis to get nutrient metals
2. Humic acids and metals returned at death.
Acids remove metals from E-layer
49
Animal Activities in A horizon
Source Runk/Schoenberger/Grant Heilman
50
Eluviation Illuviation
51

• Soil types
• The characteristics of each soil type primarily
  depend on the prevailing climatic conditions
• Three very generic soil types Zonal
  Classification
• Pedalfer Moist All Year
• Accumulation of iron oxides and Al-rich clays in
  the B horizon. Brown B horizon
• Best developed under forest landscapes
• Pedocal Low Precipitation High Evaporation
• White calcium carbonate (caliche) in B horizon
  not dissolved
• Associated with dry grasslands and dry brush
  vegetation
• Laterite e.g. Monsoonal Climate (type of
  pedalfer)
• Alternating Hot dry and Cool Wet tropical
  climates
• Intense chemical weathering
• Red Iron oxide horizons not distinct
• Very thick soils but thin useful topsoil-Trees
  are Buttressed

52
pedalfer
pedocal
laterite
tropics
Shortgrass
Evergreen forests
53
Pedocal with Caliche in the B horizon
O A E B C
Organic Activity Leached Accumulation Crushed
Rock
54
Caliche
Source Teaching Collection/University of
Washington, Department of Geological Science
55
Laterite in Sarawak, Borneo
Source Fletcher Baylis/Photo Researchers, Inc.
56
Soil Erosion

• Soil erosion
• Recycling of Earth materials
• Natural rates of soil erosion depend on
• Soil characteristics
• Climate
• Slope
• Type of vegetation

57
Erosion
Headed for the Sea
Source Ramesh Venkatakrishnan
58
Soil

• Soil erosion
• In many regions the rate of soil erosion is
  significantly greater than the rate of soil
  formation
• Farmers now level fields with lasers to slow loss
  of topsoil
• Terraces