Transcript Chapter 5 Weathering and Soils

Weathering and Soils
Earth’s surface processes
First some definitions:
• Weathering – Physical breakdown and
chemical alteration of rock at Earth’s
surface
• Erosion – The transport of rock fragments
Preview of Mechanical Weathering
Source: Tom Bean/DRK Photo
Mechanical Weathering
• Mechanical weathering – breaking of rocks
into smaller pieces
• Four types of mechanical weathering
1. Frost wedging – freezing and thawing of water in
cracks disintegrates rocks
Frost
Wedging
Evidence of
Frost
Wedging in
Wheeler
Park,
Nevada
Source: Tom Bean/DRK Photo
Rockfall caused by frost wedging
Thermal Expansion and Contraction and Salt Weathering
Weathering
• Mechanical Weathering (continued)
2. Unloading or Mechanical Exfoliation
Igneous rocks at Earth’s surface peeling like
layers off an onion due to reduction in pressure
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
Unloading - Exfoliation of a Pluton
Mechanical Exfoliation in
Yosemite National Park
Source: Phil Degginger/Earth Scenes
UnloadingExfoliation
of granite
Thermal Expansion and
Contraction
Source: Tom Bean
Tree Roots
Growing in
Rock
Fractures
Animal
Burrows
Source: Runk/Schoenberger/Grant Heilman
Increase in surface area by
mechanical weathering
Joint-controlled weathering
in igneous rocks
Definition: Joints vs Faults
Increases surface area
for chemical weathering
Chemical Weathering
• Breaks down rock and minerals
• Important agent in chemical
weathering is water (transports ions
and molecules involved in chemical
reactions)
• The ions form the cements in
Sedimentary Rocks
• The salt in the ocean
Dissolution of Calcite
• Dissolution
–By carbonic acid, CO2 in water
CO2 + H2O => H2CO3
H2CO3 => H+ + HCO3–Soluble ions contained in
underground water
Dissolution
Weathered and
Unweathered
Limestone
Boulders
Source: Ramesh Venkatakrishnan
Oxidation
• Chemical reaction where
compound loses electrons
–Important in breaking down
mafic minerals (contain Fe)
–Rust- colored mineral (Fe2O3 )
from weathering of Basalt
[which contains Olivine (Fe, Mg) SiO4 ]
Oxidation of Basalt
Rust (Iron Oxide) forms
Hydrolysis
• Water makes
+
H
and
OH
ions
–Hydrolysis is the reaction of any
substance with water
–Water’s ions replace different ions in
a mineral
–Feldspars, most abundant crust
minerals, become fine clay particles.
– clays are light weight, flat plates,
easily transported by streams
Hydrolysis – Feldspar to Clay
Feldspars become
Mechanical fracture due to chemical weathering
24
Hydrolysis: Angular Boulder
Decomposes and Rounds
Hydrolysis:
Angular Boulder
Decomposes
and Rounds
Source: Paul McKelvey/Tony Stone Images
Hydrolysis
Cleopatra’s
Needle,
(Egypt)
Granite in a
Dry Climate
Source: New York Public Library, Locan
History and Genealogy Division
Hydrolysis
Cleopatra’s
Needle,
(Central
Park, NYC)
Granite in a
Wet Climate
Source: Runk/Schoenberger/Grant Heilman
Vegetation and Soil Development
Plants use Hydrolysis to get nutrient metals out of minerals
Calcite (Limestone)
No solids
Ca ++
CO3--
Weathering
• Rates of weathering
• Mechanical weathering aids chemical
weathering by increasing surface area
• Others factors affecting weathering
• Rock characteristics e.g. minerals
– Marble and limestone easily dissolve in weak
acidic solutions -Dissolution
Factors related to bedrock composition
Weathering-resistant sandstone (mostly quartz) yields little soil
Soil
Soil
Soil
Iron-rich
basalt
Chemical weathering
by dissolution
Limestone
Chemical
weathering
by oxidation
Chemical
weathering
by hydrolysis
Feldspar-rich
granite
Weathering
• Other factors affecting weathering
• Rock characteristics continued
– Silicate minerals weather in the reverse order as their
order of crystallization (Bowens Reaction Series)
– Olivine least stable, conditions of formation least like
surface.
– Quartz (sandstone) most stable
• Climate
– Temperature and moisture most crucial factors
– Chemical weathering most effective in warm, moist
climates
Bowens Reaction Series and Weathering
Soils
Also very important in recognizing past climates
Soil
• Soil - combination of mineral and organic
mater, water, and air
• It is that portion of the
regolith (weathered rock and
mineral) that supports the
growth of plants
Components in soil
that support plant growth
Soil
• Factors controlling soil formation
• Parent material
–parent material is the underlying
bedrock - composition affects soil
types
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
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
Variations in soil
development
due to topography
Note location of agriculture
Soil Profile
• The soil profile
• Soil forming processes operate
from the surface downward
• Vertical differences are called
horizons – zones or layers of soil
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
ONLY
ACTIVE
EDUCATORS
BECOME
CHAMPIONS
An idealized
soil profile
Remember the different horizons
ONLY
O
ACTIVE
A
EDUCATORS
E
BECOME B
CHAMPIONS
C
Organic
Activity
Exited
Back
Crushed Rock
Animal Activities in “A” horizon
Worms ingest mineral grains because they are covered with living organisms –
their food. Their burrows, not their feeding, increase chemical weathering by
exposing the minerals to water and air
Source: Runk/Schoenberger/Grant Heilman
Eluviation & Illuviation
exited
back
Illuviation and Eluviation
Leaching and Precipitation of Iron
E
B
Source: Jens/Gutzmer/Rand Afrikaans
University/Geology
Soil Types
• Soil types
• The characteristics of each soil type
primarily depend on the prevailing climatic
conditions
• Three very generic soil types
•Pedalfer
•Pedocal
•Laterite
Temperature
Annual precipitation
Equatorial
and tropical
rain forests
Savannahs
Low-latitude
deserts and
semi-deserts
Grasslands
(steppes)
Temperate
regions
and mixed
boreal forests
30 degrees Latitude
40ºC
1800
mm
Equator to Poles Factors
Equator
Precipitation
Temperature
30ºC
Evaporation
20ºC
Increasing depth
of weathering
600
mm
10ºC
Rainforest
Shallow nutrients
Deeply weathered
bedrock
(~40 - 50 meters deep)
Bedrock at
or very near Desert
surface
Soil
US & Europe
Soil
Bedrock
Arctic
and
tundra
regions
pedalfer
Evergreen forests
pedocal
Shortgrass
laterite
tropics
Pedalfer
•Accumulation of iron oxides
and Al-rich clays in the B
horizon. Brown B horizon
•Best developed under
temperate forest landscapes
Pedocal
• White calcium carbonate
(caliche) in B horizon
• Associated with dry grasslands
and brush vegetation
Pedocal with Caliche in the B horizon
O
A
E
Organic
Activity
Leached
B
Accumulation
C
Crushed Rock
Laterite
Hot and wet tropical climates
•Intense chemical weathering
•Red Iron oxide - Topsoil not
distinct from B horizon
•Deep soil but usable
nutrients shallow
Laterite in Sarawak, Borneo
Source: Fletcher & Baylis/Photo Researchers, Inc.
Earth’s surface processes
• Erosion – the physical removal of
material by mobile agents like water,
wind, ice, or gravity
Soil Erosion
• Soil erosion
• Recycling of Earth materials
• Natural rates of soil erosion depend on
–Soil characteristics
–Climate
–Slope
–Type of vegetation
Erosion
Headed for the Sea
Source: Ramesh Venkatakrishnan
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
Soil Developed on a Lava Flow
End of Lecture 5
Source: Stanley Chernicoff/Patrick Spencer