subaqueous under
water subaerial under air
no significant change in chemistry or
mineralogy of the sediment particles
dominant only in cold and arid climates
typically occurs along with physical weathering
chemical and mineralogical compositions
of the rock or rock particles change
Rates of weathering tombstones from 1780 Figure 5.1
slate made of resistant minerals
marble easily weathered by acidic rain
Most important effect of
physical weathering increasing
surface area
which increases the rate of chemical reactions with the rock
Root wedging also the effects of lichens on bare rock
surface Figure 5.10
Ice
wedging Figure 5.5
water increases in volume by 9% when it
freezes
water may be in pore spaces, or in
fractures, or along bedding planes
repeated freeze and thaw breaks the rock
into smaller pieces
Thermal
expansion and contraction
flaking of the outer surface of the rock
heat gradient within the body of the rock
weathering in place produces rounded boulders Figure 5.2
Physical weathering different rocks, different rates
Figures 5.3 & 5.4
In arid regions, sandstones and
limestones produce cliffs
shales break down easily and form slopes
Release
of overburden pressure
large-scale sheeting or exfoliation Figures 5.7, 5.8, 5.9
particularly obvious in granite domes
(from batholith)
exfoliation sheet jointing
, release of pressure by
erosion
top of the batholith looks like an onion peeling off in layers
Salt
weathering
in desert environments, salts crystallize
in pore spaces of rocks outward pressure
gypsum roses inside sandstone
common with intermittent rain and high
evaporation
minerals dissolved from surface, precipitate
about 1 meter down as water evaporates
c
aliche
(kah LEE chee) a common desert soil
Wetting and drying
especially important in rocks with a clay component
alternating stresses:
compressional / tensile
Expandable clays
Bentonite
water fits in between clay mineral layers
bentonite forms from the weathering of rhyolitic volcanic
ash
large areas of the U.S. West are covered by thick ash
layers
especially near Yellowstone caldera
bentonite hills common in Colorado, Wyoming
, South Dakota
sparse
vegetation, cracks when dry, expanded and slippery when wet
occurs nearly everywhere on
Earth (and other planets as well)
almost always occurs with
physical weathering
primarily caused by the interaction
of rocks with water and dissolved gases
O2 and CO2 are the most important
occurs slowly, rate
increases with temperature
Hydrolysis
acid reacts with silicate minerals
breakdown of silicate, forms silicic acid
release of metal cations and silica
if Al is present, forms clay minerals
** primary process in the decomposition
of silicate rocks **
Hydration
water molecules added to a mineral
Oxidation
oxygen reacts with ferromagnesian minerals
Combined form oxyhydroxides rust
Example: Red sandstone hematite
Chemical weathering of ferromagnesian minerals
Example: Tropical laterite in Brazil
Dissolution
and re-precipitation
highly soluble minerals
calcite, dolomite (types of limestone)
gypsum, halite (types of salts)
commonly occurs in a vertical sequence
dissolution near land surface,
precipitation farther down
may occur with just water, but enhanced
with acid in the water
Re-precipitation forms cements between
mineral grains
most common cements:
silica (SiO4) and calcite (CaCO3)
Producing acid rain Box 5.1
Ion
exchange
alteration of one clay mineral to another
by exchanging ions
examples:
Na-clay + H+ => H-clay + Na+
Ca2+ replaced by Na+
(water softening)
In general, clay minerals are hydrous
alumino-silicates
that is, made of SiO4 (silica) AlO6 (alumina) plus water in the mineral
Three
groups of clay minerals by formation process:
primary igneous minerals micas
metamorphic minerals chlorite
secondary weathering products smectite
shows crystals of quartz feldspar
amphibole mica magnetite
almost
all of the feldspars have turned into clay minerals
most
of the ferromagnesian minerals have oxidized
quartz
grains are relatively unweathered
not much left after weathering of basalt
almost entirely ferromagnesian minerals (olivine &
pyroxene)
breaking down rocks to produce rock fragments
(clasts) and new minerals
determined in part from the thoroughness of
weathering process before erosion
related directly to grain size and cementation of
parent rock
coarse-grained rocks yield particles of individual
minerals
fine-grained rocks yield particles of rock fragments
Oxidized chemically bonded with oxygen
Reduced no oxygen, lower valence
For example, iron (Fe):
Oxidized Fe3+ or Fe (III) Fe2O3
Reduced Fe2+ or Fe (II) found in
ferromagnesian minerals
4 Fe + 3 O2 => 2 Fe2O3
iron
oxygen iron oxide
Forms
the mineral hematite
As this oxidation reaction occurs,
the Fe changes valence
from Fe (II) to Fe (III),
which have
different physical properties
Forms a hydrated iron oxide
Fe2O3 n H2O
where n is
1, 2, or 3
Minerals:
limonite
(group)
goethite
Acids
are the most effective agent for weathering rocks
Produce hydrogen ions H+
which substitute easily for larger
metal cations Ca2+ Na+ K+
This changes the chemical composition and atomic structure, and
creates a new mineral
Carbon
dioxide dissolved in water
CO2 + H2O => H2CO3 => H+ + HCO3
carbonic acid bicarbonate
This
process occurs naturally in rain, to produce a weak acid
and in soils to form a stronger acid (about
equivalent to vinegar)
From
anthropogenic pollutants:
Burning
coal produces sulfuric acid
SO2 + H2O => H2SO3
Automobile
exhaust produces nitric acid
2 NO2 + H2O => HNO2 + HNO3
Oxidation
of pyrite FeS2
pyrite has reduced iron and sulfur
in the presence of water and oxygen,
this produces sulfuric acid
FeS2 + H2O + O2 => H2SO4 + Fe (OH)3
If
not buffered by carbonate rocks (limestone), the pH of streams can drop below
4 (which kills fish and other aquatic
organisms)
Another problem mobile
metal ions
many heavy metals, that are toxic to humans and other organisms, are dissolved more easily in acidic ground water, which allows the metals to move away from the contaminated site
Chemical
weathering of feldspar
2
K Al Si3O8 + 2 H+ + 2 HCO3 + H2O =>
potassium carbonic acid
feldspar
Al2 Si2 O5 (OH)4 + 2 K+ + 2 HCO3 + 4 SiO2
clay mineral soluble ions dissolved
silica
tetrahedron polygon
with four faces
octahedron polygon with eight faces
silica one silicon atom with four oxygen atoms
alumina one aluminum atom with six oxygen atoms
Sheet silicates
building blocks
Connected alumina octahedra, attached at corners to for a sheet
Layer of silica units share oxygen
atoms with the alumina sheet
Combined tetrahedra & octahedra
form a two-layered sheet
The clay mineral kaolinite
most abundant in humid tropics and
subtropics
related to intensity of soil-forming processes
A
mudpot at Yellowstone Figure 5.13
Pyrite oxidation produces sulfuric acid,
which breaks down silicate rocks
Figure 5.15
Acidic rain from CO2
Higher concentration of CO2 in the soil
Acidic ground water alters rock fragment with
feldspar
produces
clay minerals
Soluble ions and silica are carried away with ground
water
Cl chloride
SO42 sulfate
HCO3 bicarbonate
Na+ sodium
Ca2+ calcium
Mg2+ magnesium
K+ potassium
***
these ions are from the dissolution of the minerals in rocks ***
But
not:
Fe
Al Si
***
because Fe and Al form oxidized compounds that have low solubility
and Si is used as a nutrient by diatoms (a
type of phytoplankton) ***
Clay minerals retain water
that is available for use by plants,
and plants can take up nutrients by ion exchange
*** know horizons A, B, and C ***
in order, down from the soil surface:
O
organic matter, such as leaf litter on the forest floor
A organic matter mixed with mineral material
E
a leached zone at the base of A, caused by downward percolating water
B accumulation of clay minerals, Fe oxides, and calcite
C fragments mechanically weathered from bedrock, or the
unweathered sediment
(parent material for the soil)
Minerals from evaporation
Anhydrite CaSO4
(with no water)
Gypsum CaSO4 H2O (water in
the mineral)
Almost all silicates and cations removed
only
minerals left are iron oxides, aluminum oxides, and kaolinite clay
very
poor for agriculture
Formation of bauxite aluminum ore
Bauxite forms small nodules
Factors: climate, parent material, slope, time