Weathering: the decay of rocks
and the source of sediments in
sedimentary rocks
Geology 200
Geology for Environmental Scientists
Physical Weathering
• Ice Wedging (Geo-wedgies!) - the most
effective form of p
physical
y
weathering.
g Ice
can exert 1500 lbs/in2. Most effective with
p freeze-thaw cycles.
y
multiple
• Sheeting or exfoliation - expansion joints
• Thermal expansion/contraction
• Plant roots
• Animal burrows
Fig. 10.2a. Ice wedging
Physical weathering of columnar basalt
Weathering by
exfoliation,
Half-Dome at
Yosemite
National Park
Fig 10
Fig.
10.4.
4 Sheeting in granite
Exfoliation: horizontal joints formed by
sheeting
h i as overburden
b d pressure is
i released.
l
d
Chemical Weathering
• Two variables control most aspects of
chemical weathering.
g
• Water - the more precipitation, the greater
the rate of weathering.
weathering Dry climates have a
slow rate.
• Temperature - warm,
warm wet climates have
the greatest rate of chemical weathering.
Very cold climates have a slow rate.
rate
Fig. 10.20: Precipitation, temperature, and rates of
weathering are related to latitude (precipitation and
vegetation are reversed).
Fig 10.19. Climate and weathering
Chemical Weathering - processes
• Dissolution
• Hydration
• Oxidation
O id ti
Dissolution
• W
Water
t will
ill dissolve
di l many minerals
i
l because
b
it is
i
a bipolar molecule. It acts to loosen the bonds
off ions
i
att the
th surface
f
off minerals.
i
l Salt
S lt andd
gypsum are easily dissolved.
• Carbon dioxide dissolved in water forms
carbonic acid, which dissolves calcite.
H2O + CO2
H2CO3
CaCO3 + H2CO3 Ca+2 + 2HCO3- (bicarbonate)
Dissolution
• Some silicates, such as pyroxene, will also
dissolve in carbonic acid:
MgSiO3 + H20 + 2H2CO3
Mg+2 + 2HCO3- + H4SiO4 (silicic acid)
Hydration
• A mineral reacts with either the H+ or the
OH- ((hydroxide)
y
) from water to pproduce a
new mineral. Aluminum silicates do not
dissolve in water. Feldspars
p weather this
way to form clay; e.g. plagioclase to clay:
•
((equation
q
not balanced))
NaAlSi3O8 + H20 + H2CO3
Na+ + HCO3- + H4SiO4 + Al2Si2O5(OH)4
Oxidation
• The chemical combination of oxygen with a
mineral. Important
p
in weatheringg iron-rich
silicates: olivine, pyroxene, amphibole,
biotite. Final p
product is hematite or limonite.
2Fe2SiO4 + 4H2O + O2
2Fe2O3 + 2H4SiO4
Fe2O3 + H2O
2FeO(OH) (limonite)
Results of Chemical Weathering
•
•
•
•
•
Tables 10.1 and 10.2: Review
Na K,
Na,
K Ca,
Ca and Mg are removed into solution
solution.
Al and Si are concentrated in clays.
F iis incorporated
Fe
i
d into
i
oxides.
id
Clays and oxides are in equilibrium with the
earth’s surface.
Figure 10.8
10 8 - Susceptibility to
weathering
• Least stable: halite, gypsum, pyrite, calcite,
dolomite
• Olivine, Ca-plagioclase, pyroxene,
amphibole biotite
amphibole,
biotite, Na
Na-plagioclase
plagioclase, K
Kfeldspar, muscovite
• Most stable: quartz,
quartz clay,
clay aluminum oxides
(bauxite), iron oxides.
Quartz
Fig. 10.8. Relative stability of minerals
Weathering of silicate rocks is related
to Bowen’s Reaction Series or
Temperature of Crystallization
Weathering susceptibility follows the same sequence as
Bowen’s Reaction Series or Temperature of
Crystallization
Weathering of Major Rock Types
• Granite - physically weathers by exfoliation
to form domes;; chemicallyy weathers to
quartz grains and clay. This is the source of
qquartz sand grains.
g
• Basalt - weathers totally to clay and iron
oxides; forms red or brown soils
soils.
River Sand from
eroded Granite
Weathering of Major Rock Types
• Sandstone - quartz grains are highly
resistant to weathering
g and are recycled
y
into
new sand deposits; cement type, calcite,
iron oxide,, or silica,, controls erosion of
sandstones.
• Limestone - weathers rapidly in moist
climates often forming karst features such
as caves and sinkholes; forms cliffs in arid
regions.
Seneca Rocks -- vertical beds of the Tuscarora Sandstone
Karst forms
in humid
climates
Limestones
form cliffs in
arid
id climates,
li t
such as the
Red Wall Ls
in the Grand
Canyon
Weathering of Major Rock Types
• Shale or mudstone - weathers rapidly
because it is fine-grained
g
and soft. The
clays are transported in water by suspension
to form muds which can later become
mudstones and shales.
Pennsylvanian age mudstones
and shales at Goshen Road
Differential Weathering
• Various rocks weather at different rates.
This creates topography
p g p y where the hills,,
ridges, or mountains are capped by resistant
rock types.
yp
Most of the ridges
g of the
Appalachians are capped by sandstone.
Products of Weathering
• Rocks weathered into spherical shapes
• Regolith,
Regolith including soil
soil, produced
• Ions go into solution
Shape of Weathered Rocks
• Fractures and joints provide opportunities
for weathering
g
• Bedding planes - rocks break into slabs or
sheets
• Cleavage or foliation - metamorphic rocks
break along these planes
• Shattering - in dense rocks like quartzite
Shape of Weathered Rocks
• Granular disintegration - sandstone or
g
granites
• Spheroidal weathering - angular fragments
become rounded as spheres have the least
surface area per unit volume
• Exfoliation - breaking into concentric layers
Boulders showing spheroidal weathering
Regolith
• A blanket of loose decayed rock debris
derived from the bedrock beneath it. The
thickness may range from a few cms. to
100s of meters depending
p
g on climate,, type
yp
of rock, and length of time for weathering.
Regolith is thickest where there is abundant
pprecipitation
p
and warm temperatures
p
(precipitation
(p
p
and vegetation are reversed).
Soil
• The uppermost layer of regolith. Composed
of weathered rock and clays, plus
decomposed
deco
posed oorganic
ga c matter.
atte .
• Soil horizons:
A0, A1, A2: organic
i layer,
l
humus
h
layer,
l
zone of leaching
B: subsoil, zone of accumulation of clays
and oxides
C: weathered bedrock
Fig.
g 10.14
Common soil profile
S il Horizons
Soil
H i
Soil and Climate
• S
Subtropical
bt i l soils
il are often
ft highly
hi hl weathered
th d
and are red because of iron oxides.
• Tropical soils are highly weathered and are
called laterites. They can be up to 60 m
thick! They often accumulate aluminum
oxides forming the mineral bauxite.
• Deserts and permafrost - thin soil
• Temperate - best agricultural soils
Soil and Climate: tropical on the
left, temperate on the right
Soils: Parent Rock
• Good soils form on limestone and mafic
igneous
g
rocks. Many
y pplant nutrients are
released by chemical weathering.
• Poor soils form on quartz
quartz-rich
rich rocks like
sandstone, quartzite, or quartz-rich granites.
Relatively few nutrients released for plants.
plants
Ions in Solution
• Ions dissolved in water are invisible
pproducts of chemical weathering.
g Each yyear
the world’s rivers carry about 4 million
metric tons of dissolved material to the
ocean (Table 10.2).
Rates of Weathering
• Controlled by the rock type and climate.
• Note the different rates of weathering of
gravestones. Choose granite over marble.
• Even the pyramids in a desert climate show
substantial effects of erosion.
• Volcanic
V l i rocks
k in
i tropical
i l environments
i
weather at a very rapid rate, >50 cm per
1 000 yrs.
1,000
Weathering
g of tombstones: Granite on the left,
Marble (calcite) on the right (both 1888)
Weathering of the Pyramids
in Egypt.
Egypt Less than 5000
years.