Weathering
involves physical, chemical and biological processes, but the two major type is
Physical
(mechanical) and chemical
Mechanical
weathering:
It is
strictly a physical process, involving no change of chemical composition
Ice
or frost wedging:
Water
penetrates easily into the openings of the rocks from rain or melting snow, than
when freezes expands (stress is about 110 kg/cm2= to drop a 98 kg
iron ball from a height of 3m).
Over a
period of time the rock will be literally hammered apart.
For
these we need:
1. enough moisture
2. falling, rising temperature
3. rocks with cracks or weakening (problem on highways)
Sheeting
Rocks
formed under confining pressure, when eroded up to the surface got released from
the pressure and tend to expand. This internal pressure will break the rocks
into fragments. Expansion joints will form parallel to the surface, and the
process is called sheeting.
(Highway......
rocks taken away --- sheeting can cause problems rock slumping)
Thermal
expansion
Daily
temperature cycle, in hot deserts
expansion
- contraction
Organic
Activity
root
action
animal
burrow
CHEMICAL
WEATHERING
During
chemical weathering rocks are decomposed, the internal structures of minerals
are destroyed, and new minerals are created.
Water
is the primary source of chemical weathering
-as part of the weathering or
-as carrying material for dissolved ions
on the other hand it removes the weathered particles, to expose the next
fresh rock layer
The
degree and the rate of chemical weathering is essentially influenced by the
amount of precipitation.
chem w. is not that imp. in deserts.
Major
processes are
1.
DISSOLUTION
You can
imagine dissolution easily
(salt
or sugar in water)
Most
water-soluble mineral is Halite. This is caused by the mineral structure, and
the fact that the water has polar molecules.
Lots of
mineral are insoluble in dest. water, this fact is abruptly changing by the
presence of different weak acids:
carbonic acid:H2O+CO2
HCl: think of acid test
H2SO4
Ions
from such a dissolution makes the water HARD
Ions of
hard water reacts with soap and makes insoluble compound (not desired)
2.
HYDROLYSIS
The
chemical union of water and a mineral is called HYDROLYSIS.
This is
not an adsorption process as the pores of a sponge sucks up water, but a
specific chemical change where a new mineral is forming.
In
hydrolysis the ions of the mineral react with H+
and OH- ions of the water to produce
new minerals.
ex.
Feldspar
NaAlSi3O8 (Na plagioclase)
to weather this we need: CO2
water
and of course the feldspar
CO2
will react with the water to form carbonic acid
H2O+ CO2
= H2CO3
4NaAlSi3O8+4H2CO3+
18H2O = 4Na+ + 4HCO3-
+
(plag)
(carb.acid)
(diss. comp.)
8H4SiO4 + Al4Si4O10(OH)8
(clay min.)
As you
can see the clay mineral has no Na+ in
it, on the other hand it contains (OH) in its structure.
The
original framework silicatestucturewill become sheet silicate. Some silica is also released from the feldspar, which in the
water forms weak acid, and may then be carried away from the site of reaction.
3.
OXIDATION
oxidation
in an other word is called rusting.
4Fe+3O2=Fe2O3
Oxidation
involves every process where cations loose electrons from their outer electron
shell to Oxygen.
For
oxidation most of the time the Cation was released from a silicate structure by
Hydrolysis
When in
coal mining or volcanic area pyrite is exposed it will go through oxidation
2FeS2
+ 4H2O= 2FeO(OH) + 2H2SO4
+ H2
Big
problem with soils around these mines. Acidifying
groundwater.
The
activity of chemical weathering rises with rising temperature.
it
is most intensive in warm hot tropical weathering
GEOMETRY
OF ROCK INTEGRATION
The
Mechanical breakdown and the shape of the weathered rocks are usually inherited
from the pattern of joints, bedding, and cleavage of the original rocks,
First
usually the edges and faces became weathered
Spheroidal weathering
During
this PROCESS successive shells will separate down from the rock (spelling off)
Weathering
of different rocks
Important
factors:
1.Mineral composition
Check in headstones in old cemetery!
Quartz is very stable
olivine, pyroxene very unstable
IMPortant!
Stability of minerals is the OPPOSITE of BOWEN REACTION
SERIES
Texture is also very important (porosity, permeability)
2.Climate
3.Topography (Amount of rocks exposed)
examples:
GRANITE
Feldspar - clay min Ca goes first,
Na later
pyroxene-amphib. mica - clay + Limonite
Quartz remain unaltered (light redish soil)
Basalt
Soon goes into clay and iron oxides (red-brown soil)
Sandstone
Quite resistant
Limestone
Humid clim
dissolves away easily, cave systems
aride clim.
resistant, forms cliffs
Shale
weathers
away quickly because it is fine grained and soft. However it is not changing,
probably just transported away.
DIFFERENTIAL
WEATHERING
Different
rock masses weather very differently.
More
resistant zones will make cliffs, and stand out as ridges. The weaker zones tend
to become walleyes or depressions
SOIL
Regolith
and SOIL
Regolith
is coming from the Greek word REGO
meaning
blanket!!!!!
It is a
layer of soft desintegrated rock material formed in place and covers the fresh
rock underlies beneath it.
The
thickness of the regolith is variable from some cm to hundreds of meters, dep.
on the composition of rocks, climate, + the length of time the weathering takes
place.
UPPERMOST LAYER OF REGOLITH IS
CALLED SOIL
Composed of rock fragments, minerals and decomposed organic matter (HUMUS)(50%), and pore space filled by
air and water
(50%)
Humus
is very important, because it helps to retain the water in the soil zone. On the
other hand it provides the nutrients for plants.(5%)
Soil water: is not pure water, but contains lots
of
very important ions, nutrients (25%)
Soil air: supply the oxygene plants need (25%)
Soils
formed in place are termed:
Residual soils
Soils
developed on unconsolidated deposits are termed:
Transported soils
SOIL
PROFILE
Transition
of the upper surface of the soil down to the bedrock.
Soil
Profile contains a constant sequence of layers, or horizons, distinguished by
composition, color and texture.
O
HORIZON
thin surface of leaf mold (forest)
A
horizon.
topsoil layer, light leached layer (zone of leaching) (ions and fine
grained parts are washed out downward) ELUVIATION
B
horizon.
Subsoil, contains Fine clays washed down from the topsoil. This is the zone
of accumulation and commonly reddish. ILLUVIATION
A
and B together is called solum or true soil
C
horizon:
Zone of partly
disintegrated and decomposed bedrock, It is gradually grades into the fresh
unaltered bedrock.
The
soils which has no well developed zones are immature
The
type of zones, and the thickness of soil is essentially depend on the following
factors:
1.Climate
2.Parent rock material
3.Plants and animals
4.topography
1.Climate:
Desert, arctic region:
Physical weathering major,
Organic matter is minimal (no humus)
SOIL: thin, mostly rock fragments
Equatorial regions
Chemical weathering is dominant
Thick soil develops rapidly (60m).Rarely it
even
can be
150m.
2.Parent
rock material:(ex.)
Quartz sandstone: thin infertile soil
3.Plants
and animals:
organic decay----humus (mostly plants)
Swamp almost 100%
Desert 1%
4.Topography
It effects the rate of erosion and drainage
Slope:On steep slope no too much soil, few water, not
well dev.soil.
SOIL
TYPES IN THE US
PEDalfER
pedon=soil (greek)
Al-alumina
Fe-iron
Lots
of Fe, and Al-clays, mostly occurring underneath of forests in mid-latitude
areas, with more than 65 cm annual rainfall (acidic conditions (from organic
decomposition)
PEDOCAL
Accumulation
of calcium carbonate
Dry
climate, grasslands smaller amount of clay, bec. dry climate
CALICHE
Pure
caCO3 layer arid-semi-aride climate
LATERIT
Hot
wet climate of the tropics
Great
amount of silica is dissolved---
Bauxite
Problems
in soil management
SOIL
EROSION
More dangerous lately, bec. of forest destroying
(plants are protective)
Soil
removement can happen by:
Wind
water
Eroded soil is moved to lakes rivers,
shrink the reservoir capacity
Pollution carried by soil into reservoirs drinking water
becomes toxic