First part of
Weathering is the
name of the process in which the original rocks disintegrate and decompose. It
is somewhat similar to the equilibrium processes we have learned in metamorphic
rocks; Some minerals such as igneous and metamorphic minerals are not stable
under surface condition (STP), and they will essentially alter to adjust to the
Weathering involves physical, chemical and biological processes, but the two major types are:
(mechanical) and chemical
It is strictly
physical process, involving no change of chemical composition
1. Ice or frost
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).
1. enough moisture
falling, rising temperature
with cracks or weakening (problem on highways)
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
rocks taken away --- sheeting can cause problems rock slumping)
cycle, in hot deserts
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
of the weathering or
carrying material for dissolved ions
on the other hand
it removes the weathered particles, to expose the next fresh rock. The degree
and the rate of chemical weathering is essentially influenced by the amount of
Therefore, chemical weathering can not be very important in deserts.
Major processes of chemical weathering are:
It is easy to imagine dissolution.
(salt or sugar in
water-soluble mineral is Halite. This is caused by the mineral structure, and
the fact that the water has polar molecules.
think of acid test
Ions from such a
dissolution makes the water HARD
Ions of hard water
reacts with soap and makes insoluble compound (not desired)
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 mostly clay.
2 (H+ + HCO3- )
framework silicate structure will become sheet silicate. It is true for most
silicate minerals. Some silica is also released from the silicate mineral, which
in the water forms weak a acid, and may then be carried away from the site of
Not every hydrolysis will produce kaolinite. Other clay minerals might form depending on the original silicate mineral.
oxidation in an
other word is called rusting.
every process where cations loose electrons from their outer electron shell to
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
4H2O= 2FeO(OH) +
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
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
PROCESS successive shells will separate down from the rock (spelling off)
headstones in old cemetery!
Quartz is very stable
olivine, pyroxene very unstable
Stability of minerals is the OPPOSITE
of BOWEN REACTION SERIES
is also very important (porosity, permeability)
3.Topography (Amount of rocks exposed)
- clay min Ca goes first,
pyroxene-amphib. mica - clay + Limonite
remain unaltered (light redish soil)
goes into clay and iron oxides (red-brown soil)
dissolves away easily, cave systems
resistant, forms cliffs
weathers away quickly because it is
fine grained and soft. However it is not changing, probably just transported
masses weather very differently.
zones will make cliffs, and stand out as ridges. The weaker zones tend to become
walleyes or depressions
Regolith is coming from the Greek word REGO:meaning blanket
It is a layer of
soft disintegrated 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 .
of rock fragments, minerals and decomposed organic matter (HUMUS)
(50%), and pore space filled by air and water .
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,
Soil air: supply the oxygene plants need (25%)
Soils formed in
place are termed:
developed on unconsolidated deposits are termed:
the upper surface of the soil down to the bedrock.
contains a constant sequence of layers, or horizons, distinguished by
composition, color and texture.
surface of leaf mold (forest)
layer, light leached layer (zone of leaching) (ions and fine grained parts are
washed out downward) ELUVIATION
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
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:
Physical weathering major,
Organic matter is minimal (no humus)
SOIL: thin, mostly rock fragments
Chemical weathering is dominant
Thick soil develops rapidly (60m).Rarely it even can be 150m.
sandstone: thin infertile soil
decay----humus (mostly plants)
effects the rate of erosion and drainage
steep slope no too much soil, few water, not well developed soil.
IN THE US
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)
grasslands smaller amount of clay, bec. dry climate
layer arid-semi-aride climate
Hot wet climate
of the tropics
Great amount of
silica is dissolved---
lately, because of forest destroying (plants are protective)
can happen by:
soil is moved to lakes rivers,
the reservoir capacity
Pollution carried by soil into reservoirs drinking water becomes toxic