weathering classification, products and geological products

1. Weathering

2. Introduction
• Weathering term is applied to the combined
action of all processes causing rocks to be
disintegrated physically and decomposed
chemically because of their exposure at or
near the earth surface.
• It is an initial stage in the process of
denudation and is considered as a static part
of the general process of erosion.

4. • An essential feature of weathering is that
affect rocs insitu and doesn’t involve transport
of the degraded rock materials.
• An weathered soft surface layer is called
regolith. Regolith grades down from top to
bed rock.
• Weathering helps the erosion in considerable
extent.

6. Factors affecting weathering
• Major factors are as following, they are,
a)Climate
b)Topography
c)Structure, texture & mineral
composition
d)Vegetation cover
e)Time

7. a)Climate
• Climate is defined by meteorological elements
lie temperature, moisture, humidity, rainfall,
atmospheric pressure & wind , etc.
• It determines weather mechanical
disintegration or chemical decomposition &
speed of process.
• Physical disintegration predominates in arid or
tropical climatic condition. i.e. in drier &
colder region.

9. • Rock decomposition is more active in humid
regions. i.e. in moist, warm, & low-lying areas.
• The thermal expansion & contraction are
expected to cause mechanical destruction of
the rock.
• Besides, alternate freezing & thawing, due to
temperature fluctuation also shatters the rock.
• Temperature also affects the rate of chemical
reaction.

11. • In polar & mountain region water traps on the
pore spaces and intermittent freezing split the
rocks.
• Water is an active chemical agent, lead to the
chemical decomposition of rocs, thereby
eliminating all the unstable minerals in rocks.

13. b)Topography
• Erosive power & weathering increases in high
relief and decreases in low relief.
• Relief controls the degree of weathering on
the region.
• Based on the topography the weathering
areas can be easily identified to locate the
weathering products lie soil & regolith.

15. c)Structure,
• The presence of joints, cracks and fissures in
the igneous rocks, bedding in sedimentary as
well as foliations of metamorphic rocks
facilitates maximum weathering.
• Water when percolates in rock it will
disintegrate or decompose the rock.

17. Texture,
• Texture of a roc determines hardness and
strength of the rocks.
• Fine grained rocks most susceptible to
weathering then the coarse grained rocks.
• Also that soft rocks weather more than hard
rocks.

19. Mineral composition
• The stability in rocks on the silicate minerals
are based on the Bowens reaction.
• Quartz is persists in all conditions of
temperature and pressure, at last.
• Alkali feldspar stay longer than the plagioclase
in weathering.
• Basic rocks are more weathered in chemical
action than the acidic rocks.

21. d)Vegetation cover
• Surface covered by vegetation is more
protected than the bare surfaces.
• Besides, plant roots grow in fissures & joints
tend to disintegrate rocks
• Organic acids due to plant decay, which help
in the decomposition of rocks.

23. e)Time
• Longer the period maximize the degree of
weathering and greater the volume of rocks
affected by it.

25. Agents of weathering
• The principle agents are as follows are,
• Water, wind, gases lie O2 & Co2, acids, gravity,
variation in temperature, organisms etc.
• These are mainly responsible for the physical
& chemical weathering.

26. Types of weathering
• Physical weathering or mechanical process of
disintegration
• Chemical weathering or chemical process of
rock decomposition
• Biological weathering or processes of rock
decomposition associated with the activities
of organisms.

27. Physical weathering
• Mineralogical composition of roc is not
affected, Instead it is a process of reduction of
material into smaller pieces.
• It is a process of fragmentation of roc due to
some physical forces.
• They are as following,

28. a)Fluctuation in temperature
• Rock is a poor conductor of heat but the outer
shell of the rock is subjected to the
temperature fluctuation.
• The periodic expansion & contraction of rocks
lead to the flaking off of the upper layer. The
process of scaly peeling off of the rocks is
known as exfoliation or desquamation.

30. • The difference in their volumetric expansion
may also lead to the development of cracks
and gradual disintegration of rock. this process
is also known as gradual disintegration.
• The weathering due to fluctuation in
temperature is termed as thermal weathering.

31. b)Change in pressure on rocks
• Pressure exerted on the deeply buried rock by
the column of overlying material are enormous.
As a result of removal of the overlying rocks by
denudation, the rock adjust to this unloading will
result in the development of a closely spaced
joint systems and fractures parallel to the surface.
• This phenomenon of response of rocks to release
the confining pressure is called dilation.

33. c)Growth of crystals
• Water consists of minerals enter the rocks
through fracture or cracks. With the
evaporation of water, the salts it contains start
crystallizing.
• As a crystal grow it exerts large expansive
stresses that result in the rock fragmentation.
it mostly occurs in dry region.
• Crystals of gypsum, magnesium, sulphate,
phosphate, nitrates grow in this manner.

35. d)Freezing of water
• Water expands 9% in volume during freezing
will exerts enormous pressure on the walls of
the fissures or joints of rocks, when it is
trapped in the pores of rocks.
• Alternate freezing & thawing is the most
effective process of rock disintegration.
• This phenomenon is termed as Frost wedging

37. e)Frost action
• Water present in ground freezes form a ice
layers.
• Thus the thickness of the ice forcing the soil
above it upward.
• In this way the up heaved soil is disrupted by
the expansion of freezing water.
• This is also known as frost heaving.

39. Chemical weathering
• It is aprocess of mineral alteration, which
consists of a number of chemical reactions.
• Whereby the primary mineral is converted
into new compound, the secondary minerals.
• Effectiveness of chemical weathering depends
upon, 1)size of the particles,
2)composition of rocks
3)Favourable temperature and humidity

40. a)Hydration
• It refers to the chemical union of water with a
mineral.
• Certain minerals takes up water, which lead to
a change in mineral composition of rocs.
• Due to the minerals expand, causing more
stresses within the rock and cause mechanical
disintegration.
• i.e. gypsum, hematite, to limonite

42. b)hydrolysis
• Process of exchange reactions between the
bases of the minerals and the hydrogen ions
which is part of the water.
• The higher Ph value, the more dissociated is
the water and the stronger its action.
• Water is more readily dissociated when it
contains Co2.
• Feldspar reacts with water more, hydrolysis is
more in granite.

44. c)oxidation
• The process of chemical union of o2 atoms of
other metallic elements.
• Thus the minerals are altered with the
production of oxides.
• Ferromagnesian minerals lie pyroxenes,
hornblende & olivine etc. rapidly undergoes
oxidation.
• By producing a brown crust consisting largely
of oxides of iron. i.e. pyrite.

46. d)carbonation
• Process by which Co2 is added to minerals to
form certain carbonates.
• Rain water passage through atmosphere,
dissolves some Co2 present in air. It turns into
a weak acid called carbonic acid.
• It dissolves carbonates easily. Alkali metals like
sodium, potassium, calcium & magnesium.
• Feldspar to clay minerals by this process.
Effective in limestone or chalk areas.

48. e)solution
• Some of the minerals get dissolved by water
and thus removed in solution.
• Solution tae place at different rates for
different rates for different rates for different
rocks. Maximum in chlorides and minimum in
sulphates and carbonates.
• Silica undergoes solution in alkaline fluid.
• The process of removal of soluble material
from the rocks in solution is called leaching.

50. Biological weathering
• It mainly related to the biological activities of
plants, animals and organisms lie bacteria etc.
• It involves the role of plants & animals in the
breaking down of rocks through mechanical
ways as well as in the decomposition of rocks.

52. Physical breaking down of rocks
• Plant roots growing between jointed blocks
and along minute fractures of rocs exerts force
on it & tend to widen the existing opening.
• Insects like earthworm, snail etc. and
burrowing animals like rodents loosen the soil
cover and creates suitable conditions for
various geological agencies to have action on
the underlying rocks.

54. Bio chemical actions
• Plant dissolves various constituents of the rocs
with the acid evolved by the tips of their
roots. Besides, certain bacteria algae and
moose's breakup roc forming silicates directly.
• Bird droppings have been capable of
weathering limestone's.
• Dead decays of plants & animals produce
chemically active substances which capable of
bringing about rock weathering.

56. Weathering products
• A mantle of broken & decomposed material of
varying thickness & composition called the
regolith lies over the derived rocks.
• Soluble salts are produced by the solutions which
leach outs.
• Colloidal substances which are weathered
products carried away by the ground water.
• Insoluble particles lie quartz, clay, undecomposed
feldspar, zircon & tourmaline are deposited by
weathering.

58. classification
• All the weathering products are grouped into
two categories as:
• A)transported or mobile-Which transported
over varying distance by mechanical, chemical
& bio chemical means.
• B)residual or sedentary-remains of parent
rocks after weathering like bauxite or laterite.

59. Geological features
• Differential weathering give rise to features
like honeycombed rocks, hollows and niches in
rock walls.
• Boulders by spheroidal weathering, tor or
kopje- free standing outcrop capping hill or
slope.
• Mass wasting on slope produce talus or scree.

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