Tropical Geomorphology -weathering in Tropics Deep weathering Profiles in Tropics Factors Affecting Deep Weathering in Tropics Laterites solubility and mobility of the minerals in tropics

1. PART - I
LM Thakare - Tropical Geomorphology -Part 1

2. Weathering – Processes and products
• Tropical regions – zonal weathering as a phenomenon is
responds to the particular clay minerals and behavior of
iron.
• Weathering varies greatly depending on the humidity of
the region.
• Depths of the rock decay are more greater in the tropics
 30 m = humid tropics
 25 m = wet Savanna – Ruxton and Berry 1961
 6 m = dry Savanna
 < 3m = arid zones.
LM Thakare - Tropical Geomorphology -Part 1

3. Regolith: the layer of unconsolidated solid material
covering the bedrock of a planet. Unsorted material
LM Thakare - Tropical Geomorphology -Part 1

4. Climatic factors - Tropics
Temperature Precipitation Chronological
• Thermal variations due to Land –Sea distribution
• Thermal variations due to Vegetation cover (Humification)
• High temperatures 18 degree cel, 21 to 30d c
• Exothermal conditions ( atm. T)
• Endothermic (Geothermal) – Chemical reactions
• Soil T.
• Latitudinal Radiated solar energy
• Air Temperature – Reaches a max during the day of between 27 & 32ºc but
occasionally it may exceed.
• The mean minimum temperature varies from 21º & 27ºc.
Monsoon and Foehn – Chinook wind
orogenic ppt.
LM Thakare - Tropical Geomorphology -Part 1

5. Temperature
• Most of the chemical reactions in particular hydrolysis are endothermic.
• The rate of Ch. Reactions increases slowly with temp. for every rise of 10 d cel.
• This is “ Van Hoff’s Rule”.
• Tropical Weathering rates will exceed than the cool temperate areas by at least
4 times.
 Other factors like topographic factor, the supply of groundwater and
organic matter, availability of minerals.
 Humid tropics : T = 24 -26 d cel
R = 1200 – 1300 mm
organic matter = 100 – 200 ton/ha
Humid tropics : Leaching increases = 7 – 14 times
Internal soil T
LM Thakare - Tropical Geomorphology -Part 1

6. Chronological factors
• Climate Changes – Change in climate, vegetation with time.
-Fluvial conditions in Tertiary and
Pleistocene -rejuvenation of rivers
- deep weathering
• Tectonic Changes – Variation in crustal movements
and stability- gives time for weathering
Factors = T +P = 2 -weathering = 5-6 planation surfaces
(end products) – 3-4 denudation landforms -inselbergs/
monads
Knick points -waterfalls
LM Thakare - Tropical Geomorphology -Part 1

7. Environmental Factors
Biotic factors Geomorphic factors Site factors Geological factors
1. Vegetation cover
2. Canopy of the forest – protects from splash erosion, sheet e.
3. Provides organic biomass - Tropics
4. Organic acids – mobilization of minerals within the soil
(exception in Savanna)
5. moisture holding, Water holding
6. Maintain channelization by plant roots.
Types – parent
material
LM Thakare - Tropical Geomorphology -Part 1

8. Geomorphic factors
1. Land surface – rate of weathering
2. Age of land surface – ancient landforms – deep soil profile development
3. Effect of external factors - T, R, Solar energy, etc.
LM Thakare - Tropical Geomorphology -Part 1

9. Site factors
1. Free drainage system – High altitudinal areas (steep slopes) promote
mass movement and easy movement of groundwater with in the
parent rock. This helps in faster decomposition of rock.
2. Zone of leaching, groundwater- More the groundwater, more is
penetration and rapid chemical weathering. ( solubility of the minerals
- leaching)
3. Variation in the slope of land – Steep slopes = rapid runoff
Gentle slope = slow movement of
water (problem of
waterlogging)
LM Thakare - Tropical Geomorphology -Part 1

10. Geological factors
1. Rock type
2. Rock structure
3. Texture
4. Rock cracks , faults, joints
5. Thermal expansion and contraction
6. Hydrological characteristics
LM Thakare - Tropical Geomorphology -Part 1

11. Wyoming, UK is a terrain underlain by granite.
jointed basalts (volcanic flows) from
Giant's Causeway, Ireland.
Granite boulder , Hampi, Karnataka
LM Thakare - Tropical Geomorphology -Part 1

12. Solubility and mobility of the minerals
LM Thakare - Tropical Geomorphology -Part 1

13. Soil depth
LM Thakare - Tropical Geomorphology -Part 1

14. Primary minerals in the rocks
• A primary mineral is any mineral formed during the original crystallization of the
host igneous primary rock and includes the essential mineral(s)
• In ore deposit geology, endogenic processes occur deep below the earth's surface,
and tend to form deposits of primary minerals,
• exogenic processes that occur at or near the surface, and tend to form secondary
minerals.
Examples : White veins of quartz (primary/secondary sulfate mineral)
• Composition of primary rocks is determined by the chemical composition of the volcanic or
magmatic flow from which it is formed.
• Extrusive rocks (such as basalt, rhyolite, andesite and obsidian) and
• intrusive rocks (such as granite, granodiorite, gabbro and peridotite) contain primary minerals
including quartz, feldspar, plagioclase, muscovite, biotite, amphibole, pyroxene and olivine in
varying concentrations
Aa and pahoe pahoe flow
LM Thakare - Tropical Geomorphology -Part 1

15. Primary
minerals in the
rocks
LM Thakare - Tropical Geomorphology -Part 1

16. Secondary minerals in the rocks
• A mineral formed by the sub-solidus alteration of a pre-existing primary
mineral in an igneous rock.
• Minerals which have crystallized from a magma are stable only at high
temperature and can readily alter to low temperature, secondary minerals when
a fluid, e.g. water – H2O, is introduced into the rock system.
• The fluid acts as a catalyst to initiate the alteration reaction.
• Most secondary minerals are hydrated silicates.
• A typical example is the alteration of primary olivine to
secondary chlorite and serpentine.
Accessory minerals form at various times during the crystallization, but their
inclusion within essential minerals indicates that they often form at an early time.LM Thakare - Tropical Geomorphology -Part 1

17. - Majority of rock forming minerals are silicates.
- The reaction of silicate minerals depends on weathering agents.
- They are the largest and most important class of minerals and
make up approximately 90 percent of the Earth's crust.
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20. Terahedra as single, chains and sheetsLM Thakare - Tropical Geomorphology -Part 1

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22. • Silica (silicon dioxide) SiO2 is usually considered a silicate mineral. Silica is found in
nature as the mineral quartz.
• On Earth, a wide variety of silicate minerals occur in an even wider range of combinations
as a result of the processes that have been forming and
• re-working the crust for billions of years. These processes include partial melting,
crystallization, fractionation, metamorphism, weathering, and diagenesis.
LM Thakare - Tropical Geomorphology -Part 1

23. • Ionic substitution Silicon (SiO -4)atoms within these sheets
may be replaced by metal cations such as ,
Aluminium (Al +3)
Iron (Fe+3 or Fe+2 )
Magnesium (Mg+2 )
Calcium (Ca+2 )
Sodium (Na+2 )
Potassium (K+1 )
Thus silica minerals are formed and they comprise the bulk
of most common rocks.
Terahedra as single, chains and sheets
LM Thakare - Tropical Geomorphology -Part 1

24. The bonds formed between the ions may be weak at
low temperatures in the presence of H+ and OH- ions
from the water.
It is found that the higher the energy of formation = the
temperature of crystallization = rapid disintegration of
the minerals ( hydrolysis process)
Si : O2 ratio
Substitution of Si by other cations has been more = ratio is low,
Substitution of Si by other cations has been less = ratio is more
LM Thakare - Tropical Geomorphology -Part 1

25. Two groups of Silicate minerals can be identified
Ferro-magnesian (Mafic) minerals
Olivine
Isolated Silica tetrahedral are
linked by Fe +2 and Mg+2
Feldspar minerals
Al+3 can be substituted by
Ca +2,
K+, or
Na+
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27. Bowen's reaction series showing the sequence of minerals that would
be formed and removed during fractional crystallization of a melt. The
magmas relating to the crystallizing minerals are shown on the left.
1:4
1:3
2:5
3:8
Most stable
Least stable
Kaolinite
Montmerillonite
Illite
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29. Ferro-magnesian (Mafic) minerals = discontinuous series
Ex. Olivine = Fe +2 and Mg+2 = attached on the edges = easily detached/ eroded
The erodibility of Silica minerals is difficult to study
Because:
1. Rocks have no precise chemical formula
2. Any two rocks are never similar
3. There is always a slight change in mineral
Proportion of the rocks
4. Change in the rate of weathering
5. Change in the process - T and Rainfall
6. Varying rock texture
7. Varying rock structure
8. Insitu location
9. Internal stress
10. Strength of bonding
11. Factures and sutures within the rocks
partial melting, crystallization, fractionation, metamorphism,
weathering, and diagenesis.
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30. Process of weathering or breakdown of silicate
minerals depends largely on :
Hydrolysis = activity of ionized water
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32. Mobility of silica minerals
The weathering products will depend upon the weathering environment.
Mobility amongst the common cations in the silicate rocks varies widely.
Weathering is also affected by the pH of the groundwater.
Oxidation/Reduction Potential (Eh)
• Eh is a measure of the redox (oxidation-reduction) state of a solution.
• Eh is a measurement of electrical potential and expressed in volts.
• Values of Eh in nature range from -0.6 t o +0.9V,
1Most mobile : Ca+2, Na+, Mg+2 , K+
2Intermediate mobile : K+, Mg+2, Si+4, Fe+2
3Least mobile : Fe+3, Al+3LM Thakare - Tropical Geomorphology -Part 1

33. • Both K+ and Mg+2 are readily mobilized, but they
tend to recombine to form clay minerals.
• Strong leaching magnesium is normally
lost in the solution as Group I.
• Silicon has a low but significant mobility at
most pH levels.
Ex. Quartz has less solubility ( 10th times less than
olivine)
• Iron (Fe+2) mobility depends on the Eh or
Redox potential.
1Most mobile : Ca+2, Na+, Mg+2 , K+
2Intermediate mobile : K+, Mg+2, Si+4, Fe+2
3Least mobile : Fe+3, Al+3
• Iron in oxidized ferric state is immobile
• Iron in reduced ferrous form may go into
solution at common soil pH soils as
bicarbonate. LM Thakare - Tropical Geomorphology -Part 1

34. Most mobile elements are usually lost to
th site of weathering in solution, the
elements are intermediate mobility tend
to remain in solution for short periods
And are commonly redeposit in new
minerals (clays), close to the site of
weathering.
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35. Least mobile elements (Al and Fe) in the ferric stat, generally
remain as residual products of weathering as SESQUIOXIDES.
A sesquioxide is an oxide containing three atoms of oxygen with
two atoms (or radicals) of another element. For example, aluminium
oxide (Al2O3) is sesquioxide.
Sesquioxides of iron and aluminium are found in soil.
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37. Deep weathering Profiles in Tropics
Deep weathering is the weathering of underground rocks by
water and humic acids as they percolate down through soil.
• Factors Affecting Deep Weathering
Deep weathering is most common in the humid tropics because:
• Abundant vegetation provides humic acids as they decay.
• Abundant rainfall percolates through the soil and therefore increase soil moisture
accelerating deep weathering.
• High temperatures speed up the rates of chemical reactions. (after every 10 DC of soil
temp doubles th rate of weathering)
• Dense vegetation holds the weathered mantle in situ
• Occurrence of faulted rocks provides already made fractures through which acids can
penetrate.
• The deep weathered profile is more visible on igneous rock types such as granite that
are weathered in boulders. On soluble rocks such as limestone the deep weathered
profile may not be sharply visible as these rock types are completely washed away inLM Thakare - Tropical Geomorphology -Part 1

38. Capillary action
Leaching Humic acids
High T, soil moisture, humus
Groundwater flow
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43. Followed by Part 2
LM Thakare - Tropical Geomorphology -Part 1