2. Engineering Geology and Seismology Weathering and Erosion
Lecture # 5
Engineering Geology and Seismology
Weathering and Erosion
Instructor:
Prof. Dr. Attaullah Shah
Department of Civil Engineering
City University of Science and IT Peshawar

3. Week 5
Weathering and Erosion: Weathering classification: fresh, slightly weathered, moderately weathered etc. Formation of Meanders. Cycle of Erosion.

4. Weathering
Weathering is the process that produces changes in the surfaces of rocks exposed to the atmosphere.
The rocks break and undergo decay under the influence of the atmospheric agencies like wind, sun, frost, water and organisms and produce soil.
Erosion involves the transportation and removal of weathered materials from the place of their formation.
Land areas are continually being reduced and their shape modified by weathering and erosion, and the general term for this is denudation

5. Weathering Weathering is of two types:
1. Physical weathering or disintegration
2. Chemical weathering or decomposition
3. Biological Weathering or decomposition
Physical/mechanical Weathering: By this process, angular fragments of rocks are broken off from the high mountain ranges. These fragments of rock roll down the hill slope and accumulate at the bottom. The various types are:
1.1 Thermal stress
1.2 Frost weathering
1.3 Pressure release
1.4 Hydraulic action
1.5 Salt-crystal growth
1.6 Biological Weathering

6. Weathering
Chemical weathering, or decomposition, is the break-down of minerals into new compounds by the action of chemical agents; acids in the air, in rain and in river water, although they act slowly, produce noticeable effects especially in soluble rocks.
Biological weathering describes those mechanical and chemical changes of the ground that are directly associated with the activities of animals and plants

7. Physical Weathering
Heating and cooling or Thermal stress: The heating and cooling of rock masses occur due to daily and seasonal temperature changes.
The heat causes them to expand and cooling causes them to contract. The repeated expansion and contraction tend to develop cracks in the rocks. In desert areas, the coarse grained rocks, like granite, disintegrate soon into their constituent crystals, and become desert sands as a result of temperature variation.
Frost Action: The alternate freezing and thawing of water in the cracks of rocks tend to disintegrate them because water on freezing expands and therefore, exert pressure on the walls of the cracks.
Frost Wedging: rock breakdown caused
by expansion of ice in cracks and joints

8. Weathering
Thermal expansion due to the extreme range of temperatures can shatter rocks in desert environments.
Repeated swelling and shrinking of minerals with different expansion rates will also shatter rocks.
Rain may enhance this process

9. Shattered rocks are common in cold and alpine environments where repeated freeze-thaw cycles gradually break rocks apart.
Effects of freeze thaw on granite. Mt. San Jacinto,southern California, Transverse Ranges.  About 8,500 feet.

10. Pressure Release: In pressure release, also known as unloading, overlying materials (not necessarily rocks) are removed (by erosion, or other processes), which causes underlying rocks to expand and fracture parallel to the surface.
Often the overlying material is heavy, and the underlying rocks experience high pressure under them, for example, a moving glacier. Pressure release may also cause exfoliation to occur

11. 1.4 Hydraulic action:
This is when water (generally from powerful waves) rushes into cracks in the rock face rapidly. This traps a layer of air at the bottom of the cracks, compressing it and weakening the rock.
When the wave retreats, the trapped air is suddenly released with explosive force. The explosive release of highly pressurized air cracks away fragments at the rock face and widens the crack itself.
1.5 Salt-crystal growth Salt crystallization, otherwise known as haloclasty, causes disintegration of rocks when saline solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind.
These salt crystals expand as they are heated up, exerting pressure on the confining rock. Salt crystallization may also take place when solutions decompose rocks (for example, limestone and chalk) to form salt solutions of sodium sulfate or sodium carbonate, of which the moisture evaporates to form their respective salt crystals.

12. c) Plants. Plants disintegrate rocks when it grow in the cracks of the rocks. d) Organisms. such as worms, ants etc. also contribute to the disintegration of rocks.

13. possibly by frost weathering or thermal stress
A rock in Abisko, Sweden fractured along existing joints
possibly by frost weathering or thermal stress
Salt Crystallization
Comparison of un-weathered (left) and weathered
(right) limestone.
Salt weathering ofbuilding stoneon the island ofGozo,
Malta

14. Chemical Weathering
Chemical Weathering or decomposition is the process in which rocks are
broken down by chemical decay of minerals. The main agents of chemical
weatheringas follows:
Water. The chemical weathering of rocks is done mainly by rain water.
Theprocessinvolvedare:
Oxidation: The oxygen, in the presence of water, readily units with the
iron present in the minerals such as pyroxenes, amphiboles and olivine
and convert it to iron oxide (hemitite) or to hydroxides (limonite). The
oxidation of pyrite leads to the formation of limonite and weak solution of
sulphuric acid. This acid is a powerful chemical agent which attacks the
rocks and develop solution pits. The iron oxides and hydroxides are the
very common products of oxidation which impart the red and yellow
colorstosoils. 10

15. Hydration. Hydration is the process in which water molecules combine
Chemical Weathering
Hydration. Hydration is the process in which water molecules combine
chemically with the minerals to produce new compounds. The formation of
gypsum (CaSO4. 2H2O) from anhydrite (CaSO4) is a good example of
hydration. Carbonation frequently occurs together with hydration. Thus
orthoclase feldspar, a mineral which is abundant in granite, is decomposed
andconvertedinto kaolinby hydrationand carbonation.
Carbonation. When minerals containing calcium, magnesium and sodium or
potassium ions react with water containing carbon dioxide, carbonates and
bicarbonates are formed which are taken into solution. This process of
decompositioniscalledcarbonation. 11
A freshly broken rock shows differential chemical weathering
(probably mostly oxidation) progressing inward.

16. stones, dolomite, rock salt and gypsum are particularly susceptible to
Chemical Weathering
Solution. The process of solution and carbonation goes on together. The lime
stones, dolomite, rock salt and gypsum are particularly susceptible to
solutionwhenthey areattackedby water containingcarbondioxide.
Vegetation. Decay of organic matter releases certain organic acids which
increase the solvent power of water. For example, the solubility of silica,
alumina and iron is much greater in the presence of organic acids. Some
plants tend to extract certain chemical elements from rocks and thus they
contributeto theirdecomposition. 12

17. thin sheets or layers concentric with the surface, thereby producing a
Exfoliation
Exfoliation. The process in which rocks, exposed to weathering chip off in
thin sheets or layers concentric with the surface, thereby producing a
round mass. It is sometimes formed when the pressure on a rock from
overlying rock is released. The rock then expands and cracks. The
exfoliation is also caused by alternate heating and cooling of rocks by
weatherconditions. 13

18. exposed,the fasterthe weatheringwill occur
Rate of weathering
Rates of weathering will be influence by surface area exposed. More surface
exposed,the fasterthe weatheringwill occur 14

19. Some minerals are more resistant than others. For example Quartz is
Mineral Composition
Some minerals are more resistant than others. For example Quartz is
resistantto chemicalandphysicalweathering. 15

20. Deposits them elsewhere.
Erosion
Erosion is the process of weathering and transport of solids(sediment, soil,
Rock and other particles) in the natural environment or their source and
Deposits them elsewhere.
It usually occurs due to transportby wind, water, or ice; by down-slope creep of soil and other material under the force of gravity ; or by living organisms, such has burrowing animals , in the case of bio-erosion.
Erosion is distinguished from weathering, which is the processof chemical
Or physical break down of the minerals in the rocks, although the two
Processes may occur concurrently. 16
Cliff erosion in Pacifica, California

21. Erosion 1. Causes: - Areas with high-intensity precipitation,
More frequent rainfall,
Morewind, or more storms.
Sediment with high sand or silt content sand
Areas with steep slopes erode more easily,
Areas with highly fractured or weathered rock.
Porosity and permeability of the sediment or rock affect the speed with
Which the water can percolate into the ground.
Sediments containing more clay tend to erode less than those with sand or
silt.
2.Effects of Erosion: -
Approximately 40% of the world's agricultural land is seriously degraded.
Threat to biodiversity.
Loss of top soil. 17

22. Erosion Erosion Processes:1.
Gravity:
Mass wasting, also known as slope movement or mass movement,is the
geomorphic process by which soil, regolith,and rock move down slope under the force of gravity.
2.Water.
Splash erosion is the detachment and air borne movement of small soil particles
caused by the impact of rain drops on soil.
Sheet erosion is the detachment of soil particles by rain drop impact and their
removal down slope by water flowing overland as a sheet instead of in definite channels or rills.
Bank erosion is the wearing away of the banks of a stream or river.
This is distinguished from changes on the bed of the water course, which is referred to as scour.
Gully erosion, also called ephemeral gully erosion, occurs when water flows in
narrow channels during or immediately after heavy rains or melting snow.
Shore line erosion, which occurs on both exposed and sheltered coasts, primarily
also play a role.

23. Result of material movement by the wind.There are two main effects.
Erosion
Ice:
Ice erosion can take one of two forms. It can be caused by the movement of ice,
Typically as glaciers, in a process called glacial erosion. It can also be due to freeze-
Thaw processes in which water inside pores and fracturesin rock may expand cause
Further cracking.
Wind
In arid climates, the main sourceof erosion is wind.The general wind circulation
moves small particulates such as dust across wide oceans thousands of kilometers
Down wind of their point of origin, which is known as deflation. Erosion can be the
Result of material movement by the wind.There are two main effects.
-First,wind causes small particlesto be lifted and therefore moved to another
region.This is called deflation
- Second, these suspended particles may impact on solid objectscausing erosion by
abrasion(ecological succession).
Thermal
Thermal erosion is theresult of melting and weakening permafrost dueto moving
water.It can occur both along rivers and at the coast. Rapid river channel migration
observed in the Lena River of Siberia is due to thermal erosion, as these portions of
thebanks are composed of perma frost-cemented non-cohesive materials.[ 19

24. The warmer atmospheric temperatures observed over the past decades are
Effects of Erosion
Climate Change:
The warmer atmospheric temperatures observed over the past decades are
lead to a more vigorous hydrological cycle, including more extreme
Rainfall events.
Tectonic effects:
Theremoval by erosion of large amountsof rock from a particular region, and
its deposition else where, can result in a lightening of the load on the lower crust
and mantle.
This can cause tectonic or isostatic uplift in the region. Research undertaken
Since the early 1990s suggests that the spatial distribution of erosion at the
Surface of an orogen can exert a key influenceon its growth and its final internal
structure. 20

25. Bolivia sculpted by wind erosion.
Erosion due to wave pounding at Venus
Bay, South Australia.
A rock formation in the Altiplano,
Bolivia sculpted by wind erosion.
A Linxia City, China, farmer is gradually losing his land
as the edge of the loess plateau is eroded away 21
Wind-eroded alcove near Moab, Utah.

26. Water
Occurs from chemicals in the water and the force and flow of water
Chemicals break down certain rocks like limestone or chalk
Force of water creates cracks and crevasses
These get bigger and bigger as it gets eroded, particles of the rock get carried in the water and further aid erosion

27. Wind The wind picks up smaller particles of rock (pebbles and sand)
These hit landforms and take pieces off them
This friction causes the break up of landforms

28. Glacier Erosion What is a GLACIER?:
A huge mass of ice slowly flowing over a land mass, formed from compacted snow in an area where snow accumulation exceeds melting
These large bodies of ice that pick up rocks and earth and erode the earth’s surface
Powerful eroding machine, picking up pieces of rock the size of houses in some places
Glaciers are so powerful they can carve valleys and create landforms

29. Sea Erosion Salts and other chemicals in the water erode coastal rocks
These are dragged out to sea and deposited in other areas (Long shore Drift)
Wave power creates erosion. Also particles of rock and sediment aid to the erosion process

30. Soil Erosion Biggest problem for farmers
Wind and rain remove topsoil and nutrients
Makes the soil infertile and it breaks up leaving dongas

31. Many igneous rocks, particularly granite, are formed at great depth.
When they come to the surface, they expand.
Creates a morphology with exfoliation slabs – slabs sheet off in onionlike layers
Stone Mountain, Georgia
Expansion of rocks pose a risk in deep mines – explode off walls
These fractures, or joints can enhance frost wedging and other erosive processes

32. Sheet Joints (Exfoliation)

33. Half Dome, Yosemite, CA Stone Mountain, GA
Summit of Half Dome, Yosemite, shows exfoliation
Half Dome, Yosemite, CA
Stone Mountain, GA

34. Exfoliated Domes, Yosemite

35. Stone Mountain, Georgia, showing the product of exfoliation due to unloading
Summit of Half Dome, Yosemite, shows exfoliation
Stone Mountain, GA

36. Frost Wedging: rock breakdown caused by expansion of ice in cracks and joints

37. Shattered rocks are common in cold and alpine environments where repeated freeze-thaw cycles gradually pry rocks apart.
Effects of freeze thaw on granite. Mt. San Jacinto,southern California, Transverse Ranges.  About 8,500 feet.

38. Role of Physical Weathering
Reduces rock material to smaller fragments that are easier to transport
2) Increases the exposed surface area of rock, making it more vulnerable to further physical and chemical weathering

39. Surface Area and Weathering

40. Rates of weathering
Joints in a rock are a pathway for water – they can enhance mechanical weathering
Water flowing through joints attacks the rock, rounding corners.
As minerals are converted to clay, the rock expands, breaking outer layers off
Can enhance mechanical weathering

41. Feldspars to clay
+ H2CO3 (acid)

42. ‘Karst’ landforms develop in areas underlain with limestone
This photo of Lime Sink was taken on 20 July 1932, over a week after the drawdown, which occurred over the night of 9-10 July.
‘Karst’ landforms develop in areas underlain with limestone
When Lake Jackson’s sinks rupture and the lakewater drains out, the process is referred to as a drawdown. It has occurred many times during the last century, in 1900, 1907, 1909, 1932, 1935, 1936, 1957, 1982, and again in 1999.