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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 2

Week 5

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Weathering and Erosion:

Weathering classification: fresh, slightly weathered, moderately weathered etc. Formation of Meanders. Cycle of Erosion.

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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

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Weathering

• Weathering is of two types: – 1.

Physical weathering or disintegration

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2. Chemical weathering or decomposition

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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

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1.3 Pressure release 1.4 Hydraulic action 1.5 Salt-crystal growth 1.6 Biological Weathering

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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 6

Physical Weathering

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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.

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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 7

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.

Shattered rocks are common in cold and alpine environments where repeated freeze-thaw cycles gradually break rocks apart.

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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 10

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.

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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.

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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

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.

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Chemical Weathering Hydration.

Hydration is the process in which water molecules combine chemically with the minerals to produce new compounds. The formation of gypsum (CaSO 4 . 2H 2 O) 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.

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.

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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.

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

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

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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.

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Cliff erosion in Pacifica, California

Erosion

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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.

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2.Effects of Erosion: Approximately 40% of the world's agricultural land is seriously degraded.

Threat to biodiversity.

Loss of top soil.

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Erosion Erosion Processes: 1.

Gravity : Mass wasting

, also known as the force of gravity.

slope movement

or

mass movement

,is the geomorphic process by which soil, regolith,and rock move down slope under 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

as

scour

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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

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.

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 Further cracking.

Wind

glacial erosion

. It can also be due to freeze Thaw processes in which water inside pores and fracturesin rock may expand cause 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

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.

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Erosion due to wave pounding at Venus Bay, South Australia.

A rock formation in the Altiplano, Bolivia sculpted by wind erosion.

Wind-eroded alcove near Moab, Utah.

A Linxia City, China, farmer is gradually losing his land as the edge of the loess plateau is eroded away 21

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

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

• What is a GLACIER?:

Glacier Erosion

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

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

Soil Erosion

• Biggest problem for farmers • Wind and rain remove topsoil and nutrients • Makes the soil infertile and it breaks up leaving dongas

Sheet Joints (Exfoliation)

Stone Mountain, GA Half Dome, Yosemite, CA

Exfoliated Domes, Yosemite

Stone Mountain, Georgia, showing the product of exfoliation due to unloading

Stone Mountain, GA

Frost Wedging

: rock breakdown caused by expansion of ice in cracks and joints

Shattered rocks are common in cold and alpine environments where repeated freeze-thaw cycles gradually pry rocks apart.

Role of Physical Weathering

1) 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

Surface Area and Weathering

Rates of weathering

Joints in a rock are a pathway for water – they can enhance mechanical weathering

Feldspars to clay + H

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CO

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(acid)

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

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