structural geology groundwater

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Really complex

deformation in

rocks can

produce patterns

as complex as

the swirls in a

foam on a

stream, and

events in one

area don't

necessarily tell

you anything

about events

elsewhere.

Folding

Folds are defined asFolds are defined as undulations or bendsundulations or bends that arethat are

developed in the rocks of the Earth¶s crust as adeveloped in the rocks of the Earth¶s crust as a

result of theresult of the stressesstresses to which these rocks haveto which these rocks have

been subjected to, from time to time, in the pastbeen subjected to, from time to time, in the past

history of the Earth.history of the Earth.

Folds can occur in all scales!Folds can occur in all scales!

In areas of active mountain building, as for example here in the

front ranges of the Himalayas, folds bulge up the landscape. The

upward moving parts (called antiforms) create hills while the areas

that have gone down relative to the hills (called synforms) collect

detritus eroded from the surrounding countryside.

These folds from the

island of Syros only

have a wavelength of afew millimetres.

Buckled folds

� Rocks with really complex histories often

look like this. Often there is no discernible

pattern to the folding.

Parts of Fold and TerminologiesParts of Fold and Terminologies

� Limbs

� Axial plane

� Axis of the fold� Plunge of the fold

� Anticline and syncline

� Limbs are the sides of a fold

� Axial plane is the imaginary planebisecting between the two limbs of a fold

� Axis of the fold is the line of intersection of the axial plane with any bed of the fold

� When the fold axis is inclined the anglewhich it makes with the horizontal asmeasured in a vertical plane is called the

plunge of the fold

Anticline and syncline:

When the beds are up-folded into an arch like structure, it

is called an anticline

Conversely, when the beds are down-folded into a trough

like form, the structure is called a syncline.

Axial plane

Plunge - the inclination of the fold axis

Folded Structures

1. Anticlines

2. Synclines3. Monoclines

4. Basins

5. Domes

Types of foldTypes of fold

On the basis of position of Axial planeOn the basis of position of Axial plane

Symmetrical foldSymmetrical fold

Asymmetrical folds Asymmetrical folds

Overturned foldsOverturned folds

Types of folds

Symmetrical Asymmetrical Overturned

Recumbent Folds- Fold axis is horizontal

Based on degree of compression of beds

Open folds

Closed folds

Miscellaneous foldsMiscellaneous folds

� Monocline

� Homocline

� Chevron folds

Monocline

Chevron folds

Domes are groups of strata centrally uplifted from below and

sloping away in all directions. It always appears as an anticline

Basins are reverse of domes that are centrally depressed andsloping towards a common centre.

Oldest rock in center

Basins

Youngest rock in center

Causes of folding

� Lateral compression

� Intrusion of magma

� Landslides� Creeping of slopes

� Differential compaction

� Isostatic settling� Subsidence due to solution cavities

Lateral compression

Folding due to intrusion of magma

Folding due to landslides

Folding due to differential compaction

Folding due to isostatic settling

Folding due to subsidence

Rocks suitable for studying folds

Rocks suitable for the study of structures

� Folds may occur in all kinds of rocks such asigneous, sedimentary and metamorphic rocks!

� But all rocks are not suitable for studying structures

� Sedimentary rocks are ideal for study of structures

as they are characterised by the presence of beddings

� Igneous rocks are not suitable for structures as they

bear uniform textural behaviour

� Metamorphic rocks such as gneisses and schistsalso permits study of structures.

A common misconception is that rocks

can only fold when they are nearly

molten. These outcrops tell a different

story. Rocks like these limestones can

fold even at the Earth's surface -

provided they are given enough time. A

fold like this could take a hundred

thousand years to grow.

How Is Rock Deformed?How Is Rock Deformed?

� Tectonics forces continuously squeeze, stretch,

bend, and break rock in the lithosphere.

StressStress

� Uniform stress is a condition in which the

stress is equal in all directions.

± In rocks it is also confining stress because any

body of rock in the lithosphere is confined by the

rock around it.

� Differential stress is stress that is not equal in

all directions.

Differential Stress

� The three kinds of differential stress are:

± Tensional stress, which stretches rocks.

± Compressional stress, which squeezes them.

± Shear stress, which causes slippage and

translation.

Stages of Deformation

� Strain describes the deformation of a rock.

� When a rock is subjected to increasing stress, it

passes through three stages of deformation in

succession: ± Elastic deformation is a reversible change in the volume

or shape of a stressed rock..

± Ductile deformation is an irreversible change in shape

and/or volume of a rock that has been stressed beyond theelastic limit.

± Fracture occurs in a solid when the limits of both elastic

and ductile deformation are exceeded.

Ductile Deformation Versus

Fracture

� A brittle substance tends to deform by fracture.

� A ductile substance deforms by a change of

shape.� The higher the temperature, the more ductile

and less brittle a solid becomes.

� Rocks are brittle at the Earth¶s surface, but at

depth, where temperatures are high because of

the geothermal gradient, rocks become ductile.

Confining Stress

� Confining stress is a uniform squeezing of

rock owing to the weight of all of the

overlying strata.

� High confining stress hinders the formation of

fractures and so reduces brittle properties.

� Reduction of brittleness by high confining

stress is a second reason why solid rock can be

bent and folded by ductile deformation.

Ductile Brittle

Marble sample:

Strain Rate

� The term used for time-dependent deformation of

a rock is strain rate.

± Strain rate is the rate at which a rock is forced to

change its shape or volume.

� Strain rates in the Earth are about 10-14 to 10-15/s.

� The lower the strain rate, the greater the tendency

for ductile deformation to occur.

Enhancing Ductility

� High temperatures, high confining stress, and

low strain rates (characteristic of the deeper

crust and mantle):

± Reduce brittle properties.

± Enhance the ductile properties of rock.

Composition Affects Ductility (1)

� The composition of a rock has pronounced effects on

its properties.

± Quartz, garnet, and olivine are very brittle.

± Mica, clay, calcite,and gypsum are ductile.

� The presence of water in a rock reduces brittleness

and enhances ductile properties.

� Water affects properties by weakening the chemical

bonds in minerals and by forming films around

minerals grains.

ENGINEERING CONSIDERATIONSENGINEERING CONSIDERATIONS

� Change in attitude: Due to folding, same layer may be

repeated or a different layer may be encountered. If it

happens unexpectedly and the encountered layers are of

undesirable nature, the safety of the structure will be

adversely affected.

� Shattering of rocks: Folding is the response of the rocks

to the induced stresses. These stresses are often strong

enough to break or shatter or develop cracks at the

points of maximum concentration. The shattered rocksare weak in strength and pervious in character. Such

rocks cannot be trusted as roofs and floors of tunnels or

as foundations in dams. If it is not possible to avoid,

proper treatments has to be given before construction

ENGINEERING CONSIDERATIONSENGINEERING CONSIDERATIONS

� Strained nature: The stresses that have acted on the

rocks during their folding are generally absorbed by

undergoing strain. When disturbed, these rocks release

energy leading to rock bursts during tunneling or

excavations.

Attitude of beds Attitude of beds

� Attitude refers to the three-dimensional

orientation or positioning of a given

geological structures. The attitude of a

planar geological feature like a rock bed or

a joint or a fold is defined by their strike

and dip.

Dip and StrikeDip and Strike

� The dip is the angle in degrees between a

horizontal plane and the inclined plane,

measured down from horizontal.

� The strike is the compass direction of the

horizontal line formed by the intersection of a

horizontal plane and an inclined plane.

Faults are fractures

in the Earth's crust

along which one

side moves with

respect to the

other. There are

many types of faults ranging in

size from a few

tens of meters to

hundreds of

kilometers indimension.

Aerial view of the Pu`u Kapukapu fault scarp

Mount St. Helens, Washington

Scientists measure the distance between

two benchmarks spanning the fault scarp of

a thrust fault on the crater floor of Mount St.

Helens. This scarp developed on the crater floor in 1981 as magma rose into the lava

dome (backgound) before erupting onto its

surface. The pressure exerted by the rising

magma against rocks surrounding the

conduit caused the crater floor to fracture

along a plane gently inclined toward thedome.

Material above the fault (person kneeling on

upper surface) was pushed over material

below the fault (person in lower right).

Scientists measured an increasing rate of

movement of this thrust fault before twoeruptions in June and October 1981, which

helped them to predict both eruptions

accurately.

One of the world's most famous faultsknown is the San Andreas Fault in

California. It stretches 1000 km from

the Imperial Valley in southern

California, to theP

oint Arena on thenorthern coast. The fault line is also

9km in depth. It marks the boundary

between the North America and the

Pacific tectonic plates. San Andreas is

known as a strike slip fault; it hasdisplaced rocks for hundreds of miles.

Pre-earthquake view of Halape

Post-earthquake view of Halape

To describe the displacement along a dip-slip

fault, we use nomenclature that arose from

miners who excavated shafts along fault zones

The rock surface

above the mineralized

fault zone is called thehanging wall (Note the

lantern is hung on the

hanging wall)

The rock surface below

the mineralized fault zoneis the foot wall (Note the

miner is standing on the

foot wall)

Faults in which the movement is primarily

parallel to the dip (or inclination) of the fault

surface are called dip-slip f aults.

Types of f ault s

Dip-slip faults are classified as normal f aults when the hanging

wall block moves down relative to the footwall block.

Most normal faults are small, having

displacements of only a meter or so

Reverse f aults are dip-slip faults in which the hanging

wall block moves up relative to the footwall block

Strike-slip f aults exhibit mainly horizontal

displacement parallel to the strike of the fault surface

± Strike-slip faults arise from

shear stresses.

± The San Andreas is a

strike-slip fault.

± Apparently, movement

(more than 600 km) has been occurring along it for

at least 65 million years.

Fractures along which no appreciable

displacement has occurred are called joints

Columnar joints form when igneous rocks cool and develop

shrinkage fractures that produce elongated, pillar like columns

E f li ti j i t

Exfoliation joints

Gr o undwat e r

What is ground water and how does it move?

What is the importance of groundwater?

What are the advantages of groundwater?

What are aquifers and their types?

What is groundwater prospecting?

What are some environmental problems with groundwater?

What are the geological work of groundwater?

How is karst topography produced?

Advantages of Gr ound Water

It has a suitable composition in most cases and is free from turbidity,objectionable colours and pathogenic organisms requiring not much

treatment.

It is relatively much safe from hazards of chemical, radiogenic and

biological pollution to which surface water bodies are badly exposed.

Its supplies are not quickly affected by drought and other climatic

changes and hence are more dependable compared to surface

waters.

Being available locally in many cases may be tapped and

distributed at much lesser costs using very little network of pipes. In

fact, in many areas it is directly pumped up by the users.

S f d t

Sources of groundwater

eteoric water±It is the water derived mainly from rain.The water directly infiltrated or through streams (influent

streams) finally joins the groundwater.

� Connate water± This is the water that is present right

from the deposition of sediment at the time of formation

of sedimentary rocks

� Juvenile water± Water condensed from steam emanated

from molten magma.

Hydrological cycle

Zones of groundwater

W t t bl

Water table

Infl ent and effl ent streams

Influent streams Effluent streams

Influent and effluent streams

Aquifers and wells

� A well is a manmade hole in the ground

from which water can be withdrawn

� Aquifer is a formation that is capable to

yield appreciable quantity of water by

gravity

Types of aquifers

� Unconfined aquifer

� Confined aquifer

erched aquifer

Unconfined aquifer

Confined or Artesian aquifers

Figure 17.16

Perched Aquifer and Perched Water table.

Perched aquifer

CONE OF DEPRESSION

Yield of wells

Flow to a well penetrating an unconfined aquifer

Spring

Spring formed due to the presence

of a fault across a water bearing

formation.

Artesian spring formed when a pervious layer

is sandwiched between two impervious layers.

Spring formed due to slope in a valleySpring formed due to the presence

of an impervious obstruction against

an aquifer or water-table.

Example of land

subsidence due to

groundwater lowering

Caverns and Karst Topography

Figure 17.1

Fig. 20 Speleothems Fig. 21 Soda straw stalactite

Fig. 23 Curtain StalagtitesFig. 22P

opcorn Stalactites

Figure 17.24

Fig. 24 Infrared image showing an area of

karst topography

Sink hole formed due to collapse of cavern

Geysers occur where extensive underground chambers exist within hot

igneous rocks.

This geyser emits 45,000 litres of hot water and steam per hour!

structural geology groundwater

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