Geometry, description, and properties of rocks Flashcards
The branch of geology that deals with:
* Form, arrangement and internal architecture of rocks
* Description, representation, and analysis of structures from the small to moderate scale
* Reconstruction of the motions of rocks
Structural Geology
____________ provides information about the conditions during regional deformation using structures
Structural geology
- Aims at unraveling the geological context in which deformation occurs.
- Study of the origin and geologic evolution (history of motion and deformation) of large areas (regional to global) of the Earth’s lithosphere (e.g., origin of continents; building of mountain belts; formation of ocean floor)
- this operates at scales ranging from 100 m to 1000 km, and focusses on processes such as continental rifting and basins formation, subduction, collisional processes and mountain building processes etc.
Tectonics
- Both are concerned with the reconstruction of the motions that shape the outer layers of earth
- Both deal with motion and deformation in the Earth’s crust and upper mantle
- Tectonic events at all scales produce deformation structures
- These two disciplines are closely related and interdependent
Tectonics vs. Structural Geology
________ is the first stage to any regional geophysical and geochemical surveys aiming at identifying new mineralized provinces. At the mine camp scale, structural geology guide the mining process.
Structural geology
- ____________ is at the core of geotechnical site assessment for bridges, dams, tunnels, nuclear reactors, waste disposals etc. Because of the obvious relationship between faults and earthquake , structural geology is that core of earthquake prevention and earthquake seismology.
- No geological, geochemical or geophysical study can be done without the input of this.
Structural geology
Main Principle and Concepts
- Original Horizontality
- Uniformitarianism
- Superposition
- Law of Crosscutting Relationships
- was proposed by the Danish geological pioneer Nicholas Steno (1638–1686).
- This principle states that layers of sediment are originally deposited horizontally under the action of gravity.
Original Horizontality
- The principle is important to the analysis of folded and tilted.
- From these observations is derived the conclusion that the Earth has not been static and that great forces have been at work over long periods of time, further leading to the conclusions of the science of plate tectonics; that movement and collisions of large plates of the Earth’s crust is the cause of folded strata.
Original Horizontality
- This concept, outlined in the very earliest years of geology by James Hutton is the fundamental to the subject.
- It maintains that the process which are occurring presently are the same as those which operated in the past, and that the results of these processes are the same.
Uniformitarianism
- The earliest of all geological concepts and states that in a series of rock strata, the upper members of the series were formed after the lower members.
- According to the Law of ________, layer 1 was the first layer deposited, and thus the oldest layer. The last layer deposited was layer 12, and thus it is the youngest layer.
Superposition
- Movements of Earth’s crust can lift up rock layers that were buried and expose them to erosion. Then, if sediments are deposited, new rock layers form in place of the eroded layers. The missing rock layers create a break in the geologic record in the same way that pages missing from a book create a break in a story.
- A break in the geologic record is called an ________. This shows that deposition stopped for a period of time, and rock may have been removed by erosion before deposition resumed.
unconformity
There are three types of unconformities.
- nonconformity.
- angular unconformity .
- disconformity
An unconformity in which stratified (layers) of rock rests upon unstratified rock is called a
nonconformity
The boundary between a set of tilted layers and a set of horizontal layers is called an .
angular unconformity
The boundary between horizontal layers of old sedimentary rock and younger, overlying layers that are deposited on an eroded surface is called a
disconformity
According to the Law of Superposition, all rocks beneath an unconformity are ____ than the rocks above the unconformity.
older
- When rock layers have been disturbed by faults (a break or crack in Earth’s crust) or intrusions (a mass of igneous rock that forms when magma is injected into rock and then cools and solidifies), determining relative age may be difficult. In such cases, scientists may apply this.
- is that a fault or intrusion is always younger than all the rocks it cuts through above and below the unconformity.
The Law of Crosscutting Relationships
________ is study of shapes, arrangements and relationships among rocks and stresses that deform them
structural geology
– is the force applied to a plane divided by the area of the plane and it is what causes rocks to deform
Stress
3 Types of Stress
- Compression:pushed together (shortening or flattening)
- Tension:pulled apart (stretching or elongation)
- Shear stress:moved horizontally past each other (smearing)
– the result of stress applied to a body, causing deformation of its shape and/or a change in volume
Strain
When this volume of rock, or some part of it, is forced to change its location or position, it undergoes ____________;
translation
When this volume of rock, or some part of it, is forced to change its orientation, ____________;
rotation
When this volume of rock, or some part of it, is forced to change size, ________;
dilation
When this volume of rock, or some part of it, is forced to change shape, __________
distortion
Types of Rock Deformation (4)
- translation
- rotation
- distortion
- dilation
____________ of rocks is rather easy to recognize, analogous to hitting concrete with sledge hammer. Conditions of stress result in fracturing or rupturing of rocks
brittle deformation
____________is applied slowly under constant pressure, rocks return to original size and shape after stress is removed.
elastic stress
a set of conditions must be met before rocks will deform plastically relative heat, constant pressure, and time.
Plastic/ductile deformation:
- When rocks undergo permanent plastic/ductile deformation, a ________ develops.
- When rock undergo brittle deformation, a ________ develops.
FOLD; FAULT
Factors that affect deformation of rock (4)
- Lithostatic Pressure - weight of overlying rock
- Heat - causes atomic bonds to weaken
- Time - allows stress to be applied slowly or quickly
- Composition - controls rock response to stress(minerals)
- Rocks under high temperature and pressure tend to undergo ________________ when subjected to a tectonic stress.
deformation without breaking
(plastic/ductile deformation)
- Rocks under low temperature and pressure tend to ____________ when subjected to stress.
fracture (brittle deformation)
Causes of Rock Displacement/Deformation (4)
- Tectonic Plate Movement
- Subduction
- Volcanic Activity
- Intrusive Igneous Activity
– any of the internally rigid crustal blocks of the lithosphere which moves horizontally
Tectonic plate
– the process of which one crustal blocks descends beneath another such as the descent of Pacific Plate beneath the Andean Plate along Andean Trench
Subduction
“ Primary Structures: structures that are produced during formation of rock body.” (4)
- Depositional contact
- Unconformable contact
- Cross bedding
- Vesicles in basalt
“Secondary Structures: structures produced after the rock body that they affect.(4)
– Fault contacts
– Folds
– Joints and shear fractures
– Tectonite fabric (cleavage, foliation and/or lineation)
- Are produced when rocks break or bend due to applied stresses within the earth.
- Are dynamically-produced patterns or arrangements of rock or sediment that result from, and give information about, forces within the Earth.
*
Geologic Structure
Types of Geologic Structure (3)
- Folds
- Faults
- Joints
– permanent wavelike deformations in layered rock or sediments
Folds
– ** a fracture in bedrock** along which rocks on one side have moved relative to other side
Faults
– a fracture on a rock without noticeable movement/displacement
Joints
Importance of the Study of Geologic Structure
* In engineering geology , structural geology which is concerned with the **physical and mechanical properties of natural rocks. **
* Structural fabrics and defects such as faults, folds, joints, etc. are internal weaknesses of rocks which may affect ________________
stability of human engineered structure like dams, road cuts,
Importance of the Study of Geologic Structure
To the environmental geologist, hydrogeologist and hydrologist, structure sites of groundwater flow and penetration which may affect for instance,
seepage of toxic substances from waste dumps or seepage of salty water into quifers.
- Formed when beds or fabric are deformed into curved or bent geometries on virtually any scale.
- they are wavelike bends in layered rock . It represents rock strained in aductilemanner, usually under compression . this process results in shortening and thickening of crust.
Folds
Parts of a Fold (5)
- Axial Plane
- Hinge Line or Axis
- Limbs or Flanks
- Trough
- Crest
(part of the fold)
that **divides it into its two limbs. **
Axial Plane
(part of the fold)
* This Is the line formed by the intersection of the axial plane with folded surface.
* This is where folds curve.
Hinge Line or Axis
(part of the fold)
The sides of a fold or the outer parts of the fold as it moves away from the axis of the fold
Limbs or Flanks
(part of the fold)
Is the line occupying the lowest part of the fold, or more precisely the line connecting the lowest part of the same bed
Trough
(part of the fold)
Is the line occupying the highest part of the fold, or more precisely the line connecting the highest part of the same bed
Crest
Type of Folds (3)
Anticline, syncline, monoline
Upfolded forms, results in older rocks becoming enclosed within younger strata
Type of Fold
Anticline
downfolded forms, results in younger rocks becoming enclosed within older strata.
Type of Fold
Syncline
Is a bend in the strata resulting in a local steepening in the dip of the strata which is almost flat lying on both sides away from the bend – there is only one direction of dip
Type of fold
monocline
Specific Type of Anticlines and Synclines (6)
- Open/ Symmetrical
- Isoclinal
- Overturned
- Asymmetrical
- Recumbent
- Plunging
both limbs of the fold dipping at same angle away from fold axis
Specific type of anticline and syncline
OPEN/SYMMETRICAL
both limbs of any fold are parallel to each other, regardless of the dip of the axial plane
specific type of anticline and syncline
ISOCLINAL
one limb of fold has been tilted beyond vertical; axial plane is tilted
specific type of anticline and syncline
OVERTURNED
One limb of the fold dips more steeply than the other
specific type of anticline and syncline
Asymmetrical
limbs dip in same direction; the axial plane is essentially horizontal
specific type of anticline and syncline
RECUMBENT
The tilting of the fold towards the front or back end
specific type of anticline and syncline
PLUNGING
- An up-arched series of strata with beds on all sides dipping away from the center throughout 360 degrees – rocks become progressively younger away from the center of the structure
DOMES
A down-arched series of strata with beds on all sides dipping towards the center throughout 360 degrees – rocks become progressively older away from the center of the structure
BASIN
- Fracture along which no appreciable movements has taken place
- Accommodate stress during tensional and shear stresses associated with crustal movements.
- often occur in very low-stress regimes, with broad, gentle warping of earth’s crust.
- don’t show evidence of slippage whereas faults do.
- are arguably the most common geologic structure and can form in a variety of ways.
Fracture: Joint
Causes of Joints (6)
- Unloading or sheeting effects
- Stresses in cooling magma
- Tectonic stresses causing fracturing essentially contemporaneously with the tectonic activity
- Residual stress due to events that happened long before fracturing
- Contraction due to shrinkage because of cooling or desictcation
- Surficial movemens such as downhill movements of rocks or mountain glaciers
Classification of Joints
- Geometrical
- Genetic
Geochemical Classification
- Strike Joints
- Diagonal Joints
- Bedding Joints
– are those that strike parallel to the strike of the bedding of a sedimentary rock
Geochemical Classification
- Strike Joints
– are those striking in a direction that lies between the strike and direction of the dip of the associated rocks
Geochemical Classification
Diagonal Joints
– are parallel to the bedding of the associated rocks
Geochemical Classification
Bedding Joints
Genetic Classification
- Shear Fracture
- Tension Fracture
– they are **parallel to the axial planes of folds **and forms t right angle to the axis of compression when load is released
genetic classification
Release Joints
– they are perpendicular to the axes of folds resulting from slight elongation parallel to the axes of the folds
genetic classification
Extension Joints
- Is a structure with ** major displacement of rock material along a crack in a rock**
- Are brittle fractures along which sliding has taken place
- Are ruptures along which the opposite walls have past each other. The essential feature is the “Differential Movement” parallel to the surface of the structure
Fracture: Faults
Parts of a Fault (7)
- Hanging wall
- Footwall
- Fault-plane
- Fault line, fault
- strike of the fault
- dip of the fault
- hade
Classification of Fault:
Faults like joints may be classified on the basis of their geometry or their genesis. The bases of five different geometrical classifications are:
- Rake of the net slip
- Attitude of fault relative to the attitude of the adjacent rocks
- The pattern of faults
- The angle at which the fault slip
- The apparent movement of fault
– is the total displacement; it is the distance measured on the fault surface between two formerly adjacent points situated on opposite walls of the fault
Net Slip
Geometric Classification (3)
- Strike-slip fault
- dip-slip fault
- oblique fault
- Is one in which the **net slip is parallel to the strike of the fault. **
- In this fault, movement is primarily horizontal.
geometric classification
Strike-slip Fault
- Is one in which the net slip up or down the dip of the fault.
- in this faults, movement is parallel to the dip of the fault plane and typically shows a strong component of vertical motion.
geometric classification
- Dip-slip Fault
- A fault which has a major dip slip and strike slip component
geometric classification
- Oblique Fault
Genetic Classification: Dip-slip
Normal fault and reverse fault
Genetic Classification: strike-slip fault
left-lateral and right lateral
faults in which crustal block above the fault plane (hanging wall)moves uprelative to crustal block below the fault plane (foot wall).
-this faults develop inresponse to compressional stress.
Reverse Faults
faults in which crustal block above the fault plane (hanging wall)move downrelative to crustal block below the fault plane (foot wall).
-this faults develop inresponse to tensile stress.
Normal Faults
- this fault with **very low angle, or very gently inclined (<45 deg) ** fault plane. Associated with strong, horizontally oriented, compressional stresses.
Reverse Faults: Thrust Fault
are low-angle faults which the net slip is large, generally in miles
Overthrust fault
- movement along fault ishorizontalalong the fault (similar to notion of transform faults in plate tectonics), i.e.offset is parallel to the trendor strike of the fault plane.
- Forms during shearing stress
- are vertical with no hanging wall or footwall
- Neither shortening or extension occurs
- offset surface features such as streams and valleys
Strike-slip faults-
is the block above the fault
Parts of a Fault
Hanging Wall
is the block below the fault
Parts of a Fault
Footwall
is a planar fault surface
Parts of a Fault
Fault Plane
is the intersection of the fault with the surface of the earth
Parts of a Fault
Fault-line, Fault Trace or Fault Outcrop
is the trend of a horizontal line in the plane of the fault
Parts of a Fault
Strike of the fault
is the angle between the horizontal surface and the plane of the fault
Parts of a Fault
Dip of the fault
is a an angle equals 90 deg. Less than the angle of dip. It is also defines as the angle between the fault plane and a vertical plane that strikes parallel to the fault.
Parts of a Fault
Hade
Effects of geologic structures in engineering structures
- Effects of Folding
- Effects of Faulting
- Effects of Jointing
- Effects of Unconformities
Effects of Folding
- Change in attitude
- Shattering of rocks (weak in strength parameters, porous and pervious and nature)
- Strained nature
- Folded rocks are greatly strained. Their removal for tunneling can cause rock explosions.
- Along crests of folds, the rocks are in tension, therefore highly unstable.
- Along the troughs, rocks are highly compressed, hence tough, and offer greater resistance to excavation for tunneling
- Tunneling can be done along the limbs
(Effects of folding)
Location of tunnels:
- Better rock quality available
- Fractures associated with crests and troughs are absent along the limbs
- Seepage problems along the crests and troughs can be avoided
(Effects of folding)
Quarrying should be done along the limbs:
- Folds often furnish excellent conditions to trap groundwater
- Fractures present in folded strata act as channels for groundwater movement
(Effects of folding)
Groundwater occurrence:
- Ground stability depends on the mutual relation of the dip of the beds and the slope of the cutting.
- If the surface slope and the dip are in opposite directions, the ground is stable
- If the surface slope and dip collide, the ground is unstable.
(Effects of folding)
Haul roads along hill slopes
- Suitable cap rocks are also an essential requirement
- Crests of folds offer convenient places for the occurrence of ore deposits
(Effects of folding)
Oil and gas deposits are often associated with anticlines
- Fracture and shattering of rocks along fault zones
- Serves as pathways for water, causes leakage
- Fault zones lubricated with water are potential sites for further movements
- Faults bring together different rocks, hence homogeneity is lost
- Fault zones are highly undesirable for construction of roads, due to the possibility of landslides.
- However, fault zones are often rich in minerals, and favor different processes that eventually lead to mineral formation.
Effects of Faulting
- Sources of weakness in rocks, pathways for seepage of water
- Jointed rocks, lubricated by the presence of water, are susceptible to motion
- Joints are usually treated by grouting
- Only a well-jointed and porous rock can become a good aquifer or an oil and gas reservoir
- Joints and landslides:
*
Effects of Jointing
- Allow for the percolation of water
- Unconformity indicates a break in the sequence of rocks, and hence, their engineering properties
- Exhaustive studies have to be done to analyze the mechanical properties of rocks in the vicinity of an unconformity.
*
Effects of Unconformities
