Structural Geology
In this lesson, we focus on the changes in bedrock caused by powerful forces originating deep within Earth and how rocks respond to these tectonic forces.
The main purpose of this lesson is to help you recognize certain geologic structures, understand the forces that caused them, and thus determine the geologic history of an area.
Some principles discussed in chapter 8 should help you interpret the way structures develop in an area and the sequence. Recognition of unconformities as well as the principles of original horizontality, superposition, and cross-cutting relationships are as important to structural geology as they are to determining relative time.
Subsequent chapters will require an understanding and knowledge of structural geology as presented in this chapter. To understand earthquakes, for instance, one must know about faults. Appreciating how major mountain belts and the continents have evolved calls for a comprehension of faulting and folding. Understanding plate tectonic theory as a whole (chapter 19) also requires a knowledge of structural geology. (Plate tectonic theory developed primarily to explain certain structural features.) In areas of active tectonics, the location of geologic structures is important in the selection of safe sites for schools, hospitals, dams, bridges, and nuclear power facilities.
Key points
The definition of structural geology?
The definition of stress and strain?
Different types of Responses of rocks to stress (Elastic, Ductile, Brittle)?
Fold and types of folds?
Fracture? Type of fracture? Different between joint and fault?
Fault and types of faults?
to be continued
Questions
What are types of stress?
What is definition of strike and dip
What are two main types of rock behaviors
What is Anticline and what is syncline? Describe them about limbs, dipping, oder of rock layer.
What is an plunging anticline, the fold axis plunges:
What is a structural basin?
What is the definition of a normal fault/ reverse fault, strike-slip fault, oblique fault, thrust fault?
How is a fault formed?
How is a fold formed?
What are two types of fold ?
Technical terms
angle of dip
A vertical angle measured downward from the horizontal plane to an inclined plane.
anticline
An arched fold in which the rock layers usually dip away from the axis of the fold.
axial plane
A plane containing all of the hinge lines of a fold.
brittle
Cracking or rupturing of a body under stress.
compressive stress
A stress due to a force pushing together on a body.
dip-slip fault
A fault in which movement is parallel to the dip of the fault surface.
direction of dip
The compass direction in which the angle of dip is measured.
ductile
Capable of being molded and bent under stress.
elastic limit
The maximum amount of stress that can be applied to a body before it deforms in a permanent way by bending or breaking.
elastic limit
The maximum amount of stress that can be applied to a body before it deforms in a permanent way by bending or breaking.
fault
A fracture in bedrock along which movement has taken place.
fold
Bend in layered bedrock.
footwall
The underlying surface of an inclined fault plane.
geologic cross section
A representation of a portion of Earth in a vertical plane.
geologic map
A map representing the geology of a given area.
hanging wall
The overlying surface of an inclined fault plane.
hinge line
Line about which a fold appears to be hinged. Line of maximum curvature of a folded surface.
isoclinal fold
A fold in which the limbs are parallel to one another.
joint
A fracture or crack in bedrock along which essentially no displacement has occurred.
joint set
Joints oriented in one direction approximately parallel to one another.
left-lateral fault
A strike-slip fault in which the block seen across the fault appears displaced to the left.
limb
Portion of a fold shared by an anticline and a syncline.
normal fault
A fault in which the hanging-wall block moved down relative to the footwall block.
oblique-slip fault
A fault with both strike-slip and dip-slip components.
open fold
A fold with gently dipping limbs.
overturned fold
A fold in which both limbs dip in the same direction.
plunging fold
A fold in which the hinge line (or axis) is not horizontal.
recumbent fold
A fold overturned to such an extent that the limbs are essentially horizontal.
reservoir rock
A rock that is sufficiently porous and permeable to store and transmit petroleum.
reverse fault
A fault in which the hanging-wall block moved up relative to the footwall block.
right-lateral fault
A strike-slip fault in which the block seen across the fault appears displaced to the right.
shear stress
Stress due to forces that tend to cause movement or strain parallel to the direction of the forces.
source rock
A rock containing organic matter that is converted to petroleum by burial and other postdepositional changes.
strain
Change in size (volume) or shape of a body (or rock unit) in response to stress.
stress
A force acting on a body, or rock unit, that tends to change the size or shape of that body, or rock unit. Force per unit area within a body.
strike
The compass direction of a line formed by the intersection of an inclined plane (such as a bedding plane) with a horizontal plane.
strike-slip fault
A fault in which movement is parallel to the strike of the fault surface.
structural basin
A structure in which the beds dip toward a central point.
structural dome
A structure in which beds dip away from a central point.
structural geology
The branch of geology concerned with the internal structure of bedrock and the shapes, arrangement, and interrelationships of rock units.
syncline
A fold in which the layered rock usually dips toward an axis.
tensional stress
A stress due to a force pulling away on a body.
thrust fault
A reverse fault in which the dip of the fault plane is at a low angle to horizontal.
Extra readings
1. Tectonic forces move and deform parts of the earth's crust. Stress is force applied to an object, while strain is a change in size and shape or both, while an object is undergoing stress. Compressional stress produces shortening strain, tensional stress produces stretching or extensional strain, and shear stress produces shear strain: parallel movement in opposite directions.
2. A body responding with elastic strain recovers its original shape. A body responding with plastic strain does not return to its original shape, while brittle strain produces fractures. Sedimentary rocks at the earth's surface are brittle.
3. A geologic map depicts rock types and structures, and a cross section is a vertical representation of a portion of the earth. Strike is the compass direction of the line formed by the intersection of an inclined bedding plane with a horizontal plane. Dip is the angle formed by the bed and a horizontal plane, and it is always measured perpendicular to strike. Strike and dip are measured in the field with a Brunton pocket transit. Horizontal and vertical beds have special symbols.
4. Folds are bends in layered rock produced by plastic strain. The axial plane (visualized as connecting the hinge lines formed by the bending of each bed in the fold) divides the fold into limbs. An anticline is an arch in which the beds dip away from the hinge line, while a syncline is a trough in which the beds dip toward the hinge line. Anticlines have their oldest beds exposed along the hinge line, while synclines have their youngest beds exposed along their hinge line.
5. Plunging folds have hinge lines that dip and produce V-shaped patterns of exposed strata.
6. Structural domes (doubly plunging anticlines) have beds that dip away from a central point, while structural basins (doubly plunging synclines) have beds that dip toward a central point.
7. Open folds have limbs with gentle dips; Isoclinal folds have limbs parallel to one another. Overturned folds have limbs that dip in the same direction. Recumbent folds have limbs that are essentially horizontal.
8. Brittle strain produces fractures in rocks called joints, if no displacement occurs. Columnar jointing and sheet jointing were mentioned in earlier chapters. Compression produces multiple joint sets.
9. Faults are fractures along which displacement occurs. The fault surface or plane separates the two sides of the fault into a hanging wall (above the fault plane) and a footwall (below the fault plane).
10. Dip-slip faults exhibit movement parallel to the dip of the fault plane. Normal faults have a hanging wall that moved down in response to tensional stress. Blocks bounded by normal faults produce grabens and rifts, if dropped down, or horsts, if raised up. Reverse faults are dip-slip faults that have a hanging wall that moved up in response to compressional stress. A thrust fault is a reverse fault with a low angle fault plane. Strike-slip faults are associated with shearing and have no vertical displacement. Left-lateral and right-lateral movement is determined by looking at displacement across the fault plane.