Geologic Structures

Strain

a change in shape caused by stress

What produces plunging folds?

a combination of folding and tilting

What is a thrust fault?

a dip-slip fault where the dip of the fault is less then 45 degrees causing the movement of the hanging wall to be nearly horizontal to the fault surface, they are most common in convergent plate margins due to compressive forces and the abundance of folded layers

What is a normal fault?

a dip-slip fault where the hanging wall block moves down relative to the footwall block and they occur when the crust is extended or lenghtened

What is a reverse fault?

a dip-slip fault where the hanging wall block moves up relative to the footwall block and they occur during compressive shortening of the crust

What does plunging fold mean?

a fold that is tilted down into Earth

What is ductile deformation?

a type of solid state flow that produces a change in the size and shape of a rock body without fracturing, it occurs at depths where temperatures and confining pressures are high

Which type of fold has rocks folding up in the middle?

anticline

What are the two main types of folds?

anticline and syncline

What is a rift valley?

are long valleys that lie between two geological faults, they form when the intermediate land subsides as the faults on either side are pulled apart as a result of tensional forces

What are the three types of deformation that rocks can experience?

brittle deformation is dominant in the shallow crust; ductile deformatin in the deep crust and elastic deformation. Temperatures are lower near to the surface making the rock "cold" and more likely to shatter

What are joints?

brittle deformations in a rock along which there has been no movement

What is the first reaction rocks have to the stress?

by deforming elastically

Which type of force is responsible for reverse fault formation?

compressional force

What are the three types of differential stress?

compressional, tensional and shear

What are the three categories of faults?

compressional, tensional and translational

What are the three types of differential stress?

compressional, tensional, shear

During deformation of sedimentary rocks, which type of stress produces folds?

compressional, when compressional stress is exerted on rocks at depth they can be bent (ductile deformation) and folds are formed

Which type of stress on rock is uniform in all directions?

confining pressure, in situations such as deep burial, confining pressure exerts uniform stress in all directions

In which tectonic setting would you expect to observe over-thrust faults?

convergent boundaries. Overthrust faults involve both folds and large-scale thrust faults, compressional stress is exerted on rocks at convergent boundaries, reverse, thrust and overthrust faults are the common types associated with this type of boundary

Which type of stress produces most crustal deformations?

differential stress, the influence of stress in a single direction via tension, compression or shearing produces the most deformation of all crustal rocks

Which tectonic setting are folded rocks UNCOMMON?

divergent boundaries, because rocks at divergent boundaries experience tensional stresses, thus the likelihood of folds forming there is low

Folds exhibit which type of deformation?

ductile

What are folds?

ductile deformation where a bent rock layer or series of layers that were originally horizontal are subsequently deformed

What are the three types of deformation that a rock can receive due to stress?

elastic, ductile and brittle

What are dip-slip faults?

faults in which the movement is parallel to the dip (or slope) of the fault

Name four rock structures

fold, fracture, fault, joint

What are the different geological structures that can be formed as a result of deformation?

folds, faults and joints

What are rocks below and above a fault called?

footwall below and hanging wall above

Tensional stress

forces pull a body of rock outward, resulting in elongation of the body, along divergent plate boundaries where plates are moving apart tensional stresses stretch and lengthen rock bodies

Compressional stress

forces push a body of rock inward, resulting in compression of the body, in convergent plate boundaries, when plates collide the crust is usually shortened horizontally and thickened veritcally

Faults

form where brittle deformation leads to fracturing and dis placement of Earth's crust

What are faults?

fractures in the brittle upper crust where significant displacement has taken place due to rock movement

What is the orientation of a fold's hinge with respect to the orientation of plunge?

hinge line is in the direction of plunge

What is a hinge line of a fold?

line of maximum inflection that layers wrap around

Dip slip faults

movement is usually parallel to the inclination (also called dip) of the fault surface

If a fold has bee eroded to a flat surface, in general how would you know whether this fold is plunging?

non-plunging folds look like a straight line at the surface, and plunging folds look like wavy lines

Which fault would cause the hanging wall to move down relative to the foot wall?

normal fault, these are caused by tension which will pull the landscape apart

What are the three main types of dip-slip faults?

normal, reverse, thrust

Shear stress

opposing forces push different ends of the body of rock in opposite directions resulting in displacement of one end of the body with respect to the other, at transform fault boundaries shear stress causes large segments of Earth's crust to slip horizontally past one another

Which term is used to describe a fold which one limb has been tilted beyond the vertical?

overturned, when compressional stress is applied to rocks and one of the limbs is pushed far beyond the vertical it results in the formation of an overturned fold

What is a reverse fault?

produced by compressional forces and results in the shortening of a body of crust

What is a normal fault?

produced by tensional pressure and results in elongation of a body of crust

What is the definition of deformation?

refers to the change in the original shape, size or volume of a rock

If a syncline has been eroded to a flat surface, how would the rock age change as you walked along the flat surface?

rocks would be oldest on the edges and youngest in the middle

If an anticline has been eroded to a flat surface, how would the rock age change as you walked across the flat surface?

rocks would be youngest on the edges and oldest in the middle

Which type of force is responsible for normal strike-slip formation?

shear force

Which type of stress would you expect to find at a transform boundary?

shear, where two plates are sliding horizontally past each other at a transform boundary you would observe shear stress being exerted on the rocks because there is no vertical motion of the plates, the crustal thickness is unchanged

Which type of fault has NO vertical motion of rocks associated with it?

strike-slip fault

Which type of fold has rocks folding down in the middle, like a U shape?

syncline

Which tectonic stress will result in lengthening of the crust?

tension, tensional stress pulls things apart

Which type of stress would you expect to find at a divergent boundary?

tension, where tow plates are moving away from each other at a divergent boundary, you would observe tensional stress being exerted on the rocks, this would result in relatively thinner crust along the boundary

Which type of force is responsible to normal fault formation?

tensional force

Stress

the force that deforms rock

In a normal dip-slip fault describe the movement of the hanging wall relative to the footwall?

the hanging wall moves down relative to the footwall

What causes reverse and thrust faults?

the result of compressional forces that move towards each other, these forces push the blocks together and force the hanging wall over as the crust is shortened

What causes normal faults?

the result of extensional forces that move in opposite directions, lengthening the crust and causing the hanging wall to move downward into the space created by extension

What happens when stress exceeds a plastic rock's yield strength?

the rock will experience flow

What happens when stress exceeds a brittle rock's yield strength?

the rock will experience fracture

What happens after a rocks elastic limit is suppressed?

the rock will then either deform by ductile deformation or brittle deformation

How will the orientation of a plunging anticline's limbs change in the direction the fold is plunging?

they will close to a point

What is brittle deformation?

this involves the fracturing of rock and is associated with rocks near the surface

Where so valleys tend to form in a landscape?

valleys form where rock layers are essentially erodible (soft)

What is elastic deformation?

when stress is applied gradually, rocks initially respond by deforming elastically, when the stress is removed the rock will return to nearly its original shape and size

Differential stress

when stress is applied unequally in different directions

Confining pressure

when stress is applied uniformly in all directions

What is a syncline?

where the rocks are curved downward into a trough

What is an anticline?

where the rocks create an upward arch

How will rocks respond to compressional stress?

with folding and reverse faulting