Terms in this set (64)
Tectonic forces at work: Tectonic forces deform parts of the lithosphere, particularly along plate margins.
Deformation may cause a change in orientation, location, and shape of a rock body.
When studying deformed rocks, structural geologists typically refer to stress(a force per unit).
Where stress can be measured, its expressed as the force per unit area at a particular point. It is difficult to measure stress in rocks that are buried.
We can observe the effects of past stress(caused by tectonic forces confining pressure from burial) when rock bodies are exposed after uplight and erosion.
We can observe in exposed rocks the effects of forces on a rock that was stressed. Strain is the change in shape or size(volume), or both, in response to stress.
The relationship between stress and strain can be illustrated by deforming a piece of silly putty.
If the silly putty is pushed together or squeezed from opposite directions, the stress is compressive. Compressive stress results in rocks being shortened or flattened.
An elongate piece of silly putty may shorten be bending, or folding, wheres a ball of silly putty will flatten by shortening in the direction parallel to the compressive stress and elongating or stretching in the direction perpendicular to it.
Rocks that have been shortened or flattened are typically found along convergent plate boundaries(where rocks have been pushed or shoved together).
A tensional stress is caused by forces pulling away from one another in opposite directions. Tensional stress results in a stretching or extension of material.
If we apply a tensional stress on a ball of silly putty, it will elongate or stretch parallel to the applied stress. If the tensional stress is applied rapidly, the silly putty will first stretch and then break apart. At divergent plate boundaries, the lithosphere is undergoing extension as the plates move away from one another. Because rocks are very weak when pulled apart, fractures and faults are common structures.
When stresses act parallel to a plane, shear stress is produced.
Its much like putting a deck of cards in your hands and shearing the deck by moving your hands in opposite directions.
A shear stress results in a shear strain parallel to the direction of the stressed. Shear stresses occur along actively moving faults.
How do rocks behave when stressed?
Rocks behave as elastic, ductile, or brittle materials, depending on the amount and rate of stress applied, the type of rock, and the temperature and pressure under which the rock is strained.
If a deformed material recovers its shape after the stress is reduced or removed, the behavior is elastic.
Example: If a tensional stress is applied to a rubber band, it will stretch as long as the stress is applied, but once the stress is removed, the rubber band returns(recovers) to its original shape and its behavior is elastic.
However, once the stress applied exceeds the elastic limit, the rock will deform in a permanent way, just as the rubber band will break if stretched too far.
A rock that behaves in a ductile or plastic manner will bend while under stress and does not return to its original shape after the stress is removed.
Silly Putty behaves as a ductile material unless he rate of strain is rapid.
Rocks exposed to elevated pressure and temperature during regional metamorphism also behave in a ductile manner and develop a planar texture,foliation, due to the alignment of minerals.
Materials behaving in a ductile manner
A rock exhibiting brittle behavior will fracture at stresses higher than its elastic limit, or once the stresses are greater than the strength of the rock.
Rocks typically exhibit brittle at or near Earths surface, where temperatures and pressure are low. Under these conditions, rocks favor breaking rather than bending. Faults and joints are examples of structures that form by brittle behavior of the crust.
Example: A sedimentary rock exposed at Earth's surface is brittle; it will fracture if you hit it with a hammer. Sedimentary rocks become bent or deformed in a ductile way because either stress is increased very slowly or the rock was deformed under considerable confining pressure(buried under more rock) and higher temperatures.
How do we record and measure geologic structures?
The study of geologic structures is of more than academic interest.
Petroleum and mining industries employ geologist.
Understanding and mapping geologic structures is also important for evaluating problems related to engineering decisions and seismic risk(such as determining the location for building dams, large bridges, nuclear reactors, even houses, schools and hospitals).
Geologic Maps and Field Methods.
-Geologist ordinarily use observations from a number of individual outcrops (exposure of bedrock at the surface) in determining the pattern of geologic structures.
A geologic map, which uses standardized symbols and patterns to represent rock types and geologic structures, is typically produced from the field map for a given area.
On such a map are plotted the type and distribution of rock units, the occurrence of structural features(folds, faults, joints), ore deposits,.
Strike and Dip.
According the the principle of original horizontality, sedimentary rocks and some lava flows and ashfalls are deposited as horizontal beds or strata. Where these originally horizontal rocks are found tilted, it indicates that tilting must have occurred after deposition and lithification.
Someone studying a geologic map would want to know the extent and direction of the tilting. By convention, this is determined by plotting the relationship between a surface of an inclined bed and an imaginary horizontal plant.
Strike, direction of dip, and angle of dip. The line of strike is found where an inclined bed intersects a horizontal plan. The dip direction is always perpendicular to the strike and in the direction the bed slopes. The dip angle is the vertical angle of the inclined bed as measure from the horizontal.
Strike is the compass direct of a line formed by the intersection of an inclined plane with a horizontal plane. (example N50*E)
Angle of dip is measured downward from the horizontal plane to the bedding plane (an inclined plane).
Note that the angle of dip is measure within a vertical plane that is perpendicular to both the bedding and the horizontal planes.
-The dip angle is always measured at a right angle to the strike, that is perpendicular to the strike line.
The direction of dip is the compass direction in which the angle of dip is measured.
If you could roll a ball down a bedding surface, the compass direction in which the ball rolled would be the direction of the dip.
A specifically designed instrument called a Brunton pocket transit is used by geologists for measuring the strike and dip.
-the device contains a compass, a level, a device for measuring angles of inclination.
Geologic Cross Sections- represents a vertical slice through a portion of Earth. It is much lie a road cut or the wall of a quarry in that it shows the orientation of rock units and structures in the vertical dimensions. Geologic cross sections are constructed from geologic maps by projecting the dip of rock units into the subsurface
Folds are bends or wavelike features in layered rock. Folded rock can be compared to several layers of rugs or blankets that have been pushed into a series of arches and troughs. Folds in rocks often can be seen in roads cut or other exposures.
The fact that the rock is folded or bend shows that it behaved as a ductile material. The rock exposed in outcrops is generally brittle and shatters when struck with a hemmer.
The rock is not metamorphosed(most metamorphic rock is intensely folded because its ductile under the high pressure and temperature environment of deep burial and tectonic stresses.
Geometry of folds.
Determining the geometry or shape of folds may have important economic implications(because of the many oil and gas deposits and some metallic mineral deposits are localized in folded rock).
Also important in unraveling how a rock was strained and how it might be related to the movement of tectonic plates. Folds are usually associated with shortening of rock layers along convergent plate boundaries but are also commonly formed where rock has been sheared along a fault.
Because folds are wavelike forms, two basic fold geometries are common- anticlines and synclines.
An anticline is a fold shaped like an arch the oldest rocks in the center of the fold.
Usually the rock layers dip away from the hinge line(axis) of the fold.
The counterpart of an anticline is a syncline, a fold shaped like a trough with the youngest rocks in the center of the fold.
The layered rock usually dips toward the synclines hinge line.
Each anticline and adjacent syncline share a limb.
Each plane is an axial plane, an imaginary plane containing all of the hinge lines of a fold.
The axial plane divided the fold into its two limbs.
Its important to remember the anticlines are not necessarily related to ridges nor synclines to valleys because valleys and ridges are nearly always erosional features.
In an area that has been eroded to a plain, the presence of underling anticlines and synclines is determined by the direction of the dipping beds in exposed bedrock.
The oldest exposed rocks are along the hinge ling of the anticline. THis is because lower layers in the originally flat lying sedimentary or volcanic rock were moved upward and are now in the core of the anticline.
The youngest rocks, on the other hand, which were originally in the upper laters, were folded downward and are now exposed along the syncline hinge line.
So far, the folds shown have contained horizontal hinge lines.
In nature, anticlines apt synclines are apt to be plunging folds- folds in which the hinge lines are not horizontal.
On a surface leveled by erosion, the patterns of exposed strata(bees) resemble Vs or horseshoes. Plunging anticlines and synclines are distinguished form one another in the same way as are nonplunging folds- direction of dip or by relative ages of bed.
plunging synclines- contains the youngest rocks in its center or core, and the V or horseshoe pints in the direction opposite of the plunge.
Plunging anticline contains the oldest rocks in its core and the V points in the same direction as the plunge of the fold.
Structural Domes and Structural Basins:
A structural dome
a structure in which the beds dip away from a central point, and the oldest rocks are found in the center or core of the structure. In cross section, a dome resembles an anticline and is sometimes called a doubly plunging anticline.
A structural basin
the beds dip toward a central point, and the youngest rocks are exposed in the center of the structure. In cross sections, it is comparable to a syncline (doubly plunging syncline).
Domes and basins tend to be features on a grand scale( some are more than a hundred kilometers across) formed by uplift somewhat greater(domes) and less(basins) than that of the rest of the region.
folds occur in many varieties and sizes.
Open folds have limbs that dip gently, and the angle between the limb is large. All other factors being equal, the more open the fold, the less it has been strained by shortening.
-Right fold- the angle between the limbs of the fold is small.
-isoclinal fold- limbs are nearly parallel to one another, implies even larger shortening strain or shear strain.
A fold that has a vertical axial plane is referred to as an upright fold. (where the axial plane of a fold is not vertical but is inclined or tipped over, the fold may be classified as asymmetric).
-If the axial plane is incline to such a degree that the fold limb dip in the same direction, the fold is classified as an overturned fold.
Looking at an outcrop where only the overturned limb of a fold is exposed, you would probably conclude that the youngest bed is at the top. THe principles of superposition cannot be applied to determine top and bottom for overturned beds. You must see either the fold or find primary sedimentary structures within the beds such as a mud crack.
Recumbent folds are overtuned to such as extent that the limbs are essentially horizontal. Recumbent folds are found in the cores of mountain ranges such as canadian rockies, alps, himalayas, and record extreme shortening and shearing of the crust typically associated with plate convergence.
Fractures in rocks.
-if a rock is brittle, it will fracture. Commonly, there is some movement or displacement. If essentially no shear displacement occurs, a fracture of crack n bedrock is called a joint.
-If the rock on either side of a fracture moves parallel to the fracture plane, the fracture is a fault.
Most rocks at or near the surface is brittle, so nearly all exposed bedrock is jointed to some extent.
-Columnar jointing, in which hexagonal columns form as the result of the contraction of a cooling, solidifying lava flow
-Sheet jointing, a type of jointing due to expansions is caused by the pressure release due to removal of overlying rocks and has the effect of creating tensional stress perpendicular to the land surface.
Columnar and sheet joints are examples of fracture that form from non tectonic stresses and are therefore referred to as primary joints.
Joints are among the most commonly observed structures in rocks. A joint is a fracture or crack in a rock body along which essentially no displacement has occurred. Joints form at shallow depth in the crust where rocks break in a brittle way and is pulled apart slightly by tensional stresses causes by bending or regional uplift. Where joints are oriented approx parallel to one another, a joint set can be defined.
Faults are fractures in bedrock along which sliding has ten place. The displacement may be only several cm or may involved greater distances. Most faults are no longer active.
the nature of past movement ordinarily can be determined where a fault is exposed in an outcrop. geologist look for dislocated beds or other features of the rock that might show how much displacement has occurred and the relative direction of movement.
-Geologist describe fault movment in terms of direction of slippage: dip-slip fault(movement is parallel, strike-slip fault(indicates horizontal motion parallel to the strike of the fault surface, and oblique-slip fault(has both strike slip and dip slip components).
Dip Slip Faults- the movement is up or down parallel to the dip of the inclined fault surface. The side of the fault above the inclined fault surface is called the hanging wall, whereas the side below the fault is called the footwall.
Normal and reverse faults(most common type of dip slip faults) are distinguished from each other on the basis of the relative moment of the footwall black and the hanging wall block.
In a normal fault, the hanging wall block has moved down relative to the footwall block.
In reverse faults, the hanging wall block has moved up relative to the footwall block. Horizontal compressive stresses cause reverse faults. reverse faults tend to shorten the crust.
- thrust fall is a reverse fault in which the dip of the fault plane is at a low angle (30) or even horizontal. thrust faults move or thrust older rocks on top of younger rocks and result in an extreme shortening of the crust; commonly form at convergent plate boundaries to accommodate the pushing together and shortening during convergence.
Strike Slip Falls is a fault where the movement is predominately horizontal and therefore parallel to the strike of the fault. The displacement along a strike slip fault is either left lateral or right lateral and can be determined by looking across the fault.
If a person looks across the fault and sees the stream displaces to the right(RL).
Large Strike Slip Faults- san andreas fault in cali
tectonic forces result in deformation of the earths crust. Stress is a measure of the tectonic force and confining pressure acting on bedrock. Stress can be compressive, tensional, or shearing. Strained(changed in size or shape) rock records past stresses, usually as joints, faults or rocks.
A geologic map shows the structural characteristics of a region. Strike and dip symbols on geologic maps indicate the orientation of inclined surfaces such as bedding planes. The strike and dip of a bedding surface indicate the relationship between the inclined plane and a horizontal plane.
If rock layers bend(ductile behavior) rather than break,they become folded. Rock layers are folded into anticlines and synclines and recumbent folds. If the hinge line of a fold is not horizontal, the fold is plunging. Older beds exposed in the core of a fold indicate an anticline, whereas younger beds in the enter of the structure indicate a syncline. In places where folded rock has been eroded to a plane, an anticline can usually be distinguished from a syncline by whether the beds dip toward the center(syncline) or away from the center(anticline). Also the older rocks are found in the center of an eroded anticline, whereas the youngest rocks are found int eh center or core of syncline.
Fractures in rocks are either joints or faults. A joint indicates that movement has not occured on either side of the fracture; displaced rock along a fracture indicates a fault. Dip slip faults are either normal or reverse, depending on the motion of the hanging wall block relative to the footwall black. THe relative motion of the hanging wall is upward in a reverse fault and downward in a normal fault. A reverse fault with a low angle of dip for the fault plane is a thrust fault. Reverse faults accommodate horizontal shortening of the crust, whereas normal faults accommodate horizontal stretching or extensions.
In a strike slip fault, which can be either left lateral or right lateral, horizontal movement parallel to the strike has occured.
Strike slip faults accommodate shearing strain in the brittle, uppermost lithosphere, and may alos represent transform plate boundaries where plates slide past one another. Of the most famous examples of a transform fault is the san Andrea's fault. The san Andrea's fault is a right lateral strike slip fault that forms part of the boundary between the North American and Pacific plates.
True or False.
When forces are applied to an object, the object is under stress.
Strain is the change in shape or size, volume, or both while an object is undergoing stress.
stresses can be compressive, tensional, or shear.
The compass direction of aline formed by the intersection of an inclined plane with a horizontal plane is called
folds in a rock show that the rock behaved in what way
an anticline is what
a fold shaped like an arch with the oldest rocks exposed in the center of the fold.
a syncline is what
a trough shaped fold with the youngest rocks exposed in the center of the fold
a structure in which the beds dip away from a central point and the oldest are exposed in the center is a what?
Which is not a type of fold
open, isoclinal, overturned,recumbent, thrust
fractures in bedrock along which movements has taken place are called
in a normal fault, the hanging wall block has moved ____ relative to the footwall block
normal faults occur where
there is horizontal extensions
faults that typically move older rock on top of younger rock are
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