Faults And Folds Quiz - ProProfs Quiz
Fault: Fault, in geology, a planar or gently curved fracture in the rocks of the Demystified · Quizzes · Galleries · Lists · On This Day · Biographies · Newsletters sides of the fault plane usually is measured in relation to sedimentary strata or other Occasionally, the beds adjacent to the fault plane fold or bend as they resist. An online map of U.S. Quaternary Faults (faults that have been active in the last million years) is available via the Quaternary Fault and Fold Database. Folds are bends in rocks that are due to compressional forces. Folds are most visible in rocks that layered (also known as sedimentary rocks). Folds are formed .
The term antiformal anticline is used for describing an antiformal fold with the oldest rocks in the core.
The term, synfomal syncline is used when synclinal fold has the youngest unit in the core. The term, antiformal syncline is used for describing antiformal folds that have the youngest unit in the core while synformal anticline is used to describe synformal folds with oldest rock unit in the core. The following figure graphically illustrates the typical definition for a limb.
The axial plane or axial surface is at the very top or bottom in which the limbs are on either side of it. The axial plane can be defined as the plane or surface that divides the fold as symmetrically as possible. In other words, it is an imaginary line or surface of symmetry. Axial plane is indicated in green, which cut across the fold. Hinge Line, Point and Zone The hinge line, is defined as the imaginary line that connects points touches the surface of maximum curvature of a fold.
For cylindrical folds, the hinge line and fold axis are the same. The hinge zone is an imaginary plane, as opposed to a line, where maximum curvature of the fold occurs.
Geometry of Folds | Sanuja Senanayake
A hinge plane is a surface of symmetry for cylindrical folds. A hinge point is what you would observe on a cross sectional view of the hinge line on the surface of the fold.
Amplitude, Wavelength and Symmetry Fold geometry can also be categorized by describing the folding pattern using amplitude, wavelength and symmetry. The amplitude can be measured if there is a visible complete anticline-syncline pair.
Otherwise it could be a difficult task since each limb of a fold could extend further down into the subsurface making it difficult to identify the period. The amplitude of a periodic variable is a measure of its change over a single period. But we could use the inflection point on each limb to measure the amplitude.
The inflection is the point on which the slope of the limb changes its direction. Figure 1 above is a diagram of anticline — syncline — anticline labeled with parts of folds. Cylindricity The classification cylindrical and non-cylindrical folds is based on the geometry of the hinge line or fold axis Figure 1.
Folds with straight hinge lines are known as cylindrical folds while the folds with curved hinge lines are known as non-cylindrical folds. The cylindricity varies depend on the stress experienced by the fold. In nature, non-cylindrical folds are very common. However, even cylindrical folds on large scales are often classified as non-cylindrical folds. This is expected since the cylindricity is actually a mathematical model rather than an observational one. It is very difficult to observe perfect symmetry in nature.
In large scale, almost all folds are non-cylindrical very bottom fig. But in small scale, some folds are cylindrical fig a Also on Figure 1, the left anticline is non-cylindrical because the hinge line is curving. The right anticline is cylindrical because the hinge line is straight. Cylindrical verses Conical Cylindrical folds and conical folds have similar axial geometry.
However, conical folds narrow the areas as they move along the hinge.
Types of Folds
In other words, conical folds reduce the structure to a point, just like a cone, when move along perpendicular to the maximum fold surface. Cylindrical folds maintain more or less in nature the cylindrical shape as we move along the fold axis.
When working in the field, d one should not get confused between conical folds and plunging cylindrical folds. Often Geologists make the mistake of classifying plunging cylindrical folds as conical folds.
Additionally, due to the variation in stresses and the variation in material strength, ductility, etc. Kink, Chevron, Concentric and Box folds Symmetry The geometric symmetry has been used by Hudleston to classify folds. It is based on the general shape of the fold. Others have developed several mathematical formulations to describe fold geometry. This is based on several variables such as amplitude, wavelength and period. It should not be used to describe change in orientation of a single layer of the fold.
The axial surface along with the hinge line changes orientation in plunging folds. The following figure shows two 3D models of a plunging anticline and a plunging syncline. Notice the beds plunging steeply into the subsurface. Left is a 3D model of an plunging anticline and the right is a 3D model of a plunging syncline. Fleuty has classified the folds based on this concept as Upright, Plunging; Upright, Vertical; Reclined; Plunging Inclined; Horizontal Inclined and Recumbent Figure 9 based on the orientation of the hinge line.
Fleuty Folds classification. Click on the image to view the original file 1 The Hudleston classification relies on the shape and the amplitude categorization Figure Shape and Amplitude Classification; Hudleston These are just two examples because there are several different classifications in use today. But the primary objective is to classify based on the geometry and the orientation of the folds.Geomorphology - Types & Components of Folds and Faults
While ones view of folds may vary from geologist to geologist, field observations should always be recorded as to what was observed and measured in the field. Depending on whom you talk to, these classifications may be modified. One of the few universal methods employed by geologists is to measure the distance between the two limbs on a stereonet.
This will provide the information needed to determine the tightness of the fold. Dip Isogons Dip isogons are imaginary lines that can be drawn between two points of equal dip within the fold structure. They are straight lines that can be used to connect inner and outer boundaries of folded layers. The orientation of dip isogons can be used to classify folds into three classes. Imaginary lines between fold plains are used for analysis of geometry. Imaginary lines between fold plains is used for analysis of geometry.
The California Fault System shown on a generalized geologic map in California 7. Stress is the force acting on a rock or another solid to deform it, measured in kilograms per square centimeter or pounds per square inch.
Strain is the amount of deformation an object experiences compared to its original size and shape. Rocks, like any solid material, when subjected to a stress will respond with a strain. However, the character of the strain depends on the material strength of the rock.
For instance, a hard rock like granite make take on a large amount of stress without showing any significant deformation, but at at some point with increasing pressure it will shatter fracture catastrophically. On the other hand, shale, a very soft rock, will deform fold significantly before it ruptures as a fault. Crustal Tension On a regional scale, rocks are subjected to stress that may be compressional such as along a convergent plate boundary or tensional such as along a rift valley or a spreading center of a divergent plate boundary.
The faults in the vicinity of these stress forces produce faults and features that can be described as crustal shortening or crustal extension Figure Crustal compression is more likely to form thrust faults and reverse faults associated with crustal shortening, and crustal compression is typically associated with regions where mountain ranges are being pushed up.
In contrast, crustal tension is more likely to form normal faults associated with crustal extension. Continental rifting and associated crustal thinning are associated with crustal extension.
Shearing stress can result in formation of complex fault systems with many moving parts resulting in a variety of fault and fold patterns and associated landscape features discussed below. Compression results in crustal shortening whereas tension results in crustal extension.
Crustal shortening typically results in thrust faults or reverse faults. Crustal extension produces mostly normal faults. Ductile Deformation Rocks near the surface are cold, but the temperature deep down can be extremely hot. Cool rocks near the surface tend to shatter forming joints and faults when they rupture. Deep underground, the weight of overlying material adds confining pressure to hold rocks together, and if hot enough they will deform fluidly rather that fracture if heat and pressure is great enough.
Rocks under high confining pressures and temperatures at depth will bend fold and stretch, whereas cool and brittle rocks closer to the surface will break fracture under increasing pressure. An imaginary plane exists in the lower crust and upper mantle above which rocks will tend to break causing earthquakes but below which they will tend to deform like plastic under pressure.
This hypothetical boundary is called the brittle-ductile transition zone Figure This zone varies significantly with depth and subsurface temperatures from one region of the lithosphere to another, often reflecting plate boundary conditions. The deepest and strongest earthquakes typically occur where cold oceanic crust sinks deep into the asthenosphere along subduction zones. Cold ocean crust remains brittle as it sinks along a subduction zone.
A great amount of pressure builds up on the sinking slab, and where it is still cold and brittle enough to fracture, producing powerful earthquakes. Deeper in subduction zones, the sinking rock heats up and will bend or flow under the greater pressure rather than fracture.
The upper crust behaves in a brittle fashion, fracturing under strain producing earthquakeswhereas at depth rocks deform plastically, folding or flowing under pressure rather than fracturing no earthquakes. Diagram illustrating the epicenter and focus of an earthquake along a fault.
Landforms, Mountains, Folding and Faults quiz 1. What kind of fault
Terms used to describe earthquakes illustrated in Figure An earthquake is ground shaking caused by a sudden movement on a fault or by volcanic disturbance. The focus is the point below the Earth's surface where seismic waves originate during an earthquake.
Not all faults are active or are considered earthquake faults. However, faults can remain dormant for long periods of time and can be reactivated by changing stress patterns in the crust. Below are key terms used to describe earthquake phenomena. An earthquake is the sudden and sometimes violent shaking of the ground as a result of movements within the crust associated with fault rupture or volcanic activity.
Earthquakes are described in terms of magnitude and intensity discussed below. Earthquakes can also be a result of a landslide or an explosion such as a mining or demolition blast, bomb testing, or potentially an asteroid impact, or anything that creates a shockwave. Fault creep is the gradual movement displacement displayed by a fault over time. Fault creep generally keeps pace with regional plate-tectonic related movement in a area.
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- Faults And Folds Quiz
Creep is the aseismic movement of a fault movement without detectable earthquakes. Active earthquake faults can produce both earthquakes and creep. Note that the word creep is also used for the slow movement of soil down a slope. A rupture zone is the area through which fault movement occurred during an earthquake. For large earthquakes, the section of the fault that ruptured may be several hundred miles in length. Note that ruptures associated with earthquakes may or may not extend to the ground surface.
In addition, shockwaves produced by one earthquake fault, may also set off earthquakes on other faults. A P wave P is a compressional wave is a seismic body wave that shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving. A P wave is the first shock wave to arrive from an earthquake at a distant location. A S wave S is a shear wave is a seismic body wave that shakes the ground back and forth perpendicular to the direction the wave is moving.
A seismograph is a device used to record earthquake shaking and are used to determine the distance, magnitude and intensity of earthquakes. Data from numerous seismographs linked together in networks are used to determine the focus, epicenter, extent of rupture, and amount of shaking in a region caused by an earthquake. A minimum of 3 seismographs are needed to determine the epicenter of an earthquake Figure Today there are many thousands of seismographs connected to networks that can record earthquake information from around the world.
A person who studies earthquakes and earthquakes faults is called a seismologist. Because compression P waves travel faster than shear S waves: Using a precision clock and three seismographs, the location of an earthquake epicenter can be precisely located by measuring the arrival times of the first P wave and the first S wave illustrated in Figure Suppose you feel an earthquake's P and S waves arrive 5 second apart.
P waves compression wave and S waves shear waves0 P-waves move faster than S-waves and are first to be felt, the S-waves arrive next and produce the majority of shaking in an earthquake.
At least three seismographs are needed to locate the epicenter of an earthquake. A single seismograph can only tell you how far away an earthquake occurred, but not in which direction a circle. Where three seismograph circles intersect is the location of an epicenter.
An earthquake may be described as a single shock wave or a complex pattern of shock waves taking place over a period of time. Typically in large earthquakes, there is a main event associated with the largest shock wave and significant rupture, but there there may be a series of foreshocks and aftershocks created as the fault rupture propagates through the crust, releasing or dispersing energy.
It is similar to when a rock hits a windshield on a car. Sometime small earthquakes happen before a main earthquake event—these are called foreshocks. More typically, there are often numerous smaller magnitude earthquakes after a main shock event—these are called aftershocks.
Unfortunately, it is impossible to truly define a foreshock before a larger-scale earthquake. Both foreshocks and aftershocks can take place from seconds, minutes, hours, days, weeks, years, and even longer periods associated with a main earthquake event. Can we tell if an earthquake is a foreshock to a larger impending earthquake? The answer is a definite "maybe. The seismic history of the major faults in some regions are well known. Faults, like the San Andreas Fault, are defined as segments based on earthquake history, bedrock geology, and associations with intersecting faults Figure For instance, some segments of the San Andreas Fault have experienced major earthquakes, releasing pent-up pressure on a segment or segments of the fault.
Some segments of the fault are creeping and are not likely to produce great earthquakes as in other segments in theory. Still other segments of the fault are locked up and have not experienced a major earthquake in historic times.
It is these locked up segments that are of significant concern, and any small earthquake or series of small earthquakes in the vicinity of these segments are considered possible foreshocks to a possibly larger main shock and possible aftershocks to follow. The San Andreas Fault and other faults in the California Fault System are subdivided into many segments based on seismic history, local geology, and intersections with other faults.
Earthquake magnitude M is a numeric measure that represents the size or strength of an earthquake, as determined from seismographic observations.
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The Richter scale is a numerical logarithmic scale for expressing the magnitude of an earthquake on the basis of seismograph oscillations data. Today earthquake intensity is recorded with a moment magnitude scale MMS which is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped.
Richter scale and moment magnitude scales are similar, but the MMS scale is more precise. The addition of the word great to a name implies great destruction was associated with an historically massive earthquake which, really, isn't all that great!
Interestingly, the name Great San Francisco Earthquake was applied to the earthquake on the Hayward Fault, but the name was switched to the even greater earthquake on the San Andreas Fault.
Neither fault runs through San Francisco, but that is where the destruction occurred. This network is a collaboration with many organizations and universities involved in seismic research.