Geology - Structural Geology Review ANSWERS
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GEOLOGY FINAL REVIEW Introduction to Structural Geology 1.
What is the primary focus of structural geologists when studying rock deformation? A) The economic value of the deformed rocks. B) The age of the deformed rocks relative to other rock layers (strata). C) The mechanisms, rock properties, and causes influencing the deformation. (Correct Answer) D) The types of fossils found within the deformed rocks. E) Whether the deformed rocks are igneous, sedimentary, or metamorphic. Explanation: The correct answer (C) The mechanisms, rock properties, and causes influencing the deformation is the primary focus of structural geologists when studying rock deformation because it directly addresses the processes that shaped the rocks. Let's break down why the other options are incorrect: ●
(A) The economic value of the deformed rocks: While deformation can influence the properties of rocks and potentially their economic value (e.g., creating fractures that allow fluids containing valuable minerals to flow), it's not the primary concern for structural geologists. Their focus is on understanding the deformation itself. ●
(B) The age of the deformed rocks relative to other rock layers (strata): This information can be helpful for structural geologists, but it's not their main focus. Stratigraphy, a related field, deals primarily with the study of rock layers and their ages. Structural geologists might use the age relationships of deformed rocks to understand the sequence of events that caused the deformation, but their core interest lies in the how and why of the deformation process. ●
(D) The types of fossils found within the deformed rocks: Fossils can be present in deformed rocks, but they are not the primary focus of structural geology. Fossils are more relevant to paleontology, which studies the history of life on Earth. While fossils might offer clues about the environment before deformation, they don't directly address the deformation process itself. ●
(E) Whether the deformed rocks are igneous, sedimentary, or metamorphic: This information might be helpful for understanding the original rock type and how it might have deformed differently compared to other rock types. However, structural geologists are more concerned with the deformation itself, not the initial rock classification. Therefore, option (C) accurately captures the essence of structural geology, which is to understand the mechanisms, the properties of the rocks themselves that influence how they deform, and the forces that drive the deformation in the first place. 2.
Which of the following is NOT a way that structural geology and stratigraphy can be helpful? A) Understanding Earth processes like sea-level changes and mountain building. B) Refining the geological timescale by dating rock layers. C) Predicting locations of mineral deposits. (Correct Answer) D) Identifying potential oil and gas reserves. E) All of the above are ways structural geology and stratigraphy can be helpful. Explanation: Options A, B, D, and E all highlight ways that structural geology and stratigraphy are valuable tools for geologists. They help us understand past Earth processes by looking at the structures and layering of rocks. They also play a crucial role in locating resources like oil, gas, and minerals. Option C, however, is not directly related to the information provided. While structural features can influence the movement of fluids containing minerals, predicting the exact location of a mineral deposit is more complex and involves additional factors beyond just structure. It might involve geochemical analysis of the rocks as well.
3.
A tilted rock layer has a dip of 45 degrees. What differentiates dip from dip direction? A) Dip refers to the angle of tilt, while dip direction is the compass direction the rock layer is leaning towards. (Correct Answer) B) Dip is the steepest angle of the rock layer, while dip direction is the average angle of the entire tilted surface. C) Dip refers to the color of the rock layer, while dip direction is the compass direction the rock layer was originally deposited. D) Dip is the thickness of the rock layer, while dip direction is the compass direction water would flow if poured on the layer. E) There is no difference; dip and dip direction describe the same thing. Explanation: The answer is (A) Dip refers to the angle of tilt, while dip direction is the compass direction the rock layer is leaning towards. ●
Dip: This refers to the angle in degrees (°) between a horizontal plane and the tilted surface of a rock layer. It tells you how much the layer is slanted from horizontal. ●
Dip Direction: This indicates the compass direction towards which the tilted rock layer is dipping. Imagine pouring water on the tilted surface; the dip direction tells you where the water would flow downhill. Cardinal directions (N, S, E, W) or ordinal directions (NE, SE, SW, NW) are used to describe the dip direction. While both dip and dip direction describe the orientation of a tilted rock layer, they provide different pieces of information: ●
Dip tells you the amount of tilt. ●
Dip direction tells you the compass direction of the tilt. 4.
When studying tilted rock layers, how does strike differ from dip direction? A) Strike refers to the angle of tilt, while dip direction describes the compass direction of tilt. B) Strike is the thickness of the rock layer, while dip direction is the compass direction water would flow if poured on the layer. C) Strike is the compass direction of a horizontal line along the tilted layer, and dip direction is the compass direction of the maximum tilt. (Correct Answer) D) Strike is a measure of color variation within the rock layer, and dip direction is the compass direction the rock layer was originally deposited. E) There is no difference; strike and dip direction describe the same thing. Explanation: The answer is (C) Strike is the compass direction of a horizontal line along the tilted layer, and dip direction is the compass direction of the maximum tilt. ●
Strike: This refers to the compass direction of a horizontal line created by the intersection of an imaginary flat plane and the tilted surface of a rock layer. Imagine slicing through the tilted layer with a level plane; the strike is the compass direction of the line where the cut intersects the rock surface. ●
Dip Direction: This indicates the compass direction towards which the tilted rock layer is dipping. It's similar to how dip direction works, but it refers specifically to the direction of the steepest descent of the layer. Both strike and dip direction provide compass directions related to a tilted rock layer, but they describe different aspects of its orientation: ●
Strike tells you the direction of the horizontal line along the tilted surface. ●
Dip direction tells you the direction of the maximum tilt (downward slope) of the layer. 5.
The right-hand rule is used to determine: A) The thickness of a rock layer based on its strike and dip. B) The age of a rock layer relative to other layers. C) The color and mineral composition of a rock layer.
D) The unambiguous way to report strike and dip direction to avoid confusion. (Correct Answer) E) The direction from which the rock layer was originally deposited. Explanation: The answer is (D) The unambiguous way to report strike and dip direction to avoid confusion. The right-hand rule is a convention used in geology to ensure consistent interpretation of strike and dip. It eliminates ambiguity in situations where the strike could be interpreted in two opposite directions. Here's how it works: ●
Imagine your right hand with your fingers pointing down the direction of the dip. ●
Your thumb will then naturally point in the direction of the strike. Following this rule ensures everyone interpreting geological maps understands the orientation of the rock layers in the same way. 6.
What are the THREE main types of forces that influence how a rock deforms? A) Friction, gravity, and magnetism B) Compressional, tensional, and volcanic forces C) Compressional, tensional, and shear forces (Correct Answer) D) Chemical weathering, physical weathering, and erosion E) Igneous, sedimentary, and metamorphic processes Explanation: The answer is (C) Compressional, tensional, and shear forces. The passage highlights these three types of forces as the key factors governing how rocks deform. ●
Compressional forces: These forces squeeze rocks together, often leading to folding and thickening of rock layers. ●
Tensional forces: These forces pull rocks in opposite directions, causing them to stretch and potentially break, resulting in features like faults. ●
Shear forces: These forces act parallel to a surface, causing the rock to slide past itself along internal planes of weakness. This can lead to shearing and fracturing. The other answer choices include forces or processes that are not directly related to rock deformation or are not the main types considered in this context. 7.
A rock layer is squeezed by immense pressure from tectonic plate movement. What is the difference between the stress acting on the rock and the resulting change in the rock layer? A) Stress is the pressure causing the change, and strain is the mineral composition of the rock. B) Stress is the force per unit area acting on the rock, and strain is the overall deformation or change in shape of the rock layer. (Correct Answer) C) Stress is a measure of the rock's resistance to deformation, and strain is the amount of force required to cause the change. D) Stress is the direction of the force acting on the rock, and strain is the distance the rock layer is squeezed. E) There is no difference; stress and strain are the same thing. Explanation: The answer is (B) Stress is the force per unit area acting on the rock, and strain is the overall deformation or change in shape of the rock layer. ●
Stress: This refers to the force that is applied to a rock. It's important to note that stress is considered in terms of force per unit area. Imagine squeezing a clay ball with your hand; the force you apply is the stress. ●
Strain: This is the response of the rock to the applied stress. It describes the change in size and shape that the rock undergoes due to the stress. In the clay ball example, the strain would be the flattening and elongation of the clay as you squeeze it. So, stress is the cause (force applied), and strain is the effect (deformation experienced) when rocks are subjected to forces.
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8.
Geologists use various features in rocks to understand how they have deformed. What are these features called, and what are three common examples used in structural analysis? (NOTE: FULLY AI GENERATED NOT BASED ON INFO FROM TEXTBOOK OR SLIDES). A) Igneous intrusions: Basalt, granite, and gabbro. (Incorrect - Igneous intrusions are newly formed rock bodies, not deformation features) B) Strain markers: Fossilized leaves, trilobite shells, and crinoids. (Correct Answer) C) Weathering features: Rust stains, cracks, and rounded pebbles. (Incorrect - Weathering features are caused by exposure to the environment, not deformation) D) Metamorphic minerals: Garnet, staurolite, and kyanite. (Incorrect - While these minerals can form during deformation, they are not the primary markers used to identify the type of deformation) E) Sedimentary layers: Shale, sandstone, and limestone. (Incorrect - Sedimentary layers themselves can be deformed, but they are not the markers used to analyze the deformation) Explanation: The answer is (B) Strain markers: Fossilized leaves, trilobite shells, and crinoids. ●
Strain markers: These are naturally occurring features within rocks that have been deformed and show evidence of that deformation. They help geologists understand the type and intensity of the forces that acted on the rock. Three common examples of strain markers used in structural analysis: 1.
Deformed fossils: Fossils originally with a symmetrical shape, like trilobite shells or brachiopods, can become stretched or flattened due to deformation. This change in shape provides clues about the direction and magnitude of the forces involved. 2.
Folded beds: Sedimentary layers that were originally flat and horizontal can become folded and contorted due to compressional forces. The geometry of the folds can reveal information about the style and intensity of the deformation. 3.
Cleavage: This is a secondary foliation (layered or planar structure) that develops in some rocks due to intense deformation. It can be a useful strain marker as the orientation of the cleavage planes can indicate the direction of the applied stress. These are just a few examples, and geologists use a variety of strain markers depending on the specific rock type and deformation style they are studying. 9. Please label each stain marker.
10. Rocks can deform under stress within the Earth. What is the main difference between ductile and brittle deformation? A) Ductile deformation involves slow, continuous movement of rock particles, while brittle deformation involves rapid fracturing. (Correct Answer) B) Ductile deformation only occurs in igneous rocks, while brittle deformation only occurs in sedimentary rocks. C) Ductile deformation creates rounded features like folds, while brittle deformation creates sharp features like faults. D) Ductile deformation requires very high temperatures, while brittle deformation can occur at any temperature. E) Ductile deformation is more common at the surface of the Earth, while brittle deformation is more common deep underground. Explanation: The answer is (A) Ductile deformation involves slow, continuous movement of rock particles, while brittle deformation involves rapid fracturing. ●
Ductile deformation: This type of deformation occurs when rocks behave like a plastic material and bend or flow under stress without breaking. It happens at high temperatures and pressures typically found deep within the Earth. The intense pressure allows the rock minerals to rearrange and deform over time. Visual evidence of ductile deformation includes folds, nappes, and boudinage structures. ●
Brittle deformation: In contrast, brittle deformation occurs when rocks fracture or break under stress. This happens relatively quickly and with minimal plastic flow. It's more common at shallower depths or in cooler conditions. Geological features like faults, joints, and breccia are evidence of brittle deformation. The key difference lies in the response of the rock to stress. Ductile deformation involves continuous movement and rearrangement of minerals, while brittle deformation involves breaking and fracturing of the rock. Factors like depth, pressure, temperature, and rock composition all influence which type of deformation dominates. Understanding these concepts allows geologists to interpret Earth's history, predict potential hazards, and explore for resources. 11. Does temperature play a role in how a rock deforms under stress (force)? A) No, temperature has no influence on whether a rock bends or breaks. B) Yes, higher temperatures always make rocks bend rather than break. C) Yes, higher temperatures make rocks more likely to bend and less likely to break. (Correct Answer) D) Yes, lower temperatures make rocks more likely to bend and less likely to break. E) The answer depends on the specific type of rock, not the temperature. Explanation: The answer is (C) Yes, higher temperatures make rocks more likely to bend and less likely to break. The passage explains that temperature plays a crucial role in rock deformation. Rocks behave differently under stress depending on the temperature: ●
Shallow Depths (Lower Temperatures): At shallower depths, the temperature is relatively low. This makes the rocks more rigid and brittle. Under stress, they are more likely to fracture and break rather than bend plastically. ●
Greater Depths (Higher Temperatures): Deeper within the Earth, temperatures are significantly higher. This intense heat allows the rock minerals to become more flexible and rearrange themselves under stress. As a result, rocks at these depths are more likely to bend and fold without breaking, exhibiting ductile behavior. Additional Points: ●
The text mentions "force type" as a potential factor, but in this context, temperature is the main focus. The type of force (compressional, tensional, etc.) also plays a role in deformation, but temperature's influence is significant. ●
Not all rocks at shallow depths will break; some may still show some degree of bending depending on the specific rock composition and the duration of stress.
So, temperature is a key factor influencing how rocks respond to stress. Higher temperatures promote ductile deformation (bending) by making the rocks more plastic. 11. In addition to temperature and pressure, what other factor can influence how a rock deforms under stress (force)? A) The color of the rock B) The age of the rock layer relative to other layers C) The composition of the rock unit (Correct Answer) D) The presence or absence of fossils within the rock E) The depth at which the rock was originally formed Explanation: The answer is (C) The composition of the rock unit. The passage highlights four key factors that govern rock deformation style: 1.
Type of force applied: (compressional, tensional, or shear) 2.
Magnitude of force applied: (pressure) 3.
Temperature of the rock: (higher temperatures promote ductile deformation) 4.
Composition of the rock unit: (certain minerals deform differently) Rock composition plays a crucial role because different minerals have varying degrees of strength and flexibility. Rocks rich in minerals like quartz and feldspar are generally harder and more brittle, while rocks with softer minerals like mica or clay might exhibit more ductile behavior even at shallower depths. The other answer choices are not directly related to the rock's deformation behavior: ●
Color, age of the rock layer, presence of fossils, and depth of formation don't directly impact how a rock deforms under stress. These factors might be relevant for other geological studies, but not for deformation style. 12. Geologists use the term competence to describe a rock's ability to resist deformation. Knowing this, which rock type would be considered more competent: sandstone or shale? ●
A) Sandstone (Correct Answer)
●
B) Shale ●
C) There is no difference; both are equally competent. ●
D) The answer depends on the specific type of sandstone and shale being compared. Explanation: The answer is (A) Sandstone. The passage defines competent rocks as those made up of strong materials, like well-cemented sedimentary rocks or intact igneous rocks. These rocks can resist deformation (bending, breaking) under stress. Incompetent rocks, on the other hand, are composed of weak materials and are more prone to deformation under stress. Shale, which is a fine-grained sedimentary rock often formed from mud, is an example of an incompetent rock. Sandstone, on the other hand, is typically a more cemented and coarser-grained sedimentary rock, often composed of quartz grains. This makes it generally more resistant to deformation compared to shale. Additional Points: ●
While the information provided suggests sandstone is generally more competent than shale, it's important to note that specific variations can exist. A very tightly cemented shale might be more competent than a poorly cemented sandstone. However, in general, due to their different compositions and grain sizes, sandstone is often considered the more competent rock type. 13. Geologists use both fractures and faults to understand how rocks have been stressed. What is the key difference between a fracture and a fault? ●
A) A fracture is a large crack that goes all the way through the Earth, while a fault is a small, shallow crack. ●
B) A fracture is a sign of volcanic activity, while a fault is caused by tectonic plate movement.
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●
C) A fracture does not show any movement of rock, while a fault shows some displacement of rock on either side of the crack.
(Correct Answer)
●
D) A fracture is always filled with mineral deposits, while a fault is always empty. ●
E) Fractures are more common in sedimentary rocks, while faults are more common in igneous rocks. Explanation: The answer is (C) A fracture does not show any movement of rock, while a fault shows some displacement of rock on either side of the crack. Both fractures and faults are cracks in rock caused by stress. However, the key difference lies in the movement of the rock: ●
Fracture (Joint): These are relatively small cracks in rock that do not show any significant displacement of the rock on either side. They can form due to various stresses, including cooling and shrinkage of rock. ●
Fault: These are also cracks in rock, but they show evidence of displacement along the crack. The rocks on either side of the fault have moved relative to each other. Faults can range in size, from small features to massive plate boundaries. The information also highlights the importance of faults for understanding larger-scale geological processes. Since faults involve movement, they can provide clues about tectonic plate movement and mountain building events. Fractures, while helpful for understanding stress, might not always be directly linked to these "big picture" problems. 14.
Reverse faults are a type of geologic feature. What describes a reverse fault and the forces that create it? (NOTE: FULLY AI GENERATED NOT BASED ON INFO FROM TEXTBOOK OR SLIDES). A) A reverse fault is a large, gaping crack in the Earth's surface caused by volcanic eruptions. (Incorrect - Reverse faults are not associated with volcanoes) B) A reverse fault forms when rock layers are pulled apart by tensional forces, creating a gap between them. (Incorrect - Tensional forces separate rock, not push them together) C) A reverse fault forms when two rock layers are pushed together by compressional forces, causing one layer to override the other. (Correct Answer) D) A reverse fault forms due to the weathering and erosion of rock layers at different rates. (Incorrect - Weathering and erosion don't create faults) E) A reverse fault is a specific type of fold in sedimentary rock layers. (Incorrect - Folds bend rock layers, reverse faults involve displacement) Explanation: The answer is (C) A reverse fault forms when two rock layers are pushed together by compressional forces, causing one layer to override the other. ●
Reverse fault: This type of fault forms when rocks are subjected to compressional forces. These forces squeeze the rocks together, causing one layer of rock to be thrust upwards and overriding the other layer along a dipping fault plane. The direction of the dip indicates the direction of the overriding block. ●
Compressional forces: These are forces that squeeze or push rocks together. They are a major driving force behind mountain building processes and can create various types of faults, including reverse faults. The other answer choices describe features or processes not related to reverse faults and their formation: ●
Volcanic eruptions and tensional forces create different features (volcanic craters, normal faults) ●
Weathering and erosion break down rock but don't cause faults ●
Folds involve bending of rock layers, not displacement like in reverse faults 15. Normal faults are a type of fault found in many geological settings. What characterizes a normal fault and the forces that cause it? (NOTE: FULLY AI GENERATED NOT BASED ON INFO FROM TEXTBOOK OR SLIDES).
A) A normal fault forms when two rock layers are squeezed together by compressional forces, with one layer folding over the other. (Incorrect - Compression creates reverse faults, not normal faults with folding)
B) A normal fault is a large crack in the Earth's surface that remains relatively stable with minimal displacement. (Incorrect - Normal faults involve displacement) C) A normal fault forms when rock layers are pulled apart by tensional forces, causing the block above the fault to move down relative to the block below. (Correct Answer) D) A normal fault is a specific type of igneous intrusion that forms between layers of sedimentary rock. (Incorrect - Normal faults are tectonic features, not igneous intrusions) E) A normal fault develops due to the hardening of molten rock underground, pushing existing rock layers upwards. (Incorrect - Normal faults involve movement down, not up) Explanation: The answer is (C) A normal fault forms when rock layers are pulled apart by tensional forces, causing the block above the fault to move down relative to the block below. ●
Normal fault: This type of fault forms when rocks are subjected to tensional forces. These forces pull rocks in opposite directions, causing them to stretch and potentially break. A normal fault has a dip direction that is typically away from the block that has moved down. ●
Tensional forces: These are forces that pull rocks apart or stretch them. They are responsible for the formation of features like rift valleys and mid-ocean ridges, and can also create normal faults. The other answer choices describe features or processes not related to normal faults and their formation: ●
Compression creates reverse faults, not normal faults with folding. ●
Normal faults involve significant displacement. ●
Igneous intrusions and hardening of rock don't create faults related to tension. 16. Strike-slip faults are another major type of fault movement. What characterizes a strike-slip fault and the forces that cause it? A) A strike-slip fault forms when rock layers are pushed together by compressional forces, with one layer overriding the other at an angle. (Incorrect - Compression creates reverse faults, not strike-slip) B) A strike-slip fault forms when rock layers are pulled apart by tensional forces, creating a large gap between them. (Incorrect - Tension creates normal faults, not strike-slip) C) A strike-slip fault develops due to the hardening of molten rock underground, pushing existing rock layers horizontally past each other. (Incorrect - Hardening rock doesn't create faults, and the movement isn't vertical pushing) D) A strike-slip fault forms when two blocks of rock slide past each other horizontally, with minimal vertical displacement. (Correct Answer) E) A strike-slip fault is a specific type of fold that occurs in sedimentary rock layers due to compression. (Incorrect - Strike-slip faults involve horizontal movement, not folding) Explanation: The answer is (D) A strike-slip fault forms when two blocks of rock slide past each other horizontally, with minimal vertical displacement. ●
Strike-slip fault: This type of fault occurs when rocks are subjected to shearing forces. These forces act parallel to a surface, causing the rocks on either side to slide past each other laterally, in a horizontal direction. The resulting fault plane is typically vertical or nearly vertical. ●
Shearing forces: Imagine placing a deck of cards on a table and pushing one half of the deck horizontally relative to the other half. The force you apply is similar to a shearing force. In geology, these forces can cause significant horizontal displacement along strike-slip faults. The other answer choices describe features or processes not related to strike-slip faults and their formation: ●
Compression creates reverse faults, tension creates normal faults. ●
Hardening rock doesn't create faults, and the movement isn't vertical pushing. ●
Strike-slip faults involve horizontal movement, not folding. 17. Please label the followin
g
1.
2. 3. 1.
_________________ 2. _____________________ 3. ________________________________ 18. Horst and graben features are commonly found in areas undergoing extensional tectonic forces. What do these features represent, and what causes them to form? A) Horst and graben are volcanic features: horsts are cinder cones and grabens are calderas. (Incorrect - These are volcanic landforms, not related to extension) B) Horst refers to a mountain range formed by folding, and graben is a valley formed by erosion. (Incorrect - Folding and erosion create different features) C) Horst and graben are large, uplifted plateaus (horst) and adjacent basins (graben) formed by continental collisions. (Incorrect - Continental collisions create compression, not extension) D) Horst and graben are uplifted blocks (horst) and down-dropped blocks (graben) of the Earth's crust, both created by normal faulting due to tensional forces. (Correct Answer) E) Horst is a specific type of mineral deposit, and graben is a hot spring opening on the Earth's surface. (Incorrect - These are not geological features related to faulting) Explanation: The answer is (D) Horst and graben are uplifted blocks (horst) and down-dropped blocks (graben) of the Earth's crust, both created by normal faulting due to tensional forces. ●
Horst and graben: These are topographic features associated with extensional tectonic settings. ○
Horst: An elongated block of the Earth's crust that has been uplifted relative to the surrounding area. It is bounded on both sides by normal faults with the blocks dipping away from the horst. ○
Graben: A valley formed by a down-dropped block of the Earth's crust, also bounded by normal faults with the blocks dipping towards the graben. ●
Normal faulting and tensional forces: The key to understanding horst and graben formation is normal faulting. When tensional forces pull the Earth's crust apart, these normal faults develop. The blocks between the faults can move vertically, with some blocks being uplifted (horst) and others subsiding (graben). The other answer choices describe features or processes unrelated to horst and graben formation: ●
Volcanic features, folding, and continental collisions are caused by different geological processes. ●
Horst and graben are not mineral deposits or hot springs. 19. Rocks can fold under pressure. What are some key characteristics used to describe folds? A) Folds are always large, mountain-sized features. (Incorrect - Folds can vary in size)
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B) Folds can be categorized by their overall shape (anticline or syncline) and the symmetry of their limbs. (Correct Answer) C) Folds only form in cold, brittle rock layers. (Incorrect - Warm rock is more prone to folding) D) The axial plane of a fold is a line that runs along the top of the fold. (Incorrect - The axial plane cuts through the center of the fold) E) Folds are always oriented horizontally in rock layers. (Incorrect - Folds can have different orientations) Explanation: The answer is (B) Folds can be categorized by their overall shape (anticline or syncline) and the symmetry of their limbs. The passage highlights two key features used to describe folds: ●
Shape: ○
Anticline: A fold with a convex upward bend, resembling an upside-down "U" or an "A". The layers of rock dip away from the central axis in opposite directions. ○
Syncline: A fold with a concave downward bend, resembling a "U" or "V". The layers of rock dip inwards towards the central axis. ●
Symmetry: ○
Symmetrical fold: The angles between the fold limbs (sides) and the axial plane (imaginary surface running through the fold's center) are similar on both sides. ○
Asymmetrical fold: The angles between the fold limbs and the axial plane are different on opposite sides. The other answer choices describe characteristics that are not always true for folds: ●
Folds can range in size from microscopic to mountain-sized. ●
Warm rock is more likely to fold than cold, brittle rock. ●
The axial plane cuts through the center of the fold, not along the top. ●
Folds can have different orientations within rock layers. Understanding these characteristics helps geologists interpret the forces that deformed the rocks and the overall geological history of an area. 20. Geologists can sometimes use eroded folds to determine the age of rock layers. If an anticline has been eroded, where would you expect to find the oldest rocks exposed at the surface? A) Along the sides of the fold, furthest from the center. (Incorrect - Rocks get younger outwards in an anticline) B) At the base of the fold, beneath the layers. (Incorrect - Erosion exposes underlying layers) C) In the center, or along the axis of the fold.
(Correct Answer) D) Scattered randomly across the entire eroded fold area. (Incorrect - Erosion exposes a pattern) E) It's impossible to determine the age of rocks in eroded folds. (Incorrect - Erosion reveals the age order) Explanation: The answer is (C) In the center, or along the axis of the fold. The passage explains how erosion can reveal the age sequence of rocks in folded structures: ●
Anticline: In an anticline, the oldest rock layers are located at the center or axis of the fold. As you move away from the center (outwards), the exposed rock layers become progressively younger due to the folding process. Erosion can wear away the upper layers, exposing this age sequence at the surface. The other answer choices are incorrect because: ●
The sides and further outward areas of an eroded anticline will expose younger rocks. ●
Erosion removes overlying layers, revealing the rocks beneath. ●
Scattered distribution wouldn't show a clear age pattern. ●
Erosion can reveal the age sequence in folds, not hide it entirely. By understanding the characteristic age patterns in eroded anticlines and synclines, geologists can reconstruct the geological history of an area and map the sequence of rock layers.
21. Similar to anticlines, eroded synclines also reveal a distinct age pattern. If a syncline has been eroded, where would you expect to find the youngest rocks exposed at the surface? A) In the center, or along the axis of the fold. (Incorrect - Youngest rocks are at the center in synclines) B) Along the sides of the fold, furthest from the center. (Correct Answer) C) At the base of the fold, beneath the layers. (Incorrect - Erosion exposes underlying layers) D) Scattered randomly across the entire eroded fold area. (Incorrect - Erosion exposes a pattern) E) It's impossible to determine the age of rocks in eroded folds. (Incorrect - Erosion reveals the age order) Explanation: The answer is (B) Along the sides of the fold, furthest from the center. The passage builds on the concept of eroded folds revealing rock age patterns. Unlike anticlines, synclines have a different age sequence: ●
Syncline: In a syncline, the youngest rock layers are located at the center or axis of the fold. As you move away from the center (outwards), the exposed rock layers become progressively older due to the folding process. Erosion can wear away the upper layers, exposing this age sequence at the surface. The other answer choices are incorrect because: ●
The center of an eroded syncline will expose the youngest rocks. ●
Erosion removes overlying layers, revealing the rocks beneath. ●
Scattered distribution wouldn't show a clear age pattern. ●
Erosion can reveal the age sequence in folds, not hide it entirely. By understanding the contrasting age patterns in eroded anticlines and synclines, geologists can analyze exposed rock layers and infer the fold type (anticline or syncline) that was present before erosion. This helps in reconstructing the geological history of the region.