Geology - Structural Geology Review ANSWERS
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Geology
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Apr 26, 2024
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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.
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.
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