_Lab 8 - Rock Deformation

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San Diego State University *

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1306

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Geology

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Dec 6, 2023

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pdf

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12

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Name: Angelica Romero Structural Geology Introduction Structural geology is a subdiscipline of geology that investigates rock deformation (bending and breaking or folding and faulting) on both the small (microscopic) and large (mountains, oceans, and continents) scale. For the purposes of this lab we are going to focus on the large-scale structural features associated with folding and faulting of rocks and their identification. Structural geologists (you) seek to use their observations at the surface of the Earth to interpret what the structures look like below the surface, which is hard to observe directly. In this lab we will discuss two different ways of looking at the Earth, map view and cross-sectional view (figure 1). Map view is what you see when you look straight down onto the surface of the Earth or as in figure 1A the top of a cake. Cross sectional view is what the crust looks like if you were to take a slice out of the Earth or cake (figure 1B). We use cross sections to project the structures we observe at the Earth’s surface into the subsurface. Figure 1: A. Map view of the top of a cake as an analogy for map view of the Earth. B. A slice of the cake in A is shown as an example of a cross-sectional view. Without cross sections we would not have a way to project what we observe on the surface of the Earth into the subsurface. Notice that the slice of cake has 5 horizontal layers and the oldest layer is at the bottom and that the youngest layer is at the top. Strike and Dip In our discussion of geologic time in lab 7 we learned six principles of relative dating, one being original horizontality. The idea that all rocks on the Earth are originally deposited in essentially horizontal layers and that if the rocks are currently not horizontal then some younger event must have disturbed their original horizontality. Globally geologists have decided on a map symbol that allows them to display the orientation (or attitude) of a rock unit on a map. We call this symbol strike and dip and it’s used for 1
non horizontal or dipping rock units or beds (figure 2). Figure 2. A. Map view of a geologic map with three units, one being the oldest and 3 being the youngest. Strike and dip symbol annotated. B. Cross sectional view of A. Notice that unit 1 is below units 2 and 3 which is consistent with unit 1 being the oldest. C 3D block diagram that combines A and B. The strike bar is always parallel to the intersection of the dipping plane with horizontal (figure 2A) and the dip tick points in the direction the beds are dipping. Notice in figure 2C that the strike bar is parallel to the contact between the orange and brown units or units 1 and 2 and the dip tick is pointing in the same direction that all the units (1, 2, and 3) are dipping. Without the strike and dip symbol in figure 2A we would not be able to draw the cross section in figure 2B. In this lab you will not have to measure or draw a strike and dip symbol, but you will need to use them to interpret structures. Folds: Anticline and Syncline Folds, no matter the type, form under compression like that found along convergent boundaries where two tectonic plates are moving towards one another. However, compression can occur along transform boundaries too and therefore folds can form here as well. There are several kinds of folds but in this lab, we are going to discuss two types. 2
Anticlines: You can imagine an anticline as a hill. The hinge (also known as an axis) of the anticline is analogous to the top of the hill (figure 3B) and the limbs are analogous to the sides of the hill. Therefore the hinge is a topographic high and the limbs are a topographic low as they dip away (figure 3C) from the axis of the fold. Eventually the hill will erode, exposing the older layers in the center of the anticline. The map symbol for an anticline is a line that runs down the axis of the fold with arrows pointing away from each other (figure 3C). Figure 3: A. Three horizontal units with 1 being the oldest and 3 being the youngest. The tree is growing on the Earth’s surface. B. Anticline forms under compression. Notice the topographic high or hill that forms. C. Over time the topographic high will erode to a flat surface exposing the older unit 2 in the center of the anticline. Strike and dip symbols with opposing dips indicate the limbs of the fold dip away from each other. Along the axis of the fold is the map symbol for an anticline. Notice the tree in C is gone and a new younger tree is growing. Synclines form along with anticlines under compression (figure 4B). You can imagine a syncline as a basin or valley. The hinge (also known as an axis) is analogous to the bottom of the valley and the limbs are analogous to the sides of the valley. Therefore the hinge is a topographic low while the limbs are topographic highs as they dip towards each other (figure 4C). As time progresses the high part of the limbs will erode, exposing older units on the outside and younger units in the center of the fold (figure 4C). The map symbol for a syncline is a line down the axis of the fold with two arrows pointing towards each other (figure 4C) 3
Figure 4: A. Three horizontal units with 1 being the oldest and 3 being the youngest. The tree is growing on the Earth’s surface. B. Syncline forms under compression. Notice the topographic low or basin that forms. C. Over time the topographic high will erode to a flat surface leaving the youngest unit (3) in the center of the syncline. Strike and dip symbols with dips towards the center of the fold indicate the limbs of the fold dip towards each other. Along the axis of the fold is the map symbol for a syncline. Below is a fold found along the northwestern coast of Africa near Algeria. Use this screenshot from Google Earth to answer questions 1 – 4. The fold axis is depicted with the white line, and the strike and dip of both limbs of the fold are included. 1. By examining the dip directions on the strike and dip symbols, which direction is the north limb (upper half) dipping (north, south, east, or west)? What about the south limb (lower half)? The north limp is going north and the south limb is going south. 2. What is the dip direction on either side of the fold axis (white line)? Using a pen or pencil, draw an arrow on either side of the fold axis indicating the dip direction of each limb. Refer to figures 3C and 4C for examples. 3. Based on your answers to questions 1 and 2 what type of fold is this, syncline or anticline? The fold is anticline. 4
4. Would the limbs on this fold form a topographic high or a topographic low? The limbs form a topographic low. Below is another fold found along the central portion of Iran. Use this screenshot from Google Earth to answer questions 5 – 8. The fold axis is depicted with the black line, and the strike and dip of both limbs of the fold are included. 5. By examining the dip directions on the strike and dip symbols, which direction is the east limb (upper half) dipping (north, south, east, or west)? What about the west limb (lower half)? East is pointing west while west is pointing east. 6. What is the dip direction on either side of the fold axis (black line)? Using a pen or pencil, draw an arrow on either side of the fold axis indicating the dip direction. Refer to figures 3C and 4C for examples. draw them point to eachother 7. Based on your answers to questions 1 and 2 what type of fold is this, syncline or anticline? The fold is syncline. 8. Would the limbs on this fold form a topographic high or a topographic low? 5
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