Lab 5

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Emporia State University *

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Apr 3, 2024

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Name _ Emma Brady ______ Lab 5: Interpretation of Geologic Blocks ES 111 Introduction to Earth Science Laboratory ______________________________________________________________________________ Geologic Blocks Watch Pre-Lab Video : https://goo.gl/nK9Kk6 -- Please watch the first 1:30 minutes of this video for the introduction to rock stress (Figure 1 below), and then skip to the 6:28 minute mark to see explanation of material we will cover in this lab (we will not cover strike/dip in this lab). Figure 1 Structural Geology : Anticlines and Synclines During mountain building, formerly flat-lying layers of sedimentary or volcanic rocks are often deformed into a series of waves called folds (see Figure 2 below). Anticlines and synclines are the two most common types of folds. Rock layers that fold upward, forming an arch , are called anticlines . Often associated with anticlines are downfolds, called synclines . Anticlines and synclines often occur together in the same locale when the crust is crumpled or compressed (see “Folding” example in the bottom left image of Figure 1 above). To understand folds and folding, it is important to become familiar with the geological terminology involved. During deformation, each layer can be described as being bent around an imaginary axis called a hinge line (Figure 2). Folds are also described by their axial plane, which is an imaginary plane that divides the fold as equally as possible into two halves, called limbs . In a symmetrical fold , the limbs (each side of the fold) are mirror images of each other and diverge at the same angle. In an asymmetrical fold , the limbs each have different angles of dip . A fold in which one limb is tilted beyond the vertical is referred to as an overturned fold (see Figure 2).

Anticlines are shaped like an “A” (this is a good trick to help you remember…) Figure 2 -- An anticline is a structure in which the oldest strata are found in the center of that structure. This most typically occurs when originally horizontally bedded strata are folded upwards . Furthermore, with a structural syncline, the youngest strata are found in the center or middle of the structure. This occurs most commonly when originally horizontally bedded strata are downfolded (shaped more like a “U” or “V”). At-Home Experiment : Take 3-4 pieces of notebook paper (colorful construction paper or thin sheets of crafting foam are even better) and stack them together neatly on a table to represent 3-4 horizontally bedded rock layers or strata. Then attempt to slowly compress this stack of paper (which represents layers of rock) from both ends without sharply creasing the paper. This movement should result in a series of small folds if you do this carefully and slowly, OR you can manipulate all of the layers of “rock” to all be uparched into one large anticline, or downfolded into one large syncline. If the force you apply is equal from both sides of this paper stack, the fold should look fairly symmetrical , but also try applying more compressional force from one side than the other (still keeping the paper on the table), and you should see an asymmetrical fold result, either shaped as an anticline or syncline – try to create both structures . If you keep pushing more from one side than the other (still holding onto both ends) you will produce an overturned fold (see bottom right image above in Figure 2).

Figure 3 Plunging Folds -- Folds can also be tilted or uplifted by tectonic forces on only one end of the structure which can cause their hinge lines to slope downward (see the three “plunging” images on the right side of Figure 3 above). Both images on the left side of this figure are nonplunging structures (meaning not uplifted on one end). Look at the sides of each geologic block to see how the beds are either horizontal (as with the nonplunging structures), or dipping down at an angle (as with plunging structures). Folds of this type are said to be plunging . As shown in Figure 3, the outcrop pattern of an eroded plunging anticline “points” in the direction it is plunging. The opposite is true of a plunging syncline (bottom right image). ______________________________________________________________________________ Nonplunging Folds vs. Plunging Folds

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Figure 4 Block A Block B Figure 4 above illustrates an eroded anticline and an eroded syncline . Use this figure to answer the following questions. The numbers listed on each geologic unit match up within the block itself -- so on Block A, all rock units listed with a #4, as an example, are all the same rock unit and are all the same age, just exposed either on the surface or in cross-section on the end of the block. These numerical labels are very helpful as you learn how erosion and weathering, and of course tectonic uplift, can alter the surface or top-plan view , as well as the cross-sectional view . By cross-section, I mean the side of these blocks that are facing you where you can see into the ground – these can be either naturally formed (such as when a river erodes down into a valley) or they can be man-made (such as with a road cut created to build a new highway, for instance). ______________________________________________________________________________ Lab 4: Questions to Answer -- Please use colored text for all of your answers. Each question below is worth one (1) point unless otherwise stated. 1. In the “pre-lab video” linked above, what does the narrator say about how brittle rocks react to stress compared to how ductile rocks react to stress when they are each compressed ? Go back and watch this portion of the video again. (2 points) Brittle rocks are breaking while ductile rocks are flowing. Brittle rocks react to stress by reverse faulting, normal faulting, or strike-slip faulting. Ductile rocks will fold, stretch, or shear. 2. For each block diagram in Figure 4, label the type of fold as either an anticline or a syncline. Block A) Syncline Block B) Anticline

3. Do the rock layers in an anticline dip toward or away from the axial plane (the axial plane is represented by the blue sheet in Figure 4)? [ Hint : Dip is the angle of each limb relative to the horizon . So think about this to help you answer this question… if you were to pour a bottle of water down the surface of one of the limbs of a fold, where would the water flow -- toward or away from the axial plane? Water will naturally follow the direction of dip ! This is certainly not the technical definition of structural dip, but it is a helpful trick to help you figure out dip direction. Use this trick for the next question as well.] The rock layers in the anticline dip away from the axial plane. 4. Do the rock layers in a syncline dip toward or away from the axial plane (the axial plane is represented by the blue sheet in Figure 4)? The rock layers in the syncline dip towards the axial plane. 5. Is the syncline shown in Figure 4 symmetrical or asymmetrical? How do you know this? The syncline shown in Figure 4 is symmetrical. I know this because the dips are mirror images of one another. 6. Is the anticline shown in Figure 4 symmetrical or asymmetrical? How do you know this? The anticline shown in Figure 4 is also symmetrical because the limbs and dips are mirror images of one another. 7. In Figure 4, are these folds plunging or nonplunging folds? Address both geologic blocks (A & B) and explain how you know whether they are plunging or nonplunging folds. (2 points) Block A – Non-plunging. The hinge line is completely parallel with the earth’s surface. Block B – Non-plunging. Although the hinge and axis line looks like it may be tilted, there is no V-shape or u-shape outcrop pattern.

8. The Principle of Superposition states that in rock strata (layers) that have not been overturned, the oldest rocks are found at the bottom of the rock sequence, and the rocks get progressively younger toward the surface. With this in mind, list the rock layers in each block diagram, by number , from oldest to youngest . (2 points) Block A (list oldest to youngest): 5, 4, 3, 2, 1 Block B (list oldest to youngest): 1, 2, 3, 4, 5 9. Complete the following statements that describe what happens to the ages of the surface rocks as you move further away from the axial plane of each of the following structures. a. On an eroded syncline , as shown in Figure 4, do the rocks exposed at the surface get older or younger further from the (blue) axial plane? The rocks get older the further they are away from the axial plane. b. On an eroded anticline , as shown in Figure 4, do the rocks exposed at the surface get older or younger further from the (blue) axial plane? The rocks get younger the further away they are from the axial plane. Figure 5 Anticlines and synclines are somewhat linear features caused by compressional forces , but two other types of folds, domes and basins , are roughly circular features that result from vertical displacement . Upwarping of sedimentary rocks produces a dome, whereas a basin is a downwarped structure. See the cross-section and top-plan views of a dome and basin in Figure 5. 10. In a dome, are the oldest surface rocks found near the center or at the flanks of the dome?

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In a dome, the oldest surface rocks will be found near the center. 11. In a basin, are the oldest surface rocks found near the center or at the flanks of the basin? In a basin, the oldest surface rocks are going to be found near the flanks or edges.

Lab 5

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