Geology 101 Staple this part to part one of lab 6 and turn in Lab 6, part two: Structural geology (analysis) Recall that the objective of this lab is to describe the geologic structures of Cougar Mountain Regional Park. In the first part, you obtained structural (attitude) data for geologic formations (mappable units of rock) of that area. Write three working hypotheses of how Cougar Mountain came to be. In other words, suggest three ways in which Cougar Mountain could have formed, given the data you have already collected. By performing the analysis, as well as getting some background information, you will be able to infer the most consistent hypothesis by the end of this lab.Most geological processes take place over thousands or millions of years and so have very little direct effect on our lives. In future labs, you will explore the more sudden (and catastrophic) geological events, such as earthquakes and landslides. In this lab, you will see how to use the results (hidden in the rock record) of many sudden events to reconstruct geologic history. Part of this analysis is structural geology. Stress and Strain Tectonic forces move and deform the Earth’s lithosphere. These movements create stress in the lithosphere, which is measured in units of force (such as Newtons, or N). In turn, the rocks accommodate the stress by changing the space they occupy. This change in size is called strain, which is measured in units of volume (such as meters-cubed, or m3). There are three basic types of stress: compression, extension, and shear. • Compression results in rocks (as small as a layer and as large as a plate) being pushed together • Extension (or tension) results in rocks being pulled apart • Shear results in rocks sliding past one another Regardless of the type of stress, the amount of stress can result in different strain response by the rocks: elastic deformation, plastic deformation and brittle failure. • With a small amount of stress, the deformed rock returns to its original shape after the stress is removed. This is elastic deformation. A good example of this is isostatic rebound, when an area, which had been under a thick contine4ntal ice sheet, returns to its original elevation after the ice sheet has melted. • With a bit more stress, the deformed rock rebounds somewhat when the stress is removed, but some permanent strain remains. This is plastic strain (the word “plastic” refers to the moldable property of the rock— hardly an everyday experience!). Folds are examples of plastic strain in a large volume of rock. • After a lot of stress, the deformed rock will fracture or break, and the rock’s volume does not return at all to its original size. This is brittle failure. A fault is an example of brittle failure in a large volume of rock. 1. What sort of rocks will structural geology be most effective on (recall the empha-sis on layers)? Indicate your answer by circling the most appropriate rock types. Intrusive igneous Extrusive igneous Clastic sedimentary “Other sedimentary” Foliated metamorphic Non-foliated metam.Folds Folds are warped or bent layers of rock, usually consisting of two limbs dipping (tilting) in opposite directions. In simple folds, the axial plane is the surface that passes through the points of maximum curvature of the fold. The expression of the axial plane on the surface of the land is the fold axis (or hinge line), which is usually a straight line. See the diagram below. There are two fold types: anticlines, in which the strata (layers) dip away from the fold axis; and synclines, in which the strata dip towards the fold axis. Note the terms anticline and syncline do not necessarily refer to landforms such as hills and valleys; rather, the terms describe the cross-sectional appearance of the rock layers (remember that erosion will alter the topography at the surface!). Thus, geologists can recognize an eroded syncline or anticline, even if the surface is eroded flat, by observing the orientation and relative ages of the strata.2. a. Put attitude symbols (the little “T”s — don’t worry about the exact dip angle) on the top surface of the two block diagrams below. b. Which numbered layer corresponds to the oldest rock unit of the anticline? c. Which numbered layer corresponds to the youngest rock unit of the anticline? d. Which numbered layer corresponds to the oldest rock unit of the syncline? e. Which numbered layer corresponds to the youngest rock unit of the syncline? The fold axis can also be tilted, leading to plunging folds. When planed off by erosion, as shown in the diagram, the fold pattern on the ground can look odd, but still allow the reconstruction of the eroded fold. 3. Complete the blank side of each of the two block diagrams by filling in the layers that should be visible; use the existing patterns to indicate layer types.Faults Faults are breaks or fractures in rocks along which movement of one side relative to the other has occurred. Breaks or fractures in rocks which exhibit no relative motion are called joints. While the expression of a fault (such as the San Andreas Fault in California or the Straight Creek Fault near Marblemount) on the surface is usually linear, the actual break or fracture is a planar surface, called (not shockingly) a fault plane. The hanging wall of a fault rests on or lies above the fault plane; the footwall supports the hanging wall and is therefore underneath the fault plane. Note that this terminology does not necessarily make sense for every type of fault. The upthrown block (or overriding plate) is the side that moves upward relative to the other side and the downthrown block is the side that moves downward. Once again, this terminology does not necessarily make sense for every type of fault. Note also that the footwall is not always the upthrown block, as shown in the diagram. The slip is the distance measured along the fault plane that one side of the fault has moved relative to the other. There can be vertical and horizontal components to the slip. The slip rate is the slip divided by the time interval. Faults are classified according to their relative movement, which can be up/down, right/left or a combination. • In normal faults, the hanging wall moves down relative to the footwall. It is called “normal” because the superposition rule is preserved: younger rocks remain above older