Geology 101 Name(s): Lab 7: Structural geology and geological maps Structural geology is the study of the orientation and relationship of rock units. A rock unit is a lithologically similar layer or body of rock, and different rock units are given names referred to as formations. As you have seen from the “Dating Methods” 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.How geologists measure geological structures Clearly, a horizontally-bedded set of rocks will tell a different geological story than a tilted set of rocks. To convey the data that would allow geologists to formulate an interpretation, a set of standard symbols and measurements was developed. The two principal measurements of any geological structure (for instance, a fault or a layer of rocks) is the strike and the dip of a layer of bedded rocks. Both measurements can be made with a Brunton compass; both measurements have a compass orientation (i.e., such as N 30 W (or NW 30) which means “30 degrees west of due north”). In addition, the dip requires the measurement of an angle (reported in degrees off of horizontal). But what are they? The strike of a structure is the compass orientation of any horizontal line on a planar surface. The dip is the angle of slope at right angles to the strike. Note that the strike orientation will always be 90° (perpendicular) off of the dip orientation. A way to think about strike and dip is to consider how water behaves. Imagine a boat ramp sloping toward a lake. The water dripping off the ramp into the lake will be going in the direction of the dip; the slope angle of the boat ramp is the magnitude of the dip. The water level of the lake, which makes a horizontal line across the planar surface of the boat ramp, is the strike (once you measure its direction). “Attitude” is the term used to describe a feature’s strike and dip at a particular point. An attitude symbol looks like a short-legged “T” with the arms of the T representing the direction of the strike and the short leg of the T pointing in the direction of the dip (towards lower elevation). A number is written next to the short leg of the T, which represents the magnitude of the dip in degrees. 1. a. What sort of rocks will structural geology be most effective on (recall the emphasis 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. b. Examine the foamboard model of a tilted layer set up on the back table. Using the Brunton compasses provided, record the strike and dip of the board. Strike ____________ Dip __________ ________ (compass orientation) (angle in degrees) (simplified compass orientation) c. Given the north arrow shown, draw the attitude symbol for the board. ↑ NFolds and faults 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 (don’t worry about the exact dip angle) on 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.