Using the power of Google Earth, GeoTours take you on flyovers of key locations discussed in the text.
StudySpace student website and ebook icons in each GeoTour connect your to the text and online review materials.
Dialogue boxes accompanying each site include text, figures, photos.
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Geotour 9: Mountains and Structures
Calculating Strike and Dip from Flat Irons
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| 1a. Check and double-click the polygon “Flat Iron” in the Problem 1 folder to fly to the northeastern flank of the Wind River Mountain range in Wyoming. You may recall from the sedimentary rock worksheet that tilted sedimentary rocks typically erode to form asymmetric ridges called flat irons—a relatively planar surface with a pointed or rounded edge pointing in the direction opposite of the tilt (geoscientists thought that this geometry resembled the flat bottom of a laundry iron, hence the name). In this view, the polygon outlines one of the many flat irons that border the uplift (flat irons are common in folded/tilted sedimentary strata). To make it easier to see, you might consider temporarily increasing the vertical exaggeration to 2 in Google Earth™.
Turn on the Problem 1a-strike placemark. You probably recall from your text that geoscientists describe the orientation of structures by measuring the strike and dip of the rock layers. The strike is the angle between an imaginary horizontal line on the structure and the direction of true north. To determine an approximate strike, use the Measuring Tool (Line tab, use whatever units the elevation in the Info Bar at the bottom of the screen is using, preferably meters) to determine the elevation at the point of the placemark icon by hovering the crosshairs over it and looking at the Info Bar (1846 m). Now draw a line from the Problem 1a-strike placemark to the right. Click the second point of the line at a location at the exact same elevation. This is the horizontal strike line. What is the orientation (0-360°)?
Note: This approximation works because we’re assuming that the flat part of the flat iron
approximates a bedding plane surface.
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| 1b. Leave the 1846 m line on the screen and turn on the Problem 1b-dip placemark. Recall that dip is the angle of a structure’s slope measured in a vertical plane perpendicular to the strike. To determine this perpendicular dip direction, draw a perpendicular line from the 1846 m line to the placemark point (the strike line will disappear and you will now see only the dip line). This is the dip direction (the direction that water would flow down the flat iron). What is the orientation of the dip line (000°-360°)?
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| 1c. Note the length of the dip line in m (or the same units that your elevations are in). This length is actually a horizontal distance and not the distance along the flat iron. Also, hover over the point of placemark Problem 1b-dip to determine its elevation. To determine the amount of dip, use the following formula: dip = arctan [(elevation difference between placemark Problem 1b-dip and strike line)/length of the dip line]. What is the approximate dip angle (00°-90°)?
Just for Fun...
Determining the strike and dip of a flat iron is harder to explain in words than it is
to actually do in Google Earth! Find some other nearby flat irons (look for the most planar ones)
and try to draw a horizontal strike line. How does it compare to your answer? Do the same for
dip and see how close you come. Think about reasons why you might have differences. |
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Brittle Structures-Joints
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| 2a. In the Problem 2 folder, check and double-click placemark Problem 2 to fly to Arches National Park in Utah. Also, turn on the “Arches NP Geologic Map-North” map overlay and make it semitransparent. Placemark Problem 2 points to a road (dashed red line on the geologic map) that approximates the axis of a salt-filled anticline that has collapsed to form Salt Valley. Along the flanks of this anticlinal valley, some of the rock units exhibit a prominent linear pattern that reflects systematic joint sets that have developed in association with the Salt Valley anticline and collapse.
Using the geologic map overlay, what is the youngest unit near the Problem 2a placemark that can be easily observed to contain these joint sets? Note: The oldest rock unit in the map key is the Paradox Formation and the youngest is the Modern Alluvial Deposits (older rock units are on the bottom and younger ones are on top).
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| 2b. What happens to the joint sets beneath the red rock unit (Jmt) at placemark Problem 2b?
Just for Fun...
Think about your answer for 2b...does this help date the age of jointing or does it just reflect how joints develop differently in different rock types?
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| 2c. Check and double-click the placemark for Problem 2c. This flies you to an area on the NE side of Salt Valley that shows two prominent joint sets. Joint sets may intersect at a variety of angles, but most commonly are classified as being at right angles (orthogonal) or not at right angles. Which kind are these?
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| 2d. Arches NP is known for its spectacular arches that develop in the jointed areas of the park. Placemark Problem 2d flies you to Landscape Arch, the arch with the largest span in Arches NP. If the stresses that stretched the rock to form the joints were oriented perpendicular to the joint surfaces and the rock fins that contain the arches, what was the direction that the rocks were stretched?
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Brittle Structures-Faults
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| 3a. Faults are typically classified in one of three main categories: normal, reverse, and strikeslip. The following questions will take you to locales that provide examples of each type of these faults.
Check and double-click the placemarks for Problems 3a-i, -ii, and -iii to fly to Canyonlands National Park-Needles District in Utah. This area is a region in the Grabens area that is experiencing normal faulting as the rock units in the region are extending to the west. It is called the Grabens because the faulting forms fault-bounded blocks that have been downdropped (grabens) with intervening high blocks (horsts). Which of the following is the best answer for identifying the structures in the view?
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| 3b. Check and double-click placemark Problem 3b to fly to Squaw Point, OR. The placemark lies on a normal fault scarp, an offset of the land surface by slip along a fault. Which side of the normal fault is the down-dropped hanging wall?
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| 3c. Placemark Problem 3c highlights another area east of Problem 3b that has experienced extension along a series of normal faults, forming a series of half grabens. If the extensional stresses that are stretching the rocks are approximately perpendicular to the fault traces, which direction are they oriented?
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| 3d. Photo Problem 3d shows a low-angle reverse fault (the McConnell thrust fault) placing older Paleozoic rocks (gray) on top of forested Mesozoic rocks. The gray rocks are interpreted to have been uplifted over 5 km and transported over 40 km from their original site of deposition. Which direction were the rocks transported? Hint: Turn on the “Kananaskis Country Geologic Map, Canada” map overlay (triangles are on the hanging-wall side the fault).
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| 3e. Check and double-click the placemarks for Problem 3e, and turn on the “Kananaskis Country Geologic Map, Canada” overlay. Reverse faults, and particularly thrust faults, duplicate rock units by stacking the same units on top of each other over and over. How many major repetitions of the Devonian Palliser Formation (DPa, cyan colored) are seen in the rocks north of the Bow River? You can ignore the small slivers repeated by minor thrust faults.
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| 3f. Photo Problem 3f shows a zoomable photo of a reverse fault near Barrier Lake, Canada. What key characteristic shown in the photo helps you to recognize that a fault is in the photo?
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| 3g. Check and double-click placemark Problem 3g to fly to the San Andreas fault zone near the Carrizo Plain in California. This transform plate boundary is also a strike-slip fault. Given the offset stream in the view, what is the sense of offset along this fault?
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Ductile Structures-Folds
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| 4a. Folds are classified by a wide range of names, depending on their geometry: anticlines, synclines, domes, basins, and monoclines. The following questions will take you to locales that provide examples of many of these remarkable structures.
Check and double-click placemark Problem 4a to fly to the Zagros Fold Belt in Iran. What type of fold is this? Hint: Look at the flat irons on the fold limbs.
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| 4b. Check and double-click the placemarks for Problems 4b-i and 4b-ii. Folds are said to be upright in areas where their fold hinge is approximately horizontal (bedding contacts also tend to be straight and parallel to the main fold hinge) and plunging in areas where their fold hinge is inclined into the ground (bedding contacts also tend to curve and wrap around from one fold limb to another). Which of the following is correct?
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| 4c. For placemark Problem 4c, which direction is the fold plunging?
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| 4d. Check and double-click placemark Problem 4d to fly to Sheep Mountain in Wyoming. Turn on the “Sheep Mt. Geologic Map” overlay. Which of the following best describes the area around placemark Problem 4d?
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| 4e. Which direction is the fold at placemark Problem 4e plunging and what type of fold is it?
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| 4f. Which direction is the fold at placemark Problem 4f plunging and what type of fold is it?
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| 4g. What name best describes the landform that placemark Problem 4g points to? You might turn on the “Geologic Map of Quail Creek SP, UT” map overlay to assist your interpretation.
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| 4h. Which direction do the beds at placemark Problem 4h strike?
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| 4i. Which direction do the beds at placemark Problem 4i dip?
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