Activity 10.2: Visualizing How Stresses Deform Rocks A.Visualize all stresses or
ID: 235200 • Letter: A
Question
Activity 10.2: Visualizing How Stresses Deform Rocks
A.Visualize all stresses or forces involved in the following deformations &complete the table below, checking any which apply.
.
Object/Action
Confining Pressure
Directed Pressure
Compression
Tension
Shear
1.Smashed Carton
2. Sealed Soda Can
3. Stretched elastic
4. Rubbed hands
B. Illustrate these block diagrams as per instructions in 1-3 below.
a. _____________________ b. _______________________ c. _____________________
1. On the Figure above trace on the stress directions as arrows on each of the 3 folded and faulted strata. (6)
2. On each of the 2 sides of the fault blocks in the bottom row of block diagrams, put the appropriate symbols for the relative sense of fault motion across the fault. When you view a strike skip fault in cross section, the side that comes towards you is labelled with a bullseye (circle with a dot in the middle) the side that moves away is labelled with an x inside a circle. On the map view of the dip slip faults, put a U on the up-thrown side and a D on the down-thrown side. (6)
3. Below each of the 3 fault types, label the appropriate type of plate margin where these structures tend to occur: Transform/Strike-Slip, Convergent/Subduction, Divergent/Rift. (3)
C.1 All of the above diagrams a, b & c, the stresses dominantly seem to act in the horizontal plane, with a direction parallel to the Earth’s surface. In each case there is also a vertical force. Name this force. ________ _____________________________________________________________________________________ (1)
2. For each of the 3 settings, explain how the vertical force manifests and describe what part of the deformation in the block diagrams it aids or opposes.
a. ___________________________________________________________________________________ (2)
b. ___________________________________________________________________________________ (2)
c. ___________________________________________________________________________________ (2)
Activity 10.3: Map Contacts and Geological Formations
To make geological maps or cross sections and to trace out structures, we need something to trace out or to correlate. The fundamental unit of stratigraphy and mapping is called the formation. A formation must be laterally extensive, thick enough to show up on a map and generally consists of a related stack of beds or lithologies which contain fossils of the same age and a related set of environmental indicators. Formations are organized into Groups and Supergroups. Formations usually are a several metres to hundreds of metres thick and represent a few million to many tens of millions of years of geological time.
A. The photo below is near the leading edge of the Rocky Mountains in Western Montana. The Rockies formed in Late Cretaceous through Early Tertiary time because of rapid convergence between the Farallon Plate (former Eastern Pacific seafloor) and North America. These forces were so great that mountains were built from older rocks > 1000 km east of the subduction margin! Nearby, this same structure turns into the Lewis Thrust.
1 On the image below of Scapegoat Mountain trace in the upper and lower bedding plane contacts for the cliff forming Lower Paleozoic limestones entirely across the width of the photo. __________________________ (2)
2. What is the geological name for this kind of structure? (two words) __________________ ___________ (2)
3. What kind of stresses formed this structure? ________________________________________________ (2)
4. Draw in arrows for the relative directions of those stresses above and below the photo. _______________ (2)
West East
B.1 Draw in the contacts for the base of the buff coloured Cambrian Muav Limestone near the base of the canyon and the base of the white coloured Pennsylvanian Watahomigi sandstone near the top of the plateau on the oblique air photo above. _______________________________________________________________ (4)
2. Colour the region between the 2 contacts on your map above red, and above the Watahomigi as Blue.___ (2)
3. Look at your geological map here and the related images and topographic map in the manual on p. 275 to decide whether the Watahomigi Formation is deformed into a geological structure to give it this shape or whether it is flat lying. Choose one: a) Deformed b) Flat Lying (2)
4. Explain your reasoning for the shape you found and why you interpreted its origin this way. ____________ _______________________________________________________________________________________ ______________________________________________________________________________________ (2)
Object/Action
Confining Pressure
Directed Pressure
Compression
Tension
Shear
1.Smashed Carton
2. Sealed Soda Can
3. Stretched elastic
4. Rubbed hands
Explanation / Answer
Smashed carton: Compression causes the carton to sqeeze and reduce in volume i.e. smashed.
Sealed sodacan : Confining pressure will operate in sealed sodacan.
Stretched elastic: in this tension is applied which causes the elastic to stretch.
Rubbed hands: Shear stress is applied which causes the hands to move parallel to each other
B1.In faulted strata if the fault is thrust fault than the stress direction will be in opposite direction i.e compression toward the fault. But if the fault is normal it will be in opposite direction but away from the fault i.e. extension.
3. In transform fault the fault block move sideways parallel to each other and these faults are known as strike slip faults.
In subduction the fault block move towards each other known as thrust fault.
In Divergent settting fault block move away from each other known as normal fault.
C1. Vertical force is confining pressure the pressure due to weight of overlying material.
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