Structural Features in the National Parks

Folds and Faults

OBJECTIVES

• Describe the types of stress that exist within the Earth’s crust
• Explain how rocks respond to stress
• Connect stress and strain to plate tectonic boundaries
• Understand the types of folds and faults

SUPPLIES

• silly putty, rubber bands, popsicle sticks, wooden blocks.

INTRODUCTION

The Earth is an active planet shaped by dynamic internal forces. The internal heat convecting within the plastically solid mantle breaks and moves the thin surface plates. These forces push plates upward forming mountains (the Himalayans), downward producing deep ocean trenches (the Marianas trench) and spreading to form new ocean floor (Mid-Atlantic ridge) and rifts (East African Rift) (figure 1). The cause and movement of the surface plates or crust of the Earth is understood within the unifying Theory of Plate tectonics; first developed and presented by Alfred Wegener in the early 1900’s, but not fully understood and accepted until the mid-1970’s. The movement of the Earths crustal plates creates mountains, moves continents, causes earthquakes, tsunamis and volcanoes. In the process of these movements tremendous amounts of force act on the rocks. Rocks under dynamic forces, respond by stretching, folding and breaking. This deformation can produce hazardous, dramatic and beautiful scenery (figure 2). Observing and understanding these geological structures helps us determine the movement of plates in the past and future, allowing us to build structures such as bridges, power plants and skyscrapers safely. Structural Geology is the study of how the Earth’s plates move and are deformed, often in a 3-dimensional manner. Structural geologists help us understand the formation of folds in mountains and faults that produce earthquakes. Understanding these processes and their impact on Earth materials is an essential skill for engineers, energy production, seismologist, hydrologist and many more. How the rocks respond to these forces is the focus of this lab activity.

Figure 1. Artist’s cross-section illustrating the main types of plate boundaries. Cross-section by Jose F. Vigil from this Dynamic Planet – a wall map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory. Image: Public Domain https://pubs.usgs.gov/gip/dynamic/Vigil.html

Figure 2. Folded Granite with veins on Mount Rushmore National Memorial, South Dakotas Black Hills. Intense plate tectonic forces folded this rock unit that form the crystalline core of the oldest rocks in South Dakota. Image: NPS https://www.nps.gov/moru/learn/nature/geologicactivity.htm

Crustal Deformation: - Stress

Rocks are subject to stress mostly related to plate tectonic forces acting on them. When stress is applied rocks change and deform. Stress associated with simple burial of rocks is relatively equal in all directions. This is referred to as lithostatic stress and generally acts to compact the rocks into denser forms; for example, sedimentary shale being compressed into low grade metamorphic slate (figure 3).

Figure 3. When stresses are equal in all directions such as from simple burial of sediments and mostly sedimentary rocks such as shales and mudstones the forces are called lithostatic stress. Note equal size of stress arrows. Image: https://opentextbc.ca/geology/wp-content/uploads/sites/110/2015/07/image004.png

When stress is greater in one direction over another such as from mountain building forces, the rocks can deform in different ways depending on the strongest direction. Where plates are diverging the rocks are being thinned and pulled apart, this produces tensional stress. Where plates converge the rocks are shortened, compacted and squeezed, this produces compressive stress. At transform boundaries the plates move sideways in opposite directions producing shearing stress (figure 4). Stress is usually designated by vector arrows indicating the direction and strength. Longer vector arrows indicate greater force.

Figure 4. Three stress types acting on rocks. Tensional, is a pulling stress, compressional stress results from converging forces and shear stress is in opposite directions. Image USGS.gov. https://opengeology.org/textbook/9-crustal-deformation-and-earthquakes/. https://earthquake.usgs.gov/learn/glossary/images/stress_types.gif

Tectonic Plate Boundaries combined with Stress type

Forces and their corresponding stress type occur at specific plate boundaries. Look at the plate boundaries map and connect the force direction with the stress type (figure 5).

CIRCLE LOCATIONS OF THE THREE STRESS TYPES ON THE MAP BELOW.

Tensional – Compressional – Shear – Connect them to Plate boundary type.

Figure 5. Plate tectonics map. Image: Wikimedia Commons Eric Gaba (Sting - fr:Sting), CC BY-SA 2.5 <https://creativecommons.org/licenses/by-sa/2.5>, via Wikimedia Commons

Strain

Strain is the corresponding response of the rock to applied stress. As rocks first respond to stress they initially behave elastically.

An elastic strain response is not permanent, like pulling a rubber band then letting go. When the force (stress) is removed the rock returns to its original shape. If the forces continue over time the original rock may move past an elastic response and plastically deform.

Plastic deformation is permanent, like pulling on warm taffy or gum. When rocks bend, they form folds.

If conditions exceed the overall strength of the rock the strain response is brittle, and the rock breaks or fails. A brittle response produces faults (figure 6).

Figure 6. Deformation leading to plastic then brittle response. a) undeformed state, b) accumulation of elastic strain, and c) brittle failure and release of elastic strain. The black arrows represent the direction of the stress forces. Image: Opengeology.org CC BY S.A.

Your instructor has provided you with rubber bands, Silly putty and wooden sticks. Manipulate each of the three materials to experience the three strain responses. Use figure 7 to relate the materials.

LET’S PRACTICE:

ELASTIC, PLASTIC AND BRITTLE RESPONSES IN MATERIALS

Vary the rate and direction of the forces. Note how they respond.

Can you make the Silly putty and the stick behave elastically, plastically and then brittlely?

How did you vary your conditions - stress?

Figure 7. Different materials deform differently when stress is applied. Material A has relatively little deformation under large amounts of stress before fracture. B elastically deforms then brittlely fails. C significantly deforms plastically then fails brittlely. Image opentextbc.ca CC BY S.A. S. Earle

PLASTIC FOLD RESPONSE

When a rock is compressed and behaves plastically is can produce folds. This typically occurs when the rocks are warm, deeply buried, weak, and the forces are slow and spread over a long period of time. An upward fold is called an Anticline, and a downward fold is a Syncline. The plane that splits the fold into two halves is the axial plane. Folds can be symmetrical, even on both sides, or asymmetrical having one limb more steeply dipping than the other (figures 8 & 9).

Figure 8. Examples of Symmetrical, Asymmetrical and Overturned folds. Image CC BY S.A. 4.0 international license S. Earle. Opentextbc.ca/physicalgeology2ed/

Figure 9. Left-Folded sedimentary Limestone plastically metamorphosed into Marble created this striped pattern in the canyon walls of Marble Canyon, Death Valley National Park, California and Nevada. Right-Yosemite National Park plastically folded Gneiss. Image: Left NPS Dan Kish https://www.nps.gov/subjects/geology/metamorphic.htm. Right NPS John P. Lockwood https://www.yosemite.ca.us/library/geologic_story_of_yosemite/images/24-b.jpg

BRITTLE FAULT RESPONSE

A rock that behaves in a brittle manner (breaks) produces faults (figure 10). The fault types are unique to the specific directional stress. Fracturing or failure usually occurs when rocks are cold, or near the surface, and the forces applied are strong and occur over a shorter period of time. Fault type is specifically unique to the direction or type of forces that act on the rocks.

Figure 10. Faulting of Migmatites (mixed rock – a combination of Igneous and Metamorphic) white veins show offset. This is a left-lateral fault movement from a road-cut at the Diablo Lake Overlook in North Cascades National Park along Highway 20. Image: Dave Tucker, NW Geology Field Trips. https://nps.maps.arcgis.com/apps/MapJournal/index.html?appid=8e33939d87a645a7bd4587fa13452d18

Rocks experiencing Compressional stresses at convergent plate boundaries can produce Reverse faults. Rocks experiencing Extensional, pulling stress at Divergent boundaries can form Normal faults. Rocks that move past one another at Transform boundaries can produce Strike slip faults (figure 11).

Figure 11. The three main fault types with motion. Image CC BY S.A. 4.0 International license S. Earle. Opentextbc.ca/physicalgeology2ed/

LOOK AT THE IMAGES AND DETERMINE IF THE RESPONSE IS BRITTLE OR PLASTIC. Draw in the stress arrows.

Figure 12. Determine if the above images show Plastic or Brittle responses. Image:1 Ernest Tinaja https://gotbooks.miracosta.edu/gonp/bibe/html2/bb177.htm 2 Tabor et al. 2009 https://nps.maps.arcgis.com/apps/MapJournal/index.html?appid=8e33939d87a645a7bd4587fa13452d18 3NPS Public Domain https://www.nps.gov/media/photo/gallery-item.htm?pg=2122026&id=26B47B48-155D-451F-67D53839DDC9DC7B&gid=26B049C4-155D-451F-67633305F75A85B5 4 USGS J Delano & Mark Zellman https://www.nps.gov/grte/learn/nature/faults.htm

USE THE WOODEN BLOCKS TO CREATE NORMAL, REVERSE AND STRIKE-SLIP FAULTS

MAKE DRAWINGS OF THE THREE FAULT TYPES WITH DIRECTIONAL ARROWS.

Geologic time

Rock type

WHAT TYPE OF FAULT IS THIS?? Draw in the direction of stress.

Figure 13. Ketobe Knob, in the San Rafael Swell, Utah. Image CC BY NC S.A. 4.0 international R. Schott https://www.flickr.com/photos/rschott/814080386/

TABLE 1. FILL-IN. connections between Strain – Plate Boundary Type and Fold or Fault. Find a location on the plate tectonics map below where each could be found.

Figure 14: Plate Map. Image: Wikimedia Commons Eric Gaba (Sting - fr:Sting), CC BY-SA 2.5 <https://creativecommons.org/licenses/by-sa/2.5>, via Wikimedia Commons

Dante’s view in Death Valley National Park, California is producing a basin and range landscape due to the diverging mountains. What type of forces and faults are expected at this national park?

Image NPS Public Domain Dale Pate. https://www.nps.gov/subjects/geology/plate-tectonics.htm

Using the following images from several National Parks, try to identify the plate tectonic type, force directions and folds or fault that would be or have been produced.

Katmai National Park and Preserve, Alaska. Image NPS Public Domain. https://www.nps.gov/subjects/geology/plate-tectonics-convergent-plate-boundaries.htm

Shenandoah National Park, Virginia. Part of the Appalachian Mountains. Image NPS Public Domain. https://www.nps.gov/subjects/geology/plate-tectonics-collisional-mountain-ranges.htm

The Great Smoky Mountains National Park, North Carolina and Tennessee. Part of the Blue Ridge Province. Image NPS Public Domain https://www.nps.gov/subjects/geology/plate-tectonics-collisional-mountain-ranges.htm

Image represents the San Andreas Fault region that is home to several National Parks regions: Point Reyes National Seashore, Golden Gate National Recreation Area and Pinnacles National Park. Nearby parks include The Redwoods National and State Parks to the North, Yosemite, Kings Canyon and Sequoia National Parks to the East and Joshua Tree to the south.

Image USGS. Public Domain. https://www.nps.gov/subjects/geology/plate-tectonics-transform-plate-boundaries.htm

The Granites found in Yosemite are similar to the rocks found in Point Reyes National Seashore, California and were transported 300 miles northwest along the San Andreas fault. Image USGS Public Domain. https://www.nps.gov/subjects/geology/plate-tectonics-transform-plate-boundaries.htm

Grand Tetons Range, Colorado. Image: NPS Public Domain Image https://www.nps.gov/subjects/geology/plate-tectonics-continental-rift.htm By Jon Sullivan, PD Photo. - PD Photo, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3537847