Describe the features created by glacial erosion that you might see in an area where valley glaciers recently existed. Explain how each feature forms.

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Course Learning Outcomes for Unit III Upon completion of this unit, students should be able to:

5. Demonstrate how earthquakes and volcanoes are driven by various geological forces. 5.1 Summarize the evidence to support Wegener’s continental drift hypothesis, as well as the

evidence that led to the theory of plate tectonics. 5.2 Describe the different interactions of lithosphere plates and how these interactions lead to

seafloor spreading, the movement of continents, and mountain building. 5.3 Explain the causes and effects of earthquakes and how they are measured.

Reading Assignment Chapter 5: Plate Tectonics: A Scientific Revolution Unfolds Chapter 6: Restless Earth: Earthquakes, Geologic Structures, and Mountain Building United States Geological Survey. (2014). Earthquakes with 1,000 or more deaths 1900-2014. Retrieved from

http://earthquake.usgs.gov/earthquakes/world/world_deaths.php United States Geological Survey. (1999). "Hot spots": Mantle thermal plumes. Retrieved from

http://pubs.usgs.gov/gip/dynamic/hotspots.html#anchor19316266 In order to access the resource below, you must first log into the MyCSU Student Portal and access the General OneFile database within the CSU Online Library. Sections of San Andreas Fault in San Francisco area are locked up. (2014, October 22). Space Daily.

Unit Lesson

On March 27, 1964, the largest earthquake ever recorded in the United States (measuring 9.2 on the Richter scale) hit the Prince William Sounds region of Alaska. This earthquake hit at about 5:30 p.m. local time and lasted about 4.5 minutes (United States Geological Survey [USGS], 2015). Although 120 kilometers away, Anchorage sustained tremendous damage. Tremors were felt as far away as Seattle, Washington. The earthquake triggered landslides, tsunamis, and the uplift and subsidence changed the shape of the region. Earthquakes are dramatic surface manifestations of the intense pressure that can build deep within the Earth’s crust. Beneath the Earth’s surface, plates are in constant movement due to the transference of energy by heat deep within the Earth. These processes, though not visible, shape our planet. Geologists refer to these as internal processes because they are caused by the activity of

UNIT III STUDY GUIDE

Plate Tectonics and the Restless Earth

Damage to Fourth Avenue, Anchorage, Alaska, caused by the Good Friday Earthquake. (U.S.Army, 1964)

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internal layers of the Earth, mainly the asthenosphere and lithosphere. The study of the interaction of these layers is called plate tectonics. Plate tectonics is a relatively new theory. Prior to the 1960s, geologist viewed the position of oceans and continents as fixed. Although the hypothesis of continental drift was introduced in 1915, there had been little evidence to explain how continents moved. Wegner was the first to suggest a single supercontinent of all the Earth’s land: Pangea (see figure 5.2 p. 153). The breaking of Pangea is apparent when you notice how the coastlines of South America and Africa seem to fit together like puzzle pieces. Other evidence to support Pangea includes the identical fossils that are found on different continents and mountain belts that continue across the ocean. Following World War II, oceanographic exploration began in earnest, leading to the discovery of the ocean ridge system. Oceanographers eventually discovered that new crust is constantly being formed and recycled as the lithosphere moves across the hotter mantel (asthenosphere). The theory of plate tectonics describes how the lithosphere is broken into seven rigid plates, which can move apart, come together, or slide past each other. It is these movements that are responsible for mountain building, earthquakes, and volcanic activity. New ocean crust is constantly being formed at ocean ridges. Here, the molten rock from the asthenosphere surfaces and pushes the lithosphere apart, causing the ocean floor to spread. A similar process can take place below the continental crust, forming a rift valley (see Fig. 5.15 in your textbook). As the continent spreads, a trench will form between the two land masses, eventually forming a sea. If this continues, an ocean will form and the two land masses will become separate continents. As the seafloor spreads, the older crust cools and becomes denser. When the older seafloor collides with the continental crust, it will descend below the more buoyant crust and melt into the mantle, recycling the older oceanic crust (there is no oceanic crust older than 180 million years old). Often, as this crust reaches the mantle and begins to melt, it will trigger the upwelling of magma forming volcanoes. This can form either on land or in the oceanic crust (see Fig. 5.17, p. 165 in your textbook). The transform plate boundaries are where plates slide past each other. With transform boundaries, lithosphere is neither created nor destroyed, but fracture zones can form. As the plates slide past each other, they form faults (fractures in the Earth’s crust). Theses fault zones often occur in oceanic crust, but they can also occur on continental crust (the San Andreas Fault is an example of this). Often, the faults get “stuck” as they slide past each other. As pressure continues to build, the stuck faults will eventually break away, causing an earthquake. An earthquake is the “sudden and rapid movement of one block of rock slipping past another along fractures in the Earth’s crust” (Lutgens & Tarbuck, 2014, pg. 190). Where the movement occurs is known as the focus (or hypocenter). The point on the Earth’s surface directly above the focus is termed the epicenter (see Fig. 6.2, p. 191 in your textbook). Geologists can determine the focus by measuring the seismic waves that are released during an earthquake using a seismograph. Seismic waves can either be P waves, which compress and expand the material through which they pass (like a slinky), or S waves, which are more like the waves in the ocean (see Fig. 6.10, p. 196 in your textbook). Since these waves travel at different rates, geologist can determine the epicenter by how far apart these waves occur (they will be close together at the epicenter). Since the Earth’s plates are in continual motion, pressure often builds along fault lines. If this pressure is released often, through creeping fault, there is little concern for catastrophic earthquakes. However, where these faults become locked, pressure will continue to build over time. Eventually the pressure overcomes the strength of the rocks and an earthquake is produced. The damage produced by an earthquake depends on several factors: the intensity (strength) of the earthquake, the duration of the earthquake, the nature of surface materials (loose materials will vibrate more), the nature of the building materials, and construction practices. Geologist measure the strength of an earthquake by studying both the intensity and the magnitude. Intensity can be estimated by comparing the damage produced by an earthquake, while magnitude measures the energy released. The most common scale used to measure magnitude is the Richter scale, which measures the seismic waves produced during an earthquake. The Richter scale is based on logarithmic scale, so the difference between a 6.0 and a 7.0 earthquake is actually a difference of 10x more energy released. Because the damage produced by an earthquake can vary greatly depending on both geologic substrate and man- made structures, intensity is not a very reliable measurement. However, a magnitude reading does not take

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into account secondary effects of an earthquake such as landslides, fires, and tsunamis, which can make a moderate earthquake disastrous. The shifting of plates does not always lead to earthquakes. Sometimes those pressures result in the breaking or deformation of rocks. As plates press against each other, extreme pressure leads to uplift, folding, or breaking of rock. When pressures are high, deep below the Earth’s surface, rocks will deform rather than break. The overlying pressure confines the rock, which causes it to act more like hot wax or metal. Although this folding happens deep within the surface, uplift, and erosion will eventually cause these unique formations to surface. Folding can create either an anticline (folded upwards) or a syncline (pointing downwards) (see Fig. 6.34, p. 213 in your textbook). Large uplifts of rocks can also create domes, such as the Black Hills of South Dakota. Basins are formed when downwarping occurs (Lutgens & Tarbuck, 2014). At the surface, where there is less overlying pressure, the process of convergence leads to fractures in the rock (faults). Faults can often be identified where rocks are pushed up or down, relative to each other. Examples of these can be seen in Fig. 6.38 in your textbook. On a large scale, the Basin and Range (discussed in Unit II), were formed by uplifted fault blocks (horsts) and down-dropped blocks (grabens) (Lutgens & Tarbuck, 2014). Processes that form large mountain chains are a little more complex and are often a result of rising magma that melts at a subduction zone. Island Arc Mountains (such as Japan) form where two plates of oceanic lithosphere collide. As the older, denser plate descends below the other, it reaches the asthenosphere and begins to melt to form magma. This magma will then rise to the surface in underwater volcanoes and eventually grow large enough to create land masses (Fig. 6.41, p. 218 in your textbook). Sometimes, a mantle plume will form beneath the oceanic crust. This results in a hot-spot, releasing magma. This volcanic activity will eventually create an island, and as the plates slide over the hot spot, new islands will form. The Hawaiian and Aleutian Island chains are examples of this. A similar process takes place along the edge of continents, where oceanic lithosphere descends below the more buoyant continental crust. This results in volcanic activity along continental margins (like the Andes Mountains). Another mountain-building process occurs when two continents collide. This also begins with a subduction zone of oceanic lithosphere descending beneath continental crust. As subduction continues, the intervening seafloor becomes narrower. With time the seafloor disappears completely and the two continents collide. When two continents collide, subduction does not occur. Since continental crust is more buoyant than oceanic crust, the collision results in the folding and uplift of both landmasses. Because continents are equally buoyant, they collide and push each other up to form mountains. The formation of mountains is often referred to as orogenesis. In light of the theory of plate tectonics, geologists can now explain the internal processes that lead to the development of the Earth’s unique landscapes. Once you understand these basic concepts, you will also start to recognize the incredible forces that are shaping our Earth.

References

United States Army. (1964). Fourth Avenue, Anchorage, after 1964 earthquake. Retrieved from http://vilda.alaska.edu/cdm/singleitem/collection/cdmg21/id/9648/rec/1

United States Geological Survey. (2015). The great Alaska earthquake and tsunami of March 27, 1964

[Photograph]. Retrieved from http://earthquake.usgs.gov/earthquakes/events/alaska1964/ Lutgens, F. K., & Tarbuck, E. J. (2014). Foundations of Earth Science (7th ed.). Upper Saddle River, NJ:

Pearson

Suggested Reading The links below will direct you to both a PowerPoint and PDF view of the Chapter 5 and 6 Presentations. This will summarize and reinforce the information from these chapters in your textbook. Click here to access the Chapter 5 PowerPoint Presentation. (Click here to access a PDF version of the presentation.)

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Click here to access the Chapter 6 PowerPoint Presentation. (Click here to access a PDF version of the presentation.) These web resources provide more information about tectonics: United States Geological Survey. (2014). Understanding plate motions. Retrieved from

http://library.usgs.gov/photo/#/item/51db44f5e4b02290dff9f205 United States Geological Survey. (2014). Plate tectonic animations. Retrieved from

http://geomaps.wr.usgs.gov/parks/animate/

Learning Activities (Non-Graded) In this non-graded learning activity, we will look at how earthquakes are measured. Recall from the unit lesson that Intensity measures the damage caused by an earthquake, while magnitude measures the strength of the earthquake. In many cases, earthquakes of similar magnitude may have very different damage. This can be caused by the building structures, population, and side effects such as landslides, fires, and tsunamis. Research: Go to the USGS website listed below and select two of the world’s deadliest earthquakes (the year is not important) of similar magnitude with at most a difference of 0.2. (For example, earthquake A is 5.1 and B is 5.3). Remember that with logarithmic scales even small differences in numbers are usually large in reality—that is, an earthquake of magnitude 6 is 10 times that of magnitude 5. http://earthquake.usgs.gov/earthquakes/world/world_deaths.php Summarize each earthquake, include the Richter scale rating, year, and location of each, and note the damage caused. Compare each earthquake in terms of the damage caused and look at the factors that might have contributed to the damage. Why were these earthquakes so deadly (tsunami, fire, poor construction, etc.)? Identify other factors that likely caused the differences in destruction. What additional factors could have led to higher destruction in one area versus another? Non-graded Learning Activities are provided to aid students in their course of study. You do not have to submit them. If you have questions, contact your instructor for further guidance and information.