LAB #2-PLATE TECTONICS 2

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Feb 20, 2024

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R ST 4 R W . S e I3 ——— LAB #2—PLATE TECTONICS Name: L Day & Time: PURPOSE AND OBJECTIVES: This activity explores the geological evidence that was used to identify the processes that occur along the three plate tectonic boundaries (margins)—transform, divergent, and convergent. This activity focuses on the offset of geological formations along the San Andreas fault, the formation of seafloor magnetic anomalies, the distribution of earthquakes, and the rates of plate movement. KEY TERMS AND CONCEPTS: 3 * Plate Tectonics e San Andreas (transform) fault [ e Plate boundaries: e Fault offset, annual slip rate | Transform (strike-slip faulting) e Focal depth, epicenter i Divergent (seafloor spreading) e Radiometric age dating Convergent (subduction) o Volcanic ash, plutonism e Magnetic anomaly (N-R strips) e Oceanic vs. Continental crust e Magnetic Reversal o Benioff zone L e Asthenosphere BACKGROUND The theory of plate tectonics continues to evolve through evidence from many disciplines of earth and life sciences. Although the mechanism that drives the Earth’s plates is still not fully defined, the evidence that the plates have drifted tens of thousands of kilometers over millions of years is indisputable. Figure 2.1 s a generalized plate tectonic map showing the major plate boundaries around North and South America. Numbers 1, 2, and 3 refer to locations of the three plate boundaries that you will examine in this activity. TRANSFORM BOUNDARIES: SLIP RATE ALONG THE SAN ANDREAS FAULT This exercise uses Google Earth, Google Earth overlays created by various members of the scientific community, and information from the USGS website, including seismic data and maps. Be for you begin, be sure that you have all of the .kmz or .kml files downloaded from the Assignments -> Labs -> Lab2-Plate Tectonics assignment. You can save these files locally on your computer or use Google Drive to connect to these layers when using Google Earth on a smart phone or tablet it is strongly encouraged that you use a computer with Google Earth Pro installed on it*. Once you have all files, you will start by opening up Google Earth. You may drag and drop these _kml and .kmz files into Google Earth Pro. Take some time to explore what these files are displaying. Also take note that the “2.5_month_depth_animated_link.km!” and “1.0_week_depth_animated_link.kml” files perform a live query from the USGS’s Earthquake Hazards Program webpage and update every minute. Reference the following link to assist with your data exploration: https://earthquake.usgs.gov/earthquakes/feed/v1.0/kml.php.

LAB #2—PLATE TECTONICS Adding the “plate-boundaries.kmz” will show the boundaries between tectonic plates on Earth Find California and note the red line running along the eastside of the state. This is the 5an Andreas Fault which separates the Pacific plate (to the west) from the North American Plate (to the east). From here you will need to use the search for two locations. The first is Pinnacles National Park. The second is Neenach Volcanic Formation (likely shown as Neenach Volcano on Google Earth). On the Google Earth Pro toolbar, find the icon that looks like a yellow thumbtack. When hovering over this icon, it states “Add Placemark”. You will need to create, label, and drag two markers onto your Google Earth Pro map. The first marker should go on Pinnacles National Park and the second should go on Neenach Volcano. Your map should look similar to Figure 2.2 (page 7). Geological analyses indicate that the rocks these two locations are identical in chemical composition and age. Therefore, they are believed to represent the same formation. However, the two units lie on opposite sides of the San Andreas Fault and are separated by hundreds of kilometers. You will now examine these distances and relative plate motions to calculate how quickly these plates are moving past each other. 1A) Examine arrows on both sides of the San Andreas Fault that show the sense of movement. You may need to examine other portions of your Google Earth Pro map to get a sense of movement along the San Andreas. If Pinnacles National Park and Neenach Volcanic Formation were originally close together, does the sense of motion make the San Andreas a right lateral or a left lateral fault? Which compass direction is the Pacific Plate moving relgtive to the North American Plate? O POV In Tne& same A AYYows Af ’ e q —. PY 9 1B) Having created your placemla'rkei's, use the “Show Ruler” button to measure along the San Andreas to determine how far the formations have been offset. Using the conversion factor setup below, calculate the offset in kilometers and miles. You will note that you can easily change from “Yankee units” to “SI units” at the touch of a button in Google Earth Pro; however, you should be familiar with how to make a conversion from miles to kilometers (hint-hint, wink- wink, nudge-nudge). mile= |.{ Tl L 2l o« Offset = mi x 1km/0.62mi = S50 km ( 1C) Rocks from both formations have been radiometrically dated at 25 million years old. Using this age and the km offset you determined in 1B, calculate the average movement per year (annual slip rate) in centimeters for the San Andreas Fault during this time period (note that there are 100,000 cm in 1 km). Show your work. Remember: Rate = Distance/Time A Vet

1D) Measure the straight distance from San Francisco to Los Angeles. Using your measure stance, if the San Andreas Fault continues to move at the slip rate you determined in 1C many years will it be before Los Angeles becomes a suburb of San Francisco? (it is advisable th ) . , that you use cm in your answer; so, your final answer should come out as cm/yr Rate = Distance/Time Time = Distance/Rate DIVERGENT BOUNDARIES: RATES OF SEAFLOOR SPREADING FROM MAGNETIC ANOMALY PATTERNS ALONG THE MID-ATLANTIC RIDGE Carefully examine Figure 2.3, which is a map of “magnetic anomalies”; these are stripes of “normal” and “reversed” magnetic polarity that have been identified in the seafloor on either side of the Mid-Atlantic Ridge (MAR), which is indicated by a heavy black line on Figure 2.3. Along MAR, active seafloor spreading is taking place. West (left) of the MAR, new oceanic crust is being added to the South American Plate and to the east (right), it becomes part of the African Plate. Since there is no convergent boundary on either side of the Atlantic Ocean, the Atlantic Ocean is growing wider and the two continents are moving apart. Note that the MAR is offset by a series of fracture zones (abbreviated F.Z.). Each fracture zoneis a transform fault that offsets the ridge to accommodate differential rates of spreading. Along the Mid-Atlantic Ridge, basaltic magma rises from the mantle to the surface where it solidifies, forming new oceanic crust. As the basalt cools, crystals of magnetite acquire a magnetic orientation parallel to that generated by the Earth’s magnetic field from the outer core. Magnetite in modern oceanic crust has a “normal” magnetic orientation towards the north pole, just as a compass does. But, numerous times in the past, Earth’s magnetic polarity has switched: the south pole becomes “north” and the north pole becomes “south.” Basalts that erupted during those times have “reversed” magnetic orientations. The pattern of magnetic polarity anomalies (Normal-Reverse-Normal stripes) on the ocean floor was one of 23

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D BRI T e B S T Q. . e i o BES ~ SRl e R g e T NS SEE M. . YRR T LAB #2—PLATE TECTONICS Table 2.1 Magnetic Anomalies Across the Mid-Atlantic Ridge #5 2A) Distance Between Anomalies (km) 28) Age (m.y.) 20) Spreading Rate =2A/28B (km/m.y.) 2D) Setup Conversion calculations (write answers in column 2E) km x 100,000 / age in years = 2E) Spreading Rate (em/yr) #8 O #13 Average Rate = Average Rate =

T TR I & W——— v 5 LAB #2—PLATE TECTONICS "USING EARTHQUAKES TO IDENTIFY PLATEBOUNDARIES 3A) On the map below, connect the dots (as exactly as you can) ) that indicate where plate boundaries occur at Earth’s surface. Then label the East Pacific Rise, Galapagos Rise, Chile Rise, and all of the tectonic plates in this region of the world (refer to your Google Earth Pro map and downloaded layers for assistance in labeling these areas; Figure 2.1 [page 6] will also help). Guyana Vehezuela = e 500 1000 km Map of Earthquake Activity in the Eastern Pacific Ocean and South America @ Shallow-focus @ Intermediate-focus ® Deep-focus > Trench earthquakes earthquakes earthquakes 0-69 km deep 70-299 km deep 300-700 km deep (Data from U.S. Geological Survey)

— LAB #2—PLATE TECTONICS 3B) Notice line B-B’ on the map in the map above (page 10), as well as the fact that shallow, intermediate, and deep earthquakes occur along it. Volcanoes also occur at Earth’s surface along this line. Plot the locations of earthquake foci (depth of earthquake vs. its location east or west of the trench) on the cross section below using data in the accompanying table, Table 3.1, (provided by the U.S Geological Survey). Table Location East Depth of Location East Depth of Location East Depth of or West of Earthquake (or or West of Earthquake (or or West of Earthquake (or Trench volcano location) Trench volcano location) Trench volcano location) 200 km West 20 km 220 km East 30 km 410 km East 50 km 160 km West 25 km 250 km East volcano 450 km East 50 km 60 km West 10 km 260 km East 120 km 450 km East 150 km 30 km West 25 km 300 km East volcano 470 km East 180 km 0 (trench) 20 km 300 km East 110 km 500 km East 30 km 10 km East 40 km 330 km East volcano 500 km East 160 km 20 km East 30 km 330 km East 40 km 500 km East 180 km 50 km East 60 km 330 km East 120 km 540 km East 30 km 51 km East 10 km 350 km East volcano 590 km East 20 km 55 km East 30 km 390 km East volcano 640 km Eas} 10 km 60 km East 20 km 390 km East 40 km 710 km East 30 km 80 km East 70 km 390 km East 140 km 780 km East 530 km 100 km East 10 km 410 km East volcano 800 km East 560 km 120 km East 80 km 410 km East 25 km 820 km East 610 km 200 km East 110 km 410 km East 110 km 880 km East 620 km o1 vitnVe A\ (a TR X % B West Distance East or West of Trench (km) East B 200 100 Trench 100 200 300 400 ,500 600 700 800 900 TR IR IR ) 7 fal N v ¢ < 6 gl ¢ u 0 100 —ee ; — @ & € 1 @ 1 == ~@ ¢ 2 200 s = \ 3 g \ E 300 ~ o} \ ! w \ | Gupaucity S ) beatr¢ £ 400 ) - S \ 7 \ a \

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LAB #2—PLATE TECTONICS 3C) What kind of plate boundary is shown in your cross section? VA M‘*l'\\ \r\{ 3D) Draw a line in the cross section to show the probable top surface of the subducting plate. ]“\L.“'\ ",(‘\‘\. ;\‘\ //‘Q 3E) Label the part of your cross section that probably represents earthquakes in the lithosphere. \ ® o " P (efee A0 S 3F) At what depth does magma probably originate here Just above the subducting plate? ne NG vl NN NGNS Lromn Fust alosve Wbdve OO=\S0 ¥M beravse V1% bbing heated (red a5 | T Qmesclonh e +he eactnS Wou 3G) How can you tell where magma may be originating above the subductmg plate? p G Subdvet 1 - I\ whee yagma /m// argnating glooye e SUpdeTing s¢ agma | { Crarv) the Y- Hal melting of t+he Yocks e and whh that the ma GMa, f—(ﬂfu" '“*5 Carths Sreace. N ( ¥ n-t ATl 3H) What is the deepest earthquake plotted on your cross section? Do you think there is a lower limit below which earthquakes are not likely to occur? Explain your answer. (Hint: think about how pressure and temperature mfluence the behavior of rock in the upper mantle). < 4 lower Wit ~o the depth 1n which tadhauates likely geeur ¢ e | < EXAMINING EARTHQUAKESANDTECTONIC PLATES gt 1I¢<e JopthS the demp. and gressy 9€t hgh and tevocks Star+e i d Return to your analysis of the San Andreas Fault, Neenach Volcanic Formation, and Pinnacles National Park. Using the .kml and .kmz files you downloaded from Canvas, answer the following questions. 4A) How big was the most recent earthquake to occur on the San Andreas Fault as it parallels the Central VaIIey? List the magnltude depth, date, and time of this earthquake. M v Hude ],4‘, r\f"'/"(((\‘,u,)/" N ISk TiMe- §\2Z AW 48) How far from Hanford, CA was this ea,r)thquake (||st the distance in km and miles)? et 11 “,,"//}:,,:‘/" / 4C) From what you know about plate tectonics, and from what you may have experienced, did this earthquake cause any damage in the Central Valley? (We will discuss this some more when we talk about earthquake hazards). i A v/ / }‘w‘f‘\‘w[ K OGO and ( A

LAB #2-PLATE TECTONICS 2

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