Lab 8 Tectonics
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3/21/24, 5:46 PM Quiz: Lab 8 Quiz: Tectonics Lab 8 Quiz: Tectonics Started: Mar 21 at 5:03pm Quiz Instructions Introduction Note: There is not a separate student reading for this exercise. The student reading is contained within the exercise. Plate Tectonics is considered a foundational concept in geology and other Earth Sciences. It is the understanding that the Earth’s surface is broken into a number of large, rigid lithospheric plates that are able to move independently above a plastic layer in the mantle known as the asthenosphere. The plates shift in order to allow heat from the Earth's interior to escape as part of a global convection process. The more heat that is released, the faster the plates will move. The movement of these plates and their interaction at plate boundaries is responsible for many of the major features we see at the Earth’s surface. The text and exercises in this lab are intended to help improve your understanding of this important concept. Prior to starting this lab, you should read chapter 1.5, 4 (all), 5.2, and 5.8 in the lecture textbook Earth Science. The Earth’s Interior: Compositional Layers Before discussing Plate Tectonics, it is important to understand the nature of the Earth as a material body. The Earth is not a uniform sphere of rock. There are many distinct layers within the planet that can be defined both compositionally and mechanically. Some of the chemical layers have already been introduced during the study of igneous rocks. The Earth has a thin outer crust which is either composed of mafic to felsic rock. The thinner (~7km) mafic crust is made up of igneous basalt and gabbro, rocks rich in iron and magnesium and lower in silica, with an average density of approximately 3.0 g/cm3. The high density of these rocks makes them sag lower into the Earth, creating an area for ocean water to collect. Thus, the mafic crust is known as ocean crust. The thicker (~40km) felsic crust is made up of a variety of igneous, metamorphic, and sedimentary rock whose average composition is somewhere around that of granite, with an average density of approximately 2.7 g/cm3. The low density of these rocks makes them rise higher on the Earth, creating areas of high elevation that rise above the ocean waters. Thus, the felsic crust is known as continental crust. Beneath both the ocean and continental crusts is a thick (~2,900km) compositional layer known as the mantle. The dominant rocks in this layer are peridotites, with an ultramafic composition even richer in iron and magnesium than mafic rocks and higher densities closer to an average of 5.5 g/lcm?3. Finally, at the center of the Earth is a thick (2,270km) core made up of the dense metals like iron and nickel. These metallic rocks have an average density of approximately 10 g/cm?. Question 1 12 pts Based on the information found in the text above and in your lecture textbook, complete the illustration of the Earth's compositional layers below by providing the layer name, the layer composition class, and the layer density. https://collin.instructure.com/courses/924057/quizzes/2463783/take 1/11
3/21/24, 5:46 PM Quiz: Lab 8 Quiz: Tectonics Layer A This layer known as continental crust v ocean|water A / B It is composed of felsic v lts density is 27 9/cm3 v Layer B This layer known as ~oceanic crust v It is composed of mafic v Its density is 3-0 9/cm3 v 6,396 km Layer C This layer known as mantle v D It is composed of ultramafic v lts density is °-° 9/cm3 v Layer D This layer known as core v v It is composed of iron and nickel v Its density is 10 9/cm3 v Question 2 10 pts The Earth’s Interior: Mechanical Layers The Earth can be broken into more than just the traditional compositional units of continental crust, oceanic crust, mantle, and core. Our understanding of the Earth’s mechanical layers has its roots in the study of earthquakes. Earthquakes are ground vibrations caused by rapid release of stored energy along a fault. The energy released radiates outward spherically in the form of seismic waves. Four different types of seismic waves have been identified, two of which (P-waves and S-waves) are called body waves because they pass through the interior (body) of the earth. P- or primary-waves are so called because they are the first waves detected in any earthquake and they have the fastest velocities of the wave types. S- or secondary-waves are slower than P-waves, arriving shortly after a P-wave at a seismic station. All wave types travel faster through, denser and/or more rigid mediums, and slower through ductile materials. S- waves can only travel through solid materials while P-waves can travel through materials in a solid, liquid, or gaseous state. Seismologist use the various arrival times of different seismic waves that have traveled though the Earth or bounced off layers to determine the physical characteristics of the Earth’s interior, in much the same way a technician can read a sonogram from a doctor’s office. Before interpreting the seismic velocity chart, you must make sure you understand some of the basic rules as presented above: a) Which seismic wave travels faster, a P-wave or an S-wave? P-waves travel fastest v https://collin.instructure.com/courses/924057/quizzes/2463783/take 2/11
3/21/24, 5:46 PM Quiz: Lab 8 Quiz: Tectonics b) Which seismic wave can travel through all mediums (solid, liquid, and gas)? P waves fravel through allm v c) Which seismic wave can only travel through a solid medium? S waves can only travel throi v d) When seismic waves travel through a low density medium, what should happen to their speed? \Vaves slow down in low der v e) When seismic waves travel through a high density medium, what should happen to their speed? \Vaves speed up in high den v Question 3 14 pts o Below is a chart of the velocity of P- and S-waves as they travel down through the interior of the earth (velocity vs. depth chart). You will be asked to make observations of wave behavior and relate those behaviors to the "rules" identified in the previous section. The goal is to use waves behavior to identify the density of different earth units. Velocity (km/sec) https://collin.instructure.com/courses/924057/quizzes/2463783/take Layer A The P waves in this layer Speed up The S waves in this layer speed up This behavior suggests the layer is becomes more dense withd v Layer B The P waves in this layer Slow down The S waves in this layer Slow down This behavior suggests the layer is becomes less rigid v Layer C The P waves in this layer speed up The S waves in this layer speed up | This behavior suggests the layer is becomes more dense withd Layer D Th P waves in this layer ~Slow down The S waves in this layer disappear This behavior suggests the layer is liquid v Layer E 3/11
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32124, 5:46 PM Quiz: Lab 8 Quiz: Tectonics The P waves in this layer ~SPeed up v This behavior suggests the layer is is more rigid than unit above v Question 4 10 pts In the previous question you used the speed of seismic waves to interpret whether the layer was a high density material or a low density material, or its rigidity If you combine this information with an understanding of the materials that seismic waves can travel through (presenter in earlier text), you can interpret the mechanical behavior of the layer -whether the layers are high-density solids, low- density solids, or liquids. From the information above, match each layer's to its mechanical behavior. Layer A high-density solids v Layer B low-density, rigid solid v Layer C very high-density solids v Layer D liquid v Layer E low-density, deformable solic v The mechanical layers of the Earth represented in the chart on the previous page are known, from center of the Earth to the surface, as the (E) inner core, (D) outer core, (C) mesosphere, (B) asthenosphere, and (A) lithosphere . These layers are critical to the function of plate tectonics. e The inner core behaves as rigid, very high-density solid iron and nickel. e The outer core is a very high-density /iquid of iron and nickel. It is the only true liquid layer in the earth. (It is a common misconception that volcanoes are sourced by a large layer of liquid magma just beneath the crust or that it flows from this liquid core. But instead, shallow magma that feeds volcanoes comes from small, specific regions where pressures, temperatures, and fluids help the otherwise solid rock pass the melting point.) e Above the core layers is the mesosphere, which is part of the mantle and is a thick layer of ultramafic rock that behaves as a rigid, high-density solid. o Above the mesosphere, still within the mantle, is the asthenosphere. The asthenosphere is an area where the ultramafic mantle rocks are close to pressures and temperatures needed to melt rock, making them behave as a plastic or deformable, lower-density solid. e The outermost portions of the Earth behave as a rigid, low-density solid material known as the lithosphere. The lithosphere is made up of the relatively thin mafic ocean crust, the thick felsic continental crust, and the uppermost part of the ultramafic mantle. Because the lithosphere behaves as a brittle solid, it has broken into about two dozen tectonic or lithospheric plates which can move independently over the plastic asthenosphere . The plates can contain both ocean and continental crust. The density of the lithosphere varies widely with continental crust being the (~2.7 g/cm3), oceanic crust (~3.0 g/cm?3), and upper mantle rocks (~3.3 g/cmd). As lithospheric plates shift and move, tremendous stresses at the plate boundaries generate some of the most dramatic landscapes on Earth, including the deepest trenches and tallest mountains. These same forces are also associated with generating earthquakes and volcanoes. https://collin.instructure.com/courses/924057/quizzes/2463783/take 4/11
3/21/24, 5:46 PM Quiz: Lab 8 Quiz: Tectonics Plate Boundaries There are three main types of plate tectonic boundary, each defined by the sense of plate motion and the stresses involved. The boundary stresses can deform rocks and are responsible for creating many of the large scale features on the Earth’s surface like trenches and mountain chains. e o At a divergent boundary, plates experience extensional stress as they move away from each other. Divergent boundaries are considered constructive boundaries because new mafic crust is created at these sites. As the plates move apart, heat from the Earth’s interior is allowed to escape, creating conditions favorable for volcanoes. Most of the new crust that is created forms at ocean ridges in a process known as seafloor spreading. o At a convergent boundary, plates experience compressional stress as they move toward each other. As new crust is continually created, old crust must also be consumed. Convergent boundaries in the ocean are considered destructive boundaries because old crust is bent down into the mantle beneath an overriding plate in a process known as subduction. The ocean crust that gets subducted will be reincorporated into the mantle. Only ocean plates can be subducted into the mantle because they are high density. When two continental plates converge, neither can be subducted because the rock density is too low. Instead of subducting, the rocks build vertically into massive mountain ranges. o At a transform boundary, plates experience shear stress as each plate tries to slip past the other. Crust is neither being created nor destroyed - the rocks are simply slipping past each other laterally. Below is a list of the six major plate tectonic boundaries. There are two divergent boundaries (continental rift and ocean ridge), three convergent boundaries (ocean-continent convergent, ocean-ocean convergent, continent-continent convergent), and one transform boundary. More detailed information on these boundaries and their characteristics can be found in chapter 4 of your lecture textbook. Convergent type boundary Ocean Continent Convergent Bndry. Compressional stress « Shallow to deep earthquakes, high magnitude » Explosive, high viscosity felsic to intermediate, continental arc volcanoes » (ascades, Pacific Northwest Divergent type boundary Continental Rift Boundary Extensional stress Shallow earthquakes, low magnitude Gentle, low viscosity mafic volcanoes : ; Lithosphere [ \ 4 East African rift enosphefe . Divergent type boundary Ocean Ridge Boundary — Extensional stress » Shallow earthquakes, low magnitude = + Gentle, low viscosity, mafic volcanoes J « Mid-Atlantic ridge Oceanvc crust Convergent type boundary Ocean-ocean Convergent Bndry. soeal © Extensional stress - 2+ Shallow to deep earthquakes, high magnitude « Explosive, medium viscosity, mafic to intermediate, island arc volcanoes « Aleutian islands, Alaska Convergent type boundary Continent-Continent Convergent Bndry. « Compressional stress « Shallow to intermediate earthquakes, moderate to high magnitude 1 « No volcanoes * Himalayas Transform Boundary * Shear stress « Shallow to intermediate earthquakes, low to moderate magnitude Lithosphere ] « No volcanoes E.mm,, « San Andreas fault, Calif. — Question 5 24 pts Below is a hypothetical tectonic profile across the Earth’s surface. Each of the six major boundary types is represented by one of the lettered sites. After identifying each boundary by the rock types and general geometry, fill in the chart provided below the image. Use the boundary descriptions and figure on the previous page to help you. https://collin.instructure.com/courses/924057/quizzes/2463783/take 5/11
3/21/24, 5:46 PM Quiz: Lab 8 Quiz: Tectonics ocean cont. water crust If earthquakes occur at this boundary, indicate their depth |If volcanoes occur at this Boundary Name: Type of Stress: range:shallow, shallow-to- boundary, indicate if they are intermediate, or shallow-to- gentle or explosive deep Boundary Continent-Ocean Convergen Compression v | shallow to deep earthquakes + | explosive volcanic eruptions A Boundary Continent-Continent Converg Compression v || shallow to intermediate eartr v | no volcanoes v B Boundary divergent Ocean Ridge Extension v || shallow earthquakes v || gentle volcanic eruptions v C Boundary divergent Continental Rift Extension v | shallow earthquakes v || gentle volcanic eruptions v D Boundary Ocean-Ocean Convergent Extension v | shallow to deep earthquakes + | explosive volcanic eruptionsy v E Boundary Transform Shear v || shallow to intermediate eartr v || no volcanoes v F Question 6 9 pts Below is a map of recent earthquake events color coded by focal depth. Because earthquakes occur at all plate boundaries, their general occurrence can be used to identify the Earth’s various lithospheric plates. As described earlier, the specific stresses and processes at a boundary will result in earthquakes of different depths. The earthquake depth map can thus also be used to interpret boundary types and tectonic processes. After examining the map, answer the questions about the highlighted zones. https://collin.instructure.com/courses/924057/quizzes/2463783/take 6/11
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3/21/24, 5:46 PM Quiz: Lab 8 Quiz: Tectonics ¥ /’-" $ » ‘ -{'fi’ “g. i ‘ ‘. K : \4 | o — 2. . s Earthquakes>M3, 2001-2011 Ff " e ‘ S DA of o one 2 =" * ® 0-33km shallow F’ - : = e 2 A 33-100km intermediate| _ e 2 S @ 200-400km intermediate : — : : ‘ ‘ 400-700 km deep 180* 150° 120" o o -30* o 0 The depth of earthquakes in Zone 1 Subduction zone convergent v The depth of earthquakes in Zone 2 Divergent Ocean Ridge bour The depth of earthquakes in Zone 3 Continent-Continent converg v Question 7 4 pts range from shallow to deep are all shallow range from shallow to interm v . These earthquake depths are associated with v . These earthquake depths are associated with v .These earthquake depths are associated with Longitude (°E) Longitude (°W) 135 136 137 138 1390 140 141 142 143 73 72 71 70 69 68 67 66 65 &4 63 62 0 0 I o , 2 100 SRR AP - ¢ u:m 4 100 . /T AR < 3 : 'l [ ] l“é‘. B ] ' ] 200 ] - mcccpeno,.n ’ : : Pl I Rl " i E : € . =3 : o . & 00 Q--otevmg 300 - B - .. ... .. S 8' . n°. g. . o Ll & ' 400 - .: .....
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500 o .L - 00 x 59 » ! ' ' . l-: a m ' b m Lm“m . [ ' l- L] https://collin.instructure.com/courses/924057/quizzes/2463783/take 7/11
3/21/24, 5:46 PM Quiz: Lab 8 Quiz: Tectonics In the figures above, two different collections of earthquake foci depths are plotted in profile view. The location in which the earthquakes were recorded is displayed in the inset map for each data plot. Based on your knowledge of plate tectonics, what type of plate boundary formed this pattern? O Divergent @® Convergent O Transform Question 8 3 pts Seafloor Age 120E 180 120W 60W 0 60E 120E ” y w\ . X : i , son I 60N 30N /% 30N 7 o NN y oy AN : 30S 30S fi ") 60S 60S sl 120E 180 120W 60W 0 60E 120E Seafloor Age (millions of years) T . O 25 50 75 17007125 150 115 200 The colored area in the map above represents that age of the ocean crust. If you notice, there is a thin black line running through the floor of the oceans. This line represents rift valleys, the cracks in the ocean lithosphere that are found at the center of an ocean ridge. Using the maps color scale, answer the questions below. a) What is the age of the rocks at the ocean ridge rift valley? = recent (0 million years) v b) Is the ocean crust along the ocean basin margins younger or older than the rocks at the rift valley? the ocean basin margins are v c) About what age is the oldest ocean crust on the map? = 200 million years v https://collin.instructure.com/courses/924057/quizzes/2463783/take 8/11
3/21/24, 5:46 PM Quiz: Lab 8 Quiz: Tectonics Rates of Plate Movement Paleomegnetism ® In addition to ages of rocks, scientists identified that the iron rich mafic rocks of the ocean floor, much like the needle of a compass, can be magnetized according to Earth’s magnetic field. Small iron-rich crystals in liquid basalt lava align with the Earth’s magnetic field. When the lava solidifies, all the crystals preserve a record of the magnetic field known as remnant magnetism. If the rocks move due to tectonic forces, the preserved magnetic fields can be used to determine the location in which the rock originally formed. Scientists also discovered that throughout geologic time, the Earth has experienced magnetic reversals. A reversal occurs when the polarity or “charge” of the North and South Poles switches places. The present orientation, with the north “charge” in the Arctic, is known as normal polarity; during a reversal, the north “charge” is found in the Antarctic (reverse polarity). Because oceanic rocks are iron rich, a record of reversals is preserved in the ocean crust. With seafloor spreading occurring as the reversals occur, the magnetic record on the ocean floor appears as bands of normal and reverse polarity that are symmetric about the rift valley. For a better understanding, view the following video: Magnetic Reversals and Seafloor Spreading & (https://www.youtube.com/watch? v=BCzCmIdiaWQ) 5_(https://www.youtube.com/watch?v=BCzCmldiaWQ) Magnetic Orientation of Ocean Crust volcanoes mid-ocean ridge volcanoes sea-floor spreading rising magma mantle In the seafloor model above, the current magnetic field is normal, with the rocks next to the rift magnetized north to the Arctic (1 - normal polarity). Because seafloor spreading is symmetric, as rocks are magnetized some spread to the west (numbered seafloor) and some spread to the east. Seafloor spreading records the reversals as symmetrical magnetic "stripes." In the seafloor model above, there is evidence that the poles have reversed (flipped polarity) three times - from current normal polarity 1 to reversed polarity 2, from reversed 2 to normal 3, from normal 3 to reversed 4. You can determine the rate at which the ocean plate is moving away from the rift valley (R) if you know the age of the seafloor (T) and its distance (D) from the rift valley (R=D/T). The spreading rate (SR) however, is twice this amount - as this account for the movement on both sides of the rift (SR = 2*D/T) Question 9 4 pts https://collin.instructure.com/courses/924057/quizzes/2463783/take 9/11
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3/21/24, 5:46 PM Quiz: Lab 8 Quiz: Tectonics Magnetic Polarity Record y 4,crican plate A African plate v \ Present (0 m.y.) “— riftvalley —” 1 million years old }.f 4 > F & normal polarity (colors) reverse polarity (tan) scale: 10 0 10 20 3 40 50 60 70 km g A W 7 # A e Examine the hypothetical Atlantic seafloor magnetic record in the image above. Normal polarity rocks are brightly colored whereas reverse polarity rocks are tan. The ages of some of the rocks are marked in the rock column to the left of the profile. Carefully examine this image and answer the questions below. How many magnetic reversals are recorded in this segment of the ocean floor displayed above? @® 5 reversals O 6 reversals O 10 reversals O 11 reversals O 21 reversals O 22 reversals Question 10 10 pts Magnetic Polarity Record y 4,crican plate A African plate Present (0 m.y.) N rift valley > 1 million years old }.f 4 > F & normal polarity (colors) [ ] reverse polarity (tan) scale: 10 0 10 20 3 40 50 60 70 km https://collin.instructure.com/courses/924057/quizzes/2463783/take 10/11
3/21/24, 5:46 PM Quiz: Lab 8 Quiz: Tectonics The process of seafloor spreading is continuous, conveying crust laterally away from the ridge to make room for more new crust to form. If you know the age of a rock and the distance it has been conveyed, you can calculate the rate at which a plate is moving (R=D/T). The rock at point X' in the image above is on the African Plate. Using the information provided with the profile, you can determine the rate at which the African plate is moving eastward from the ridge center. The ages of some of the rocks are marked in the rock column to the left of the profile and a graphical scale is provided. (A hint for the following calculations, 1km = 100,000 cm) In the space provided below, answer the following questions: ® a) How far is the rock at point X from the rift valley? 29 km v b) How far is the rock at point X from the rift valley in centimeters? [ Select ] v c) How old is the rock at point X? 3,000,000 years v d) Given your distance and time measurements from above, what is the rate at which the African plate is moving away from the rift valley in km/year (R =D/T)? [Select] v e) What is the spreading rate of this rift (SR = 2* D/T)? [ Select] v Skills you should have: 1. Can you identify the different compositional layers of the Earth, including their name, position, relative thickness, composition, and relative density? . Do you know how the P- and S- seismic waves behave in different materials? . Do you understand how P- and S- seismic waves can be used to interpret the density and mechanical behavior of the Earth's layers? . Can you identify the different mechanical layers of the Earth, including their name, position, and mechanical behavior? . Can you identify the 6 major plate boundaries including the type of crust involved, the type of stress, and the characteristics of earthquakes and volcanoes that may be in the setting? . Do you understand the processes of subduction and seafloor spreading and how they relate to the concept of plate tectonics? . Do you understand the age distribution of rocks on the seafloor? . Can you use age and distance data to calculate the rate of plate movement? . Do you understand the driving force behind plate tectonics? o L WON © 00 N O Quiz saved at 5:46pm Submit Quiz https://collin.instructure.com/courses/924057/quizzes/2463783/take 11/11