Geo Lab #3 (1)

GEOL101 Dynamics of the Earth - Fall 2023 Name: Emily Thomson Laboratory 3: Plate Tectonics Section: Learning Outcomes: ● Identify the internal layers of the Earth based on composition and physical properties. ● Describe how regional lithosphere density and thickness affect topographic elevation. ● Identify major geographic features resulting from differing plate tectonic boundaries. ● Calculate seafloor spreading rates based on distance and age date. Slicing Up Planet Earth The Earth’s interior can be subdivided into concentric layers or shells based on chemical composition or physical properties (Figure 1). Let’s explore each of these classifications . . . in depth <insert student groan here>. Based on chemical composition, the Earth can be subdivided into three layers. The crust is the outermost compositional layer of the Earth and has two different “flavors.” Oceanic crust comprises most of the global ocean basins of the world, averages ~7 km thick, and is generally composed of basalt and similar rocks. Basalts are typically composed of 45% to 52% silicon dioxide (SiO 2 ), and tend to be enriched in magnesium (MgO) and iron (FeO) compared to continental crust. In contrast, continental crust is much thicker, averaging 45 km and up to 75 km in places, and is generally composed of granite and similar rocks. Compared to basalts, granites have more silicon dioxide (i.e., 65% to 72%) and less magnesium and iron. Because iron and magnesium are much higher mass atoms, oceanic crust tends to have a higher density

(i.e., ~3.0 grams per cubic centimeter) than continental crust (i.e., ~2.7 grams per cubic centimeter). We will return to the thickness and density differences between oceanic and continental crust as they reflect and explain some major plate tectonic processes as well as some major global elevation patterns. The mantle extends from the base of the crust down to ~2,885 km (~1,792 miles) and is lower in silicon dioxide (typically <45%) and higher in magnesium and iron than oceanic crust, which results in a density of ~4.5 grams per cubic centimeters. Much of the mantle is composed of peridotite and other “ultra-mafic” rocks (where the term mafic represents a blend of “magnesium” and a contracted form of “ferric,” which means related to iron). The core is the third and deepest compositional layer, extending from ~2,885 km down to the center of the Earth at ~6,371 km (~3,959 miles), and is composed largely of iron with nickel and sulfur. All three of these layers are based on composition as summarized on the left side of Figure 1, but note that the term “core” serves “double-duty” as it is also used for a different complementary classification based on physical properties as described below. The Earth can also be subdivided into five concentric layers based on physical property of the material, specifically whether the layer shows a rigid, plastic, or liquid response to an applied force (Right side of Figure 1). Note that these physical-property-based layers do not necessarily coincide with the composition-based layers. The rigid lithosphere consists of the outermost ~100 km of the Earth, which includes the continental and oceanic crust along with the upper portion of the mantle. Starting at ~100 km and proceeding down to ~2,885 km, the mantle material remains similar in composition, but due to increasing temperature and pressure transitions in behavior from the rigid lithosphere to the plastic asthenosphere to the rigid mesosphere . Note that plastic is not being used here in the “recycle your plastics” sense, but to describe the behavior of a material that flows and deforms in response to an applied force, but doesn’t return to its original form after removing the force (think toothpaste). Our exploration of plate tectonics will largely focus on the compositional layers of crust and mantle and physical property layers of lithosphere and asthenosphere. Below the rigid mesosphere, continuing increases in temperature and pressure produce a liquid outer core and a solid inner core. Note that circulation within the liquid outer core creates the Earth’s magnetic field, which not only allowed humans to invent compasses for navigation, but also protects life on our planet by deflecting harmful radiation from our Sun. Putting Planet Earth in Motion Now that we understand the basic nature of the lithosphere and asthenosphere, let’s take a closer look at the lithosphere. The rigid lithosphere encircling planet Earth is broken into a number of tectonic plates . In fact, there are seven major (large) plates and a number of smaller “microplates.” Each plate is in close physical contact with the surrounding plates, but the nature of this contact can change over space and time. Scientists have identified three fundamentally different types of plate boundaries, each based on the nature of the movement between the two plates: Convergent is used when plates are moving toward one another, divergent is used when plates are moving away from one another, and transform is used when plates are grinding past one another. Let’s use Google Earth to get better acquainted with where these boundaries are located and how to identify them. (If you need help using Google Earth , go to the end of the lab for tips.) 1. If you don’t already have Google Earth Pro desktop installed on your computer, download and install it for free or navigate to www.google.com/earth/versions.

of oceanic-crust-bearing lithosphere and is subducting beneath the South American plate. The South American plate, like most plates, is composed of both continental and oceanic lithosphere. What to look for where oceanic crust and oceanic crust meet at a plate boundary : In the case where the leading edge of both lithospheric plates include oceanic crust, then the older, more dense plate will descend or subduct beneath the less dense and younger plate (Figure 2b). This is because the older plate will tend to be relatively denser. A deep bathymetric furrow, or trench, commonly marks the position where the overriding and subducting plates first contact each other. Parallel to the trench and located on the overriding plate will be an oceanic volcanic arc. An excellent example of this is the Island of Japan. In Google Earth , double click the Japan pin and you will be taken to that island arc. In this scenario, the Pacific Plate is subducting beneath the Okhotsk plate. We know the Island of Japan is a volcanic arc and it is located on the Okhotsk plate, meaning it must be the overriding plate. What to look for where continental crust and continental crust meet at a plate boundary: In the case where the leading edge of both plates includes continental crust, then neither plate really subducts; instead they continue to be forced into each other causing the crust to thicken (Figure 2c). The Himalayan Mountains formed in this manner and as a result contain the tallest mountain on Earth, Mount Everest. Double click the Himalayas pin in Google Earth to be taken there. Divergent Boundaries: Along diverging boundaries (red lines), plates are moving away from one another and new oceanic-crust-containing lithosphere is forming. Such boundaries are commonly marked by mid-ocean ridges and a rift valley, and are sites (Figure 3). Double click the Mid Atlantic Ridge pin located in the Divergent and Transform Boundary folder in Google Earth to be taken to this divergent boundary in the middle of the Atlantic Ocean. Here African plate material is moving east and South American plate material is moving west. Zoom in to the pin and turn the Plate Boundary Model off by clicking the check mark to its left. You should still be able to recognize the boundary between the two plates. Transform Boundary: Transform boundaries (green and orange lines) are prominent in ocean basins where they appear to offset ridge axes. However, transform boundaries actually act to link up mid ocean ridges and allow segments of plates to move past each other laterally. Let’s stay at the Mid Atlantic Ridge pin. The transform boundary in green is

working to link up the motion of the divergent (red) boundary. If you zoom out and spin the Earth, you will see that divergent and transform boundaries are working together all through the ocean basins. Transform boundaries are not only found in the Earth’s oceans; in fact, the closest plate boundary to San Diego State University is a transform boundary and is called the San Andreas Fault to our east! Lab Activity Note that the questions start with Question 7 . . . there is no Question 1 through 6! In Google Earth , turn on the folder entitled Question 7 by clicking the box to its left and expand the folder. Double-click on the Question 7 polygon to be taken to the location shown in the figure below. Use Google Earth and the reading above to help answer the following questions. Tip: If you are having trouble viewing the area, turn off layers, such as volcanoes of the world or sea floor age by unchecking the box next to them until they are needed for later questions. Question 7.1: Zoom out in Google Earth to identify and name the tectonic plates labeled A and B in the Question 7 location. Make sure the layer Plate Boundary Model is turned on. You may need to rotate the Earth around to see the plate names that are shown in orange. A: North America Plate B: Pacific Question 7.2: What type of boundary lies between the two plates shown in the Question 7 location? Which plate do you think is being subducted? Outline your reasoning for both answers by referring to

Divergent plate boundary Question 8.3: Based on your understanding of seafloor spreading, draw an arrow in each of the circles to indicate the relative motion of the underlying lithosphere. What direction (e.g., east, west, north, or south) is Plate A moving? Plate B? Plate A is moving east and plate B is moving west, oceanic spreading ridge and oceanic transform fault Question 9: Now turn on the layer entitled Seafloor Age ; a screenshot of what you should see in Google Earth is shown to the right. The oceanic crust will be color-coded based on its age, with warmer colors (reds) represent younger (i.e., more recently formed) oceanic crust and colder colors (blues and purples) represent older oceanic crust. Follow the steps below to complete the table below. ● Expand the Seafloor Age folder and navigate to the Mid Atlantic Ridge by double clicking the pin labeled A . ● Go to the Atlantic Ocean and use the seafloor age color scale to determine the youngest oceanic crust located at Pin A and the oldest oceanic crust located at Pin B. ● Use the ruler tool to measure the distance in km from Pin A to Pin B and round to the nearest whole number (i.e., 4,792.5 km = 4,793 km). If you don’t know how to use the ruler tool, then go to the end of this handout for help. ● Calculate the seafloor spreading rate using your measured distance and the formula of Spreading Rate = Distance (km) / difference in age (yr) ● Repeat the above steps for the East Pacific Rise at C.

○ Measure to the oldest crust at D. Hint: [1 km = 100,000 cm] and [1 my = 1,000,000 yr] Oldest crust age (yr) Approximate distance from ridge to oldest crust (km) Spreading rate (cm/yr) A. Mid Atlantic Ridge 180,000,000 3,776.87km 20.98 cm/yr C. East Pacific Rise 180,000,000 9,096.18km 50.5 cm/yr Question 10: Based on your calculations in the table above, which ridge is moving faster? East pacific Rise Question 11: Notice the seafloor age pattern in the Atlantic Ocean, now navigate to the Pacific Ocean and notice its age pattern. With the seafloor ages and these patterns, do you need to calculate the spreading rate to conclude what you did in question 10, why? No because it the calculation of the spreading rate already tells you its moving faster because it is a higher number per yr Question 12: Turn off the Seafloor Age layer and turn on Question 12 . Double-click on Question 12 to move your view to the west coast of the United States. A screenshot of this view is shown below. Use it and Google Earth to answer the following questions.

The plate motions have a pattern of being divergent so they will grow farther apart Question 12.4: What type of plate boundary is located between Oakland and San Diego? This boundary is also a fault, a pretty well-known one actually. What is the name of this boundary or fault? Hint: you can find the name on page 9 of this lab. The Andreas fault Question 13.A-I: Now double-click on Question 13 in Google Earth and travel to the southern tip of South America. Use Google Earth and the knowledge you’ve gained in this lab to answer questions A through I below. For example, Question A below corresponds to where A is pointing on the figure below, and so on (ignore location E). Also note that the sketch beneath the Google Earth screenshot is not drawn to scale. A. Type of crust? Continental crust B. Type of crust? Oceanic crust C. What would you call this interval based on physical properties? Lithosphere

D. What would you call this interval based on physical properties? The mantle F. What is the name of this tectonic plate? The antarctic plate G. What is the name of this tectonic plate? The pacific plate H. Based on the boundary type between Plates E and F, what would you expect to find on the seafloor? Subduction I. Based on the boundary type between Plates E and F, what would you expect to see here? To see the two plates collide together where one is beneath the other Using Google Earth Pro Desktop:

The ruler tool is located in the menu bar at the top of Google Earth, to the right of the search bar and it looks like a small blue ruler. Click the ruler tool once to get the ruler tool dialog box. Make sure the line tab is open. You can change the units by clicking the drop-down box located to the right of ‘Map Length’. Once you have the units set, click once on one of the locations you want to measure between and then click on the other location. You do not click and drag between the two locations. The distance between your two locations will be displayed next to ‘Map Length’. To measure another distance, click ‘Clear’. To close the ruler tool, click the red x located in the top right corner of the ruler dialog box.