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Name: ___________________ Date: __________ Earth Science – 2 hours Period: __________ LAB 4-3: Seafloor Spreading INTRODUCTION: Nearly three-quarters of the earth’s lithosphere lies beneath the ocean. As a result, much of it is hidden from direct view making it difficult for scientists to study. Recent advances in technology along with underwater research vessels have allowed scientists to create detailed maps of the ocean floor and make many new discoveries about the deep ocean. These maps and discoveries have led to a better understanding of the movement of the earth’s tectonic plates under the ocean. As you have already learned, the earth’s crust is broken up into a large number of tectonic plates that are moving in relation to one another. The focus of this lab is to examine the sea floor of the Atlantic Ocean where two tectonic plates are moving apart creating a divergent plate boundary. Divergent plate boundaries are commonly found on the sea-floor underneath the earth’s oceans. Along divergent boundaries, magma from the asthenosphere is rising to the surface as the plates move apart. The magma cools and solidifies to form new oceanic crust which piles up to create an underwater mountain range called a mid-ocean ridge. Along the center of this mid-ocean ridge is a rift valley, a narrow valley that forms where the plates are separating. By studying the ocean floor in detail, scientists have found patterns in the age as well as magnetic orientation (minerals in the rock will align with the earth’s magnetic poles as the rock hardens) of the rocks on either side of the mid-ocean ridge. These observations have led scientists to the theory of sea-floor spreading which explains the mechanism behind Alfred Wegener’s theory of continental drift. In order to fully understand what is going on during sea-floor spreading, it is important to discuss the features of the ocean floor. The earth consists of two types of crust, continental and oceanic. These two types of crust have different characteristics including rock type and density. In general continental crust is composed of granite, has a low density and is generally found above sea level. Oceanic crust is composed of basalt, has a high density and is generally found below sea level. Along the coastal areas of continents there is a region where the continental crust is covered with water. This region is called the continental shelf. At the edge of the continental shelf is the continental slope, a steep downward slope where ocean depth increases dramatically. The base of this slope represents the boundary between continental and oceanic crust. Beyond the continental slope consists of a vast, flat area of the deep-ocean basin called an abyssal plain. Abyssal plains are the flattest regions on earth and cover about half of the deep-ocean basin. There are parts of the deep ocean basin that are not entirely flat. These areas are often associated with current or ancient volcanic activity. Submerged volcanic mountains called seamounts can be found scattered along the ocean floor. If these seamounts are tall enough they will rise above the ocean surface and form oceanic islands. Objective: This lab will teach you how we can use observations about the rock on either side of a mid-ocean ridge to learn how this plate boundary actually works. Materials: Colored Pencils: Red, Orange, Yellow, Green, Blue, Purple, and Brown Excel file titled “LAB4-3atlanticOceanProfile2013.xlsx” (Available on Haiku and on the coursework drive) Seafloor Spreading Lab Vocabulary: Directions – choose a term from the word bank that correctly matches each definition. Write that term on the line provided next to the proper definition. Word Bank Term: Seamount Continental Shelf Mid-Ocean Ridge Continental Slope Rift Valley Oceanic Island Divergent Plate Boundary Magnetic Orientation Abyssal Plain Definition: _____________________: A seamount that extends above the surface of the ocean. _____________________: An underwater mountain range that forms along divergent plate boundaries as magma rises to the surface and solidifies. _____________________: The area along the coast of continents where the continental crust is covered by ocean water. _____________________: A submerged volcanic mountain that can be found in the deep-ocean basin. _____________________: A narrow valley that forms along the center of a mid-ocean ridge marking the location where the plates are separating. _____________________: A pattern that has been discovered in igneous rocks resulting from the minerals aligning with the earth’s magnetic poles as the rock hardens. _____________________: A vast, flat area of the deep-ocean basin. _____________________: A steep downward slope at the edge of the continental shelf where the ocean depth increases dramatically. _____________________: A location on earth where two tectonic plates are moving apart. These are commonly found under the earth’s oceans. Procedure A: Atlantic Ocean Profile The map below shows the path a research ship took across the Atlantic Ocean. The ship sailed from the East Coast of the United States to Portugal. Along the way the ship measured the depth of the ocean and even a measurement of elevation of an island in the Azores. The data table below shows some of the data collected by this ship. Distance from US in kilometers 0 120 200 400 490 620 680 720 2000 2500 2900 3000 3090 3100 Elevation/Depth in kilometers* 0.0 -0.2 -2.7 -3.7 -3.7 -4.6 -1.8 -4.6 -4.6 -4.0 -2.7 -1.8 -4.0 -2.4 Distance from US in kilometers 3200 3500 3600 3650 4025 4050 4100 4125 4500 5000 5300 5800 6000 6075 6100 Elevation/Depth in kilometers* -2.9 -3.5 -3.7 -3.7 -4.0 -2.7 0.5 -2.2 -4.6 -5.0 -4.4 -3.7 -2.7 -0.2 0.0 *Note: Negative numbers indicate depth below sea level. Positive numbers indicate elevation above sea level. Procedure A: Atlantic Ocean Profile (Continued) Complete the following steps in order: Have one group member logon to a computer. Once logged in go to the “Coursework” drive, open the “Science” folder and the click on the folder titled “Donohue”. Open the file titled “LAB4-3atlanticOceanProfile2013.xlsx”. (There will also be a copy placed on the Haiku site on the class notes page) Transfer the elevation/depth data from the chart in your lab to the chart on this sheet. Watch the profile of the ocean floor form as you do this. It would be wise to have one group member read the data, one group member enter the data and the others check to make sure the data is entered correctly. NOTE: The profile is vertically exaggerated!!! Print out a copy of this sheet for every member in your group. Use a BLUE colored pencil and darken the 0 km elevation on your profile. This line represents sea level. Lightly shade (still in BLUE) everything below this line but above your graph line to show where the water is. Use a BROWN colored pencil to shade everything below your graph line to represent the Earth's crust. Label the following features on your profile: - Continental Shelf (There will be two of these to label) - Continental Slope (There will be two of these to label) - Seamount (Submerged) - Seamount (Island) - Mid-Ocean Ridge - Rift Valley - Abyssal Plain (There will be two of these to label) Procedure B: Southern Mid-Atlantic Ridge Use the map below to answer the questions that follow. The map shows a portion of the Atlantic Ocean floor located beneath South America and Africa. Isolines show the approximate age, in millions of years, of the ocean-floor bedrock on each side of the Mid-Atlantic Ridge. Points A, B, C, and X represent locations on the ocean floor. B C Age of bedrock (MYA) Color Code Temperature 0-10 RED Warmest 10-40 ORANGE 40-68 YELLOW 68-83 GREEN 83-118 BLUE 118+ PURPLE Coldest 1) On the diagram above, arrows pointing up ( ) represent the current orientation of our magnetic field, while arrows pointed downward ( ) indicate reversed polarity. On the diagram above, draw the correct pattern of magnetic polarity on the east side of the MidAtlantic Ridge. 2) Shade the diagram above to indicate the age of the ocean-floor bedrock according to the chart above. The abbreviation MYA stands for millions of years ago. 3) Draw arrows on the diagram to indicate the direction the plates are moving on either side of the Mid-Atlantic Ridge. 4) Describe the pattern of both the magnetic orientation and the bedrock age that can be observed on both sides of the Mid-Atlantic Ridge. 5) On the grid below, construct a line graph of bedrock age by plotting the age of the bedrock shown by each isoline that crosses line ABC. Points A and C are plotted on the grid. Connect the plots from A to C with a line. BE PRECISE!!! THIS IS SCIENCE AND ACCURACY COUNTS!!! C 6) Estimate the age of the ocean-floor bedrock at location X. ______________________________________ 7) Explain why the age of the ocean-floor bedrock increases as the distance from the MidAtlantic Ridge increases. 8) Draw a line on the graph below to show the general temperature pattern from point A to C. Ocean-Floor Bedrock Temperature A B C 9) The Mid-Atlantic Ridge separates pairs of crustal plates, such as the South American Plate and the African Plate. Identify one other pair of crustal plates separated by the Mid-Atlantic Ridge. 10)What type of plate boundary is the Mid-Atlantic Ridge? ____________________________________________ Procedure C: Mid-Atlantic Ridge Cross Section Analysis Use the cross section below to answer the questions that follow. The cross section represents a pattern of magnetic field reversals preserved in the igneous bedrock of the ocean crust east of the Mid-Atlantic Ridge. 1) Which cross section best represents the magnetic field patter west of the Mid-Atlantic Ridge 2) Identify the name of the igneous rock and the density of the rock that makes up the bedrock of the Atlantic Ocean-floor. Name of Rock: __________________________________________ Density of Rock: __________________________________________ 3) On the diagram below, draw arrows on either side of the ridge to indicate the motion of the plates. 4) On the diagram below, draw arrows in the box beneath the cross section to represent the convection current that is present beneath the Mid-Atlantic Ridge.