Investigating Seafloor Spreading

Transcription

1 Procedure Follow the steps below while taking notes in your science notebook. Part I: Constructing the Seafloor Model 1. Your teacher will show on the overhead projector an illustration of the seafloor model for you to use as a guide for creating the model (Figure 1). Look at Table 1 and use the data from the Width of Color Band on Paper (column a) to mark and color one of the long sheets of white paper provided by your teacher. Borders between colors do not need to be perfectly straight as shown in the figure, but they should be perpendicular to the paper s long edge. 2. Label each boundary between color bands with the appropriate rock age in millions of years (m.y.). Widths of the color bands vary (some are narrow, some are very wide). Using units of m.y.: Units of million years, or m.y. are used to avoid having long numbers with 7 or more digits, as with 1,000,000 yr (or one million years ). For example 23,000,000 yr would be expressed as m.y., and 131,530,000 yr = m.y. What is m.y. expressed in yr? m.y. = yr 3. Trim the paper about 5-7 cm beyond the edge of the violet color band, so that the total length of the paper is about 100 cm, or 1 m in length (including the white edge at the end). 4. Cut the paper down the middle of its length, so that you have two striped strips that are nearly identical to one another. Each piece represents a half-seafloor. 5. Cut the corrugated cardboard into two pieces so that one edge of each piece is equal to the striped paper s width (Figure 1, on the overhead projector). 6. Tape the cut edge of a cardboard piece to the white end of one paper sheet. The paper should be taped under the cardboard to hide the white edge, so that the violet color band is adjacent to the edge of the cardboard. Repeat for the other piece of cardboard and striped paper. 7. Cut the cardboard tube so that the two pieces are the widths of the half-seafloors. Tape the other red end of each striped paper to a cardboard tube (refer again to the projected figure). What does your model represent? You have now created two half-seafloors that are nearly identical to one another. The color bands indicate seafloor of varying ages (see Table 1). Each color band represents rocks with ages that range over a segment of time. For example, the YELLOW color band is made of seafloor rock with ages ranging from to million years old. The cardboard pieces represent continents and the cardboard tubes, are located at the model s midocean ridge. When the two half-seafloors are laid out together, with the mid-ocean ridge in the middles and the continents on the ends, they will represent a complete slice of an ocean basin that is bounded by continents. 8. On a long table, or the floor, arrange the model seafloor so that the striped sheets of paper lie end to end, with the mid-ocean ridge in the middle and the continents at the edges (see Figure 2, on the overhead projector). When oceanographers began to sample the seafloor and determine rock ages, mid-ocean ridge rocks were dated to be less than 1 million years old (1 S-1

2 m.y.). The type of rock collected was basalt. 9. Your teacher may provide you with a sample of basalt. Basalt is a black igneous rock with small crystals that can rarely be seen by the naked eye. Examine the sample provided by your teacher. a. How is an igneous rock formed? b. What do the small crystal sizes of basalt tell you about how it cooled? c. What does the dark color of basalt tell you about its composition? As rocks were collected farther from the ridge, scientists discovered that basalt ages were increasingly older. Eventually, enough data were collected to construct a map similar to the striped paper model. 10. As a class, decide what to name your ocean basin, and label your seafloor model. Everyone in the class has a slice of the same ocean basin. Part II: Plate Rates and Seafloor Spreading The seafloor of any ocean basin is the upper layer of a plate. New seafloor or oceanic crust is formed as the plate moves away from a fissure, or large, elongate fracture in the seafloor. As the fissure widens, magma rises up from the underlying mantle onto the seafloor. The hot magma quickly cools when it comes in contact with the cold seawater, and crystallizes into solid rock along the fissure. The mafic composition of the magma produces a mafic igneous rock with very small crystals (due to the rapid rate of cooling), forming basalt. The motion of two plates moving away from each other at mid-ocean ridges is called divergence, and results in a divergent plate boundary. This motion is called seafloor spreading, named by the two oceanographers who first studied it, Harold Hess and Donald Dietz. The rate that a plate moves, or the plate rate, can be determined using the formula for velocity: R p = D/T where: R p is the plate s rate (velocity) of motion D is the distance from the mid-ocean ridge T is time, or the age of the rock on the seafloor But there are two sides to this story! Divergent plate boundaries have seafloor on both sides of the mid-ocean ridge. In your model ocean, the two sides of the ridge are mirror images of one another, so by doubling the rate of one half-seafloor, the rate of divergence, or the seafloor spreading rate of the entire ocean basin can be calculated: R ss = 2(R p ) where: R ss is the model ocean basin s seafloor spreading rate R p is the half-seafloor s plate rate It is important to know that not all plates have the same rate of motion, so scientists calculate the rate of each of the two half-seafloors on either side of a mid-ocean ridge, then add them together, rather than doubling one half-seafloor s rate. 11. Complete Table 1 (column d) by calculating the Total Seafloor Age represented by each of the 6 color bands of your model. These ages are expressed in units of m.y. For example, the YELLOW color band total age is m.y m.y. = m.y. 12. Using the information provided in Table 2, calculate the model s scale in the form: 1 cm = km. S-2

3 This scale means that 1 cm on your model is equivalent to km in the ocean it represents. Fill in all the blanks on Table two with the value you calculate. The scale s conversion factor you will use in Table 3 is km/cm. 13. Using the scale s conversion factor from Table 2, calculate the distance represented by each color band, and complete Table 3 (column b). Complete column c by adding the distance of each color band to the distance from the previous line. In other words, this number is a cumulative distance from the mid-ocean ridge. 14. Transfer the data from Tables 1 and 3 to the appropriate columns on Table 4 (as indicated). Then, calculate the Half-Seafloor Plate Rate for each color band, completing column c. Note that rates in Table 4 should be expressed as km/m.y. 15. Because seafloor motion occurs at a rate similar to the growth of fingernails (and faster!), oceanographers most often use the units cm/yr. Thus, the data from Table 4 must be converted so that the units are cm/yr. For example, if you have calculated a rate of 40 km/m.y., the conversion would be as shown below. 40 km = 40,000 m = 4,000,000 cm = 4 cm 1 m.y. 1,000,000 yr 1,000,000 yr 1 yr Answer: 40 km/my = 4 cm/yr a. Convert your data from Table 4 to complete Table 5 (columns a, b, and c). 16. For the model seafloor in this activity, the seafloor spreading rate is twice (2x) the halfseafloor plate rate. a. Complete Table 5 (column d) by calculating the Seafloor Spreading Rate during each color band. Now your units are cm/yr. Again, in real ocean basins, rate of motion for each plate must be determined separately, and added together to calculate the ocean s spreading, or growth rate. 17. Complete Table 6 (column b) by transferring the data from Tables 3, as indicated. These data will be used to graph the seafloor s velocity (rate). 18. Using the data from Table 6, plot your data on Graph 1 with distance from ridge (km) on the y-axis (the independent variable) and age of seafloor (m.y.) on the x-axis (the dependent variable). a. Be sure to include the zero point, as it represents the mid-ocean ridge, where distance = 0 km, and age = 0 m.y. b. Note that both axes are labeled, including the units used. c. Connect data points with a line, starting at the origin (0,0). 19. Compare your graph with the Half-Seafloor Plate Rate data from Table 4 (column c). In your science notebook answer the following questions: a. What does the slope of each line segment on your graph represent? b. If one segment of the line is steeper than the other, what does that mean? c. Based on your data, did the rate of seafloor spreading remain constant through time? How do you know? d. How old is your model ocean? e. What is the distance from the ridge to the continent (in km) for your half-seafloor? S-3

4 f. What is the overall average rate of seafloor spreading for the half-seafloor? Express your answer first in km/m.y., then in cm/yr. g. What is the average rate of seafloor spreading for the entire ocean basin? h. Seafloor spreading rates of the North Atlantic Ocean have varied between 2 and 4 cm/yr, whereas the East Pacific Rise in the Pacific Ocean has a rate of over 10 cm/yr. How does your model ocean compare with these actual spreading rate averages? Part III: Simulating Seafloor Spreading 20. Roll up each half-seafloor until the cardboard continent is reached. Place a wooden dowel through each of the 2 half-seafloor tubes. Four students are needed to demonstrate the process of seafloor spreading. Select your group. Arrange the two half-seafloor rolls with the dowels as shown in Figure 3a (not as shown in Fig. 3b!). Two students face each other, and each holds the ends of two dowels. A third and fourth student each hold the end of one of the continental pieces. Place the edges of the two continental pieces together, with the paper rolled up beneath them, hidden from the surface. 21. Before unrolling the seafloor, simulate the initial rifting (fracturing) of the continent to produce a small fissure (only 4-6 inches wide). Violet paper should be exposed in the fissure. A fissure is an elongate fracture. When a fissure occurs, the heated rock below melts (due to the release of pressure) to form magma. The magma rises to fill in the crack. The composition of the magma is mafic. As the magma cools and crystallizes, basalt is formed. a. Explain what the violet paper represents. b. Based on your data table, how old is the rock that was originally formed when the fissure was produced? 22. Continue to create new seafloor by very slowly diverging the two plates. Be sure that the same color of paper (= same age of rock) is forming at the ridge at the same time. As new seafloor is formed at the ridge, call out the ages. When the divergence simulation is completed, the red seafloor will be located at the mid-ocean ridge. This model approximates the ages of the North Atlantic s seafloor. The model ocean basin is scaled to the North Atlantic as well. 23. Answer the following questions in your science notebook. a. Do the different colors represent rocks of different compositions? What composition or compositions of rocks form as an ocean floor grows? b. What type of crust is produced at the ridge? c. Where is the source of the magma that formed the seafloor rock? d. What color represents the oldest seafloor? e. What color represents the youngest seafloor? f. As distance from the ridge increases, what happens to the age of the seafloor? Summarize your observations and discoveries by answering the questions on the next page. S-4

5 Questions Use your observations of the Earth s lithospheric plates and the relationship of plate boundaries to seafloor features to answer the following questions. Your teacher will provide information of how to format your answers. 1) Examine your graph of the Half-Seafloor Plate Rate (Graph 1). During what range of time did the plate move fastest? Answer with a range of time, such as between 10 and 20 million years ago. 2) When was the plate movement slowest? Again, answer with a range of time. 3) Write a 1 page description of how your ocean grew, starting 180 million years ago. In your description, explain what occurred during seafloor spreading (including the type of rock formed) and how the spreading rates varied through time. S-5

6 DATA CALCULATION TABLES Model Ocean Basin s Name Group members Table 1. Calculating seafloor ages for each color band. Subtract the youngest age from the oldest age for each color band on your model. This Total Seafloor Age in Color Band indicates the period of time that elapsed while the plate was moving away from the mid-ocean ridge. Color Band a) Width of Color Band on paper b) Youngest Age of seafloor c) Oldest Age of seafloor d) Total Seafloor Age In Color Band (cm) (m.y.) (m.y.) (m.y.) RED ORANGE YELLOW GREEN BLUE VIOLET TOTAL Half Seafloor S-6

7 Table 2. Using the model scale. The paper model uses the scale 92 cm = 2,300 km. If: 92 cm on the model = 2,300 km in the actual ocean Then: 1 cm = km Now, enter the number above into all the blank spaces below: 1 cm on the model = km in the ocean the model represents. We can express the scale as being kilometers per centimeter. So, the scale s conversion factor is: km/cm S-7

8 Table 3. Calculating distances. Use the scale s conversion factor from Table 2 to calculate the width of seafloor for each color band segment (column b). Add the distances together (column c) so that the distance increases from the ridge along the model seafloor. Color Band a) Seafloor Width, measured on the model b) Seafloor Width, represented by the model c) Distance from ridge of oldest rock (cm) (km) (km) RED ORANGE YELLOW 7.00 GREEN BLUE VIOLET S-8

9 Table 4. Calculating half-seafloor plate rates. Calculate the model s half-seafloor plate rates through time, first using the units kilometers per million years, or km/m.y. Color Band a) Seafloor Width (km) (from Table 3) b) Age Difference (m.y.) (from Table 1) c) Half-Seafloor Plate Rate (km/m.y.) RED ORANGE YELLOW GREEN BLUE VIOLET S-9

10 Table 5. Calculating seafloor spreading rates in cm/yr. Plate rates are usually expressed using the units centimeters per year, or cm/yr. Seafloor spreading rate is two times the half-seafloor plate rate, since there are two plates involved and they are moving apart at the same rate (in your model ocean). Color Band a)seafloor Width (cm) b) Age Difference (yr) c) Half-Seafloor Plate Rate (cm/yr) d) Seafloor Spreading Rate (cm/yr) RED ORANGE YELLOW GREEN BLUE VIOLET S-10

11 Table 6. Summarizing your data. Complete the table below, using your calculations from Table 3. Use this table to make your graph, below. Color Band a) Oldest Age of Seafloor in Color Band (m.y.) (from Table 1) b) Distance from ridge of oldest rock (km) (from Table 3) RED ORANGE YELLOW GREEN BLUE VIOLET S-11

12 Distance from Ridge (km) Investigating Seafloor Spreading Graph 1. Graphing Half-Seafloor Plate Rate. Use data from Table 6 to graph the variations in half-seafloor plate rates. Seafloor Spreading Age of Seafloor (m.y.) Half-Seafloor Plate Rate S-12