Mapping the Ocean Floor Using the Hess Model Background Information Before World War II, Not Much Was Known About the Ocean Floor

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WLHS/Marine Biology/Oppelt Name ______Mapping the Ocean Floor Using the Hess Model Background Information Before World War II, not much was known about the ocean floor. The development of sonar during the war made more detailed maps of the ocean floor possible. Harry Hammond Hess, a geologist from Princeton University, was the captain of the assault transport vessel Cape Johnson. While stationed in the Pacific Ocean during World War II, Hess used the echo location (sonar) capabilities of this ship to collect data about the ocean floor. He later used this data to construct map of the ocean floor. He later used this information to construct a map of the ocean floor that led him to develop the hypothesis of the sea floor spreading. Echo location works by sending a signal out, called a ping, from a ship. That sound bounces off the ocean floor and is reflected back to the ship. By timing how long it takes to hear the echo of the sound, scientists can determine how far it is to the bottom. To determine the distance to the ocean floor, the time of the echo and the speed of sound in ocean water (1500 meters per second) must be determined. For example, if a ship sends out a ping that is reflected back in 1.34 seconds, the ping took 0.67 seconds to go down and 0.67 seconds to reflect back (1.34 sec ÷ 2 = 0.67 sec). To find the distance from the ship to the ocean floor, multiply 0.67 by the speed of sound in ocean water (1500 m/s). In this example, the ocean floor is 1005 meters from the surface (0.67 sec x 1500 m/s) = 1005m). Repeating this procedure as the shop moves forward will reveal data that can be used to map the ocean floor. Practice Mapping On the next page, are sonar data recordings from the sea floor near Guam, where the Challenger Deep is located (see map to the right). The sonar data will help you create a map of the bottom of the ocean for this area. Hypothesis: How deep do you think the Challenger Deep is? ______

Time it took for Distance from Total time for echo sound to reach Speed of sound in Distance to bottom Guam (km) to return to ship bottom ocean water in meters (Depth) Total time in (Total time ÷ 2) x seconds) (Total time ÷ 2) 1500 m/s 1500 m/s 0 0 0 1500 0 16 1.34 0.67 1500 1005 32 1.34 1500 48 5.22 1500 64 4.00 1500 80 4.00 1500 98 4.88 1500 112 4.27 1500 128 4.13 1500 144 9.47 1500 160 14.70 1500 176 12.00 1500 192 9.33 1500 208 6.80 1500 224 5.60 1500 240 5.07 1500

2) Plot the data from the Sonar Data Table in the graph below

UAF Geophysical Institute © 2001, 2007 MOD EJO 2017 Questions 1. On your graph, write “Challenger Deep” that represents the deepest part of the ocean trench. 2. How does the number for the depth of Challenger Deep you hypothesized compare to your results in the graph?

Shoe Box Model 1) On the back lab tables, you will see boxes. DO NOT open the boxes. Choose 1 box. You will notice that the box has holes on the top of the box. Each hole is 2 cm apart. 2) You will be using the ruler to measure the depth of the ocean and then graphing to your data points to map out the ocean floor. To do this, stick the ruler into each hole and push the ruler until it hits the bottom of the box. Using the cm side of the ruler, record your distance in the data table below.

Distance from shore Distance from surface Distance from shore Distance from surface (cm) (cm) (cm) (cm) 0 18 2 20 4 22 6 24 8 26 10 28 12 30 14 32 16 34

3) Graph your data from step 2. HINT: Look at your other graph to see how it should be set up. 1 cm = 1000 meters. Label your axes!

2. Once you have graphed your data, gently open the box and examine the ocean floor. Compare your graph to ocean floor in the box? How close did they match?

3. On your graph, label the following structures where applicable: continental shelf, continental slope, continental rise, abyssal plain, mid-ocean ridge, seamount, and trench.

4. What are some limitations to making sounding of the ocean floor (which is what you just modeled)?

5. What are some ways you could improve the “resolution” in how you see the ocean floor?

6. Describe how weather and tidal conditions could make using sonar as a way to map the ocean floor more difficult.

7. What types of features would be impossible to map?

8. Why do we need an accurate map of the ocean floor?