To pic : Hydrothermal Vents

Data Puzzle

Student Pages

How much heat is released by a seafloor hydrothermal vent?

H

ydrothermal vents are places where hot, mineral-laden water jets out of the sea floor, as shown in the photograph in Figure 6.1. These vents are found at places where two of the Earth’s tectonic plates are pulling apart from each other. At such places, called mid-ocean ridges, hot rocks and magma are close to the sea floor. Water from hydrothermal vents changes the chemistry of the ocean, supports chemosynthetic life forms, and transports heat from the Earth’s crust into the ocean. But just how much heat do these vents produce?

Figure 6.1

Hydrothermal Vent on Ocean Floor

Directions: Follow steps 1 to 18 below. Use additional sheets of paper as needed and answer questions in complete sentences.

Heating Up Water in Tea Kettles 1.

First, a reminder of some concepts and units we will need to solve this puzzle. We are using Source: Courtesy of the National Aeronautics and Space metric units: liters (L), milliliters (ml), grams Administration (NASA). (g), meters (m), centimeters (cm), degrees Celsius (°C), and calories (cal). Be sure to include units in all your answers. 1a. How many milliliters are in a liter? 1b. What is the density of water? (Although the density of Earth’s

EARTH SCIENCE PUZZLES:

MAKING MEANING FROM DATA

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Data Puzzle

Hydrothermal Vents

Student Pages water varies, we are making an approximate calculation so it is OK to use the density of pure water at standard pressure and temperature.) 1c. What is the temperature of water as it begins to boil? Note: Your answer is only good at sea level. At a higher pressure, the boiling temperature would be higher, too. 1d. What is specific heat? 1e. What is the specific heat of liquid water?

Discuss step 1 with your teacher before moving on to step 2. 2.

Imagine a kettle full of water at room temperature, 20º C. Now imagine heating the kettle until just before it begins to boil. (Assume no phase change occurs.) What would be the change in temperature of the water in the kettle?

3.

Recalling your knowledge of the specific heat of water, how much energy was required to heat 1 g of the water in the kettle from room temperature up to the temperature just before it began to boil (again with no phase change)? Show how you found your answer.

© Wilton Industries, Inc. Used with permission.

4a. A typical kettle holds 2.5 L of water. What is its volume in milliliters? 4b. What is the mass of the water in a full kettle? (Hint: Use your knowledge of density of water.) 5.

How much energy was required to heat the entire kettle up to the point just before it began to boil? (Hint: You have determined how much energy was required to heat each gram of water in the kettle, and how many grams of water there are in the kettle. Combine these two pieces of information to answer this question.) Now you know just how much heat is needed to bring one tea kettle to the point of boiling. Let’s see how that compares to the heat being released by one hydrothermal vent.

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Data Puzzle

Hydrothermal Vents

Student Pages

Heating Up Hydrothermal Vent Water 6.

Scientists studying hydrothermal vents use research submersibles to observe and photograph the vents and take samples of vent water. Describe observable features of the vent shown in the photograph in Figure 6.1, page 145.

7.

Scientists measured the opening of one typical hydrothermal vent to be 3 cm in diameter. Sketch a 3 cm diameter circle along with your other vent observations. What would be the most accurate analogy to describe water coming out of a vent this size? Circle your choice below. ฀ Like water out of a water pistol ฀ Like water out of a garden hose ฀ Like water out of a fire hydrant

Figure 6.2

Introduction to Steps 8–11: The water that comes out of hydrothermal vents begins as cold seawater. It seeps down through cracks in the rocks of the ocean floor, somewhat like rain seeps into the ground after a storm on land. At mid-ocean ridges where hydrothermal vents occur, hot magma is close under the sea floor. If the sinking water comes into contact with hot rocks warmed by underlying magma, it can be heated up to the temperature of a hydrothermal vent.

Changes in Seawater Temperatures (to be completed by student)

Source: Diagram by Zina Deretsky, National Science Foundation, modified from www.nsf.gov/discoveries/disc_summ. jsp?cntn_id=110976&org=NSF by Deresky with permission.

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Data Puzzle

Hydrothermal Vents

Student Pages 8.

9.

The temperature of seawater at the bottom of the ocean is approximately 2º C. Write in “2º C” in the appropriate space in the diagram in Figure 6.2 on page 148 to show the temperature of the water sinking down through cracks in the sea floor. Color the down-going flow blue to indicate cold. Scientists also use submersibles to place instruments in and around vents to look for changes in the chemistry and temperature of vent water over time. Table 6.1 shows some temperature data from a vent in the East Pacific Rise, which is 2,550 m below the ocean surface. The vent is located in a field area for biologists studying vent organisms. Scan down through the data table. Select one of the following three choices to describe the temperature of the vent water during this time interval. Defend your answer.

Table 6.1

Temperature Data From a Vent in the East Pacific Rise (to be completed by student) Month

Average Temperature

Nov.

347.0°C

Dec.

347.7°C

Jan.

347.8°C

Feb.

347.9°C

Mar.

347.9°C

฀ highly variable Average ฀ slightly variable ฀ nearly constant The temperature of this hydrothermal vent water is hot enough to melt plastic! When hydrothermal vents were first discovered, the pilots of the submersible were worried that the windows of the submersible might melt. Thankfully, the windows did not melt. 10a. Calculate the average temperature of the vent water during the fivemonth period in Table 6.1. Put your answer in the table, rounding to the nearest tenth of a degree. 10b. Record your answer on Figure 6.2 (p. 147) to show the temperature of the water coming up out of the Earth’s crust, through the vent openings, and into the ocean. Color the arrows red to indicate hot water. 11. By how many degrees did the temperature of the water change from when it seeped down through the sea floor to when it returned to the ocean as hydrothermal vent water? Show your reasoning. 12. Recalling your knowledge of specific heat of water, how much thermal energy would be required to raise the temperature of 1 g of ocean water up to the temperature of the vent water? (Hint: It may help to look back at your tea kettle calculation.) 13. What was the source of the thermal energy that caused this increase in temperature? 14. Hydrothermal vents release much more than 1 g of water. In fact, one hydrothermal vent has been estimated to expel hot water at a rate of

148

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Data Puzzle

Hydrothermal Vents

Student Pages 3 L per second. For a vent expelling water at 3 L per second, 14a. how many liters of hot water would be released in a minute? 14b. how many liters of hot water would be released in an hour? 14c. how many milliliters of hot water would be released in an hour? 14d. what is the mass, in grams, of hot water released in an hour? 15. How much thermal energy was required to heat the water released from one vent in one hour up to hydrothermal vent temperature? Show your reasoning. (Hints: You will need to combine two numbers you have already calculated. Refer to your teakettle calculation. Be careful about units; you are looking for an answer in calories.)

Is This a Lot of Heat? Congratulations on completing this calculation. Scientists make such approximate calculations to get a “sense” or a “feel” for how large or small an object is or how important or unimportant a process is. This intuitive Earth-sense is valuable, even if some of the numbers going into the calculation may not be precise. Are we dealing in this puzzle with a lot of thermal energy or not very much? How does the hydrothermal vent thermal energy compare to tea kettle energy? To Recap: In step 5, you calculated how much thermal energy is required to raise one kettleful of water from room temperature to the boiling temperature. In step 15, you calculated how much thermal energy is required to raise one hour’s worth of vent water output from normal sea floor temperature up to hydrothermal vent temperature. Now let’s move on. 16. Think about the amount of thermal energy from the Earth that was used to heat up the water that came from the vent during the time (about one hour) that you have been working on this puzzle. If that energy had been used to heat tea kettles instead, how many kettles could have been heated to the boiling point?

Table 6.2

17. One kettle can fill approximately 10 mugs. How many mugs could be filled from the number of kettles in your answer in step 16? 18. Imagine that one cold day we wanted to serve a mug of cocoa to every man, woman, and child in an entire town. Which of the towns in Table 6.2 has a population that could be served by the number of mugs heated by an hour’s worth of vent heat energy? Try not to waste any more cocoa than necessary. Explain your answer.

EARTH SCIENCE PUZZLES:

Four U.S. Towns and Their Populations Town

Population

Boise City, ID

185,787

Boston, MA

589,141

Fort Lauderdale, FL

152,397

Buffalo, NY

292,649

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