Investigation of how unbalanced forces cause changes in the speed and direction of tectonic plates, using the Hawaiian Island/Emperor Seamount Chain.

Time Frame - 30 minutes

Materials 

• Topographic map of the North Pacific seafloor based on gravity anomalies
• Map showing the age progression of the Hawaiian Islands
• Regional map with scale

Advance Preparation

Presenter/teacher will chronicle the historical development of the theory of Plate Tectonics. This will involve a discussion of the continental drift and seafloor spreading. Presenters/teachers will point out the mid-ocean ridges on a physiographic map of the ocean basins and explain their role as "spreading centers". Presenters/teachers will also point out the occurrence of deep-sea trenches and briefly explain their relevance with respect to the theory of plate tectonics. Students may be provided with background reading materials prior to this learning exercise.

Presenter/teacher will point out some of the long, straight chains of volcanic islands that are prevalent in the ocean basins (i.e., the Hawaiian Islands, the Marshall Islands, the Samoan Islands, the Emperor Seamounts, and Easter Island). Their occurrence represents the surface manifestations of volcanism that originated deep within Earth’s mantle in association with narrow upwelling systems, known as mantle plumes or hotspots. The positions of hot spots tend to remain relatively stable in comparison to tectonic plates. As the oceanic crust moves over a hotspot, successive eruptions can produce a linear series of peaks or seamounts on the moving crustal plate. Thus, hotspots are useful in tracing plate motion.

Procedure

Presenters/teachers will use the topographic map (based on gravity data) of the North Pacific Ocean floor showing the Hawaiian Island and the Emperor Seamount Chain with the age data provided to make inferences about the plate motion.

Participants/students should use the scale on the maps provided on the map, in conjunction with the age data provided, to calculate how fast the Pacific plate is moving over the Hawaiian mantle plume (hot spot).

Formative Assessment - A

1. Look at the map of the Pacific Ocean.  What pattern do you observe when you look at the Hawaiian Islands?  The Emperor Seamounts?

Answer:  The Hawaiian Islands and Emperor Seamounts form a continuous chain of volcanoes that make tracks across the Pacific Ocean.  The volcanoes are generally oriented from northwest to southeast, with a bend in the middle.

2. The youngest volcano in the Hawaiian Islands is at the southeastern end of the chain.  What can you infer about the recent direction of movement of the Pacific Plate?

Answer:  Recent Pacific Plate movement has been to the northwest.

3. Geologists believe that both the Hawaiian Islands and the Emperor Seamounts formed as a result of the same hot spot.  Based on this assumption, what can you infer about the direction of motion of the Pacific Plate?

Answer:  If both the Hawaiian Islands and the Emperor Seamounts were formed by a single, stationary hot spot, then the Pacific Plate has not always traveled in the same direction.  The Pacific Plate is currently traveling in a northwesterly direction.  When the Emperor Seamounts were formed, the Pacific Plate was traveling in a northerly direction.

4. When did the motion of the Pacific Plate change directions?

Answer: About 42 million years ago.

Formative Assessment - B

Purpose:  To simulate the creation of volcanoes over a hot spot and investigate how plate movement is related to the pattern of volcanic island formation.

Materials

• Clear plastic shoebox

• Small clear dropper bottle with small neck opening

• Red food coloring

• Hot water

• Cold water

• Styrofoam (meat tray) cut into an irregularly shaped "tectonic plate"

Procedures

1. Fill the clear plastic shoebox approximately 2/3 full with cold water.

2. Put a few drops of red food coloring and hot water into the dropper bottle.

3. Place the uncapped dropper bottle without tilting it in the center of the clear plastic shoebox making sure that the cold water in the shoebox covers the top of the bottle.

4. Float the Styrofoam "tectonic plate" in the water so that one end is directly over the bottle.

5. Carefully observe what happens to the hot (red) and cold water and where the hot water contacts the "tectonic plate."

Note:  If an air bubble becomes trapped in the mouth of the bottle, use a pencil or straightened paper clip to dislodge it.

What happens to the hot water in the dropper bottle?

Answer:  The hot water flowed out of the dropper bottle up toward the bottom of the Styrofoam "tectonic plate."

What happened to the cold water in the shoebox?

Answer:  The cold water flowed into the dropper bottle and formed a layer on the bottom of the bottle.

What is the main reason that the hot water (and molten rock) tends to rise?

Answer:  Hot water and magma are both less dense than the surrounding substances and are pushed above denser substances.

What feature forms on the Earth's surface where magma flows upward from beneath a plate?

Answer:  a volcano

6. Gently touch the top surface of the Styrofoam "tectonic plate" with one finger and try not to disturb the water in the shoebox to simulate plate motion by gently moving the Styrofoam so that a new area of the "tectonic plate" is directly over the dropper bottle.  Observe where the hot water now contacts the "tectonic plate."

7. Repeat the simulated plate motion until you run out of room in the shoebox.  Observe the pattern created among the volcanoes that would form above the plume of rising water.

8. Geologists believe that hot spots remain stationary within the mantle.  What results when a plate moves over a stationary hot spot?      Answer:  a chain of volcanoes

9. Draw an arrow on the figure below to indicate the direction of plate motion.

10. Place an X on the figure below to indicate where the next volcano is likely to form.

11. List the strengths and weaknesses of this model of the way volcanoes from over hot spots.                              

Answers may vary.  Participants may suggest that strengths include the model demonstrates that the heat source is located in one place and the plates move across that source forming a series of volcanoes; that the "tectonic plate" is rigid like the actual lithosphere; and that the "asthenosphere" in the model has the ability to flow.  It is important to be aware of weaknesses in this model which may convey misconceptions, such as that the actual asthenosphere is made up of molten or liquid material.