Magma Upwelling

1. Obtain two shoe boxes. Use masking tape and a marker to label one smaller end on each box “Mid-Ocean Ridge.” Label the top of each box “Ocean Floor” and the other smaller end of each box “Trench.”

2. Use masking tape to secure each box to a desktop so that the ends labeled “Mid-Ocean Ridge” have a space between them of about 5 cm.

3. Two students in the group obtain a set of magma strips. Each student holds onto the stick labeled “1” and lowers the rest of the magma strip into the mid-ocean ridge between the boxes. Numbered sticks face each other.

4. Model how magma from Earth’s hot interior wells up at mid-ocean ridges by working together to gently pull up on the magma strips so that both craft sticks labeled “2” appear on each side of the ridge at the same time.

Seafloor Spreading Model

Seafloor SpreadingExplore 1

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Seafloor Spreading Explore 1

5. Draw a diagram of the model. Title the diagram “Magma Upwelling” and label the mid-ocean ridge, ocean floor, and magma.

Magma Upwelling

Formation of New Oceanic Crust

1. Slide the magma strips so that the craft sticks are flat on the “ocean floor.” Continue sliding so that both craft sticks numbered “3” appear at the same time on each side of the “mid-ocean ridge.” Model how magma flows evenly to both sides of the ridge to form new ocean crust by pulling the sticks so that the same number appears on both sides of the ridge.

2. Notice the “red basalt” and “white basalt” that appear at the ridge. The “red basalt” represents magma that pushed up, cooled, and then solidified during a period when Earth’s magnetic field was in normal polarity. The “white basalt” represents magma that pushed up, cooled, and solidified when Earth’s magnetic field was in reverse polarity (if the rocks had a compass needle within the layers, it would be pointing south instead of north). This change in polarity is evidence that plates have changed position over time and supports the theory of plate tectonics.

3. Draw a diagram of the model. Title the diagram “Formation of New Oceanic Crust,” and label the stripes of basalt, the direction of motion, and indicate the polarity.

Magma

Ocean floorOcean floor

Mid-Ocean Ridge

N N S S S S N N N S S S S N N N S S S S N NFormation of New Oceanic Crust

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Seafloor Spreading Explore 1

Subduction in Trenches

1. As magma flows out of the ridge, the newly formed crust pushes older crust away from the ridge. Continue pulling the magma strips until the magma sticks slide across the ocean floor and drop off into the trenches. As the ocean crust moves under the continental crust, it creates a trench. Ocean crust is recycled as it subducts and moves toward Earth’s hot interior where the crust re-melts into magma once again.

2. Draw a diagram of the model. Title the diagram “Subduction in Trenches” and label the younger crust, older crust, and trench. Show the direction of movement.

Reflection Questions

1. Magma is pushed up through the ridges in the seafloor to form new oceanic crust. The energy from Earth’s interior is transferred as the magma rises to cause the materials to move. As each new layer emerges, the prior layer is pushed away from the ridge. If the space between the boxes is the ridge, and the craft sticks represent the magma, then which sticks represent the oldest portion, and which sticks represent the youngest portion of oceanic crust? Explain your answer using the numbers on the magma sticks.

Tren

ch

Trench

Younger crustOlder Crust Older Crust

Subduction in Trenches

The students should understand the sticks representing the oldest portion of the seafloor are the sticks marked “1,” the first sticks pushed up through the ridge. The youngest are the ones closest to the ridge. The oceanic crust is made of strips of cooled magma from the ridges, with either side being, in a sense, mirror images. By falling off the boxes, the sticks are being subducted into the trench. As new crust is created at the mid-ocean ridge, old crust is subducted under continental crust (creating a trench) and melted.

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Seafloor Spreading Explore 1

2. The addition of new oceanic crust is one mechanism that applies force to tectonic plates. The model did not accurately show the direction of force during the creation of new crust. How could you change the model to show that magma is pushed out at the ridge (rather than pulled)?

3. Compare the pattern of the “red” basalt and the “white” basalt on either side of the ridge. What do you notice?

4. What forms as the new rock material continues to move away from the ridge?

5. What will happen to the oceanic crust that is pushed down at the trench? Is crust formed or recycled at the trenches?

6. This geological process is the recycling of the ocean basins. As new crust is formed at the ridge, a process called subduction occurs at the trenches. Why must subduction occur at the trenches?

7. How does seafloor spreading provide evidence of past plate motion?

Answers will vary. Students should change the model so that magma is pushed out of the ridge, not pulled.

Students should note that the pattern is the same on both sides of the ridge.

Students should note that the ocean basin forms.

It will be melted to make magma. The crust is recycled because it melts into magma.

If subduction did not occur, then Earth would continue getting bigger because crust would be created but not destroyed.

Seafloor spreading is the driving force explaining why the plates are being pushed apart.

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