5. Seafloor Spreading - Lesson Plan
Infer plate motion
- Review convection currents in the Earth’s mantle. Discuss how convection currents form and how they move. (The heat from the core causes mantle in some areas to rise while cooled mantle near the crust sinks.)
- Discuss how things floating on top of the surface of this mantle move. (At the surface, stuff just above the places where hot mantle rises gets pushed away from each other and stuff where the cool mantle sinks gets pulled down or pushed against the edge of something solid.)
- Tell students you are going to switch gears briefly.
- Review the causes of earthquakes. Discuss how earthquakes are caused when two plates collide or rub against one another. For a while, the plates get stuck even though the pressure is building up all the time and finally, when the plates release, you get an earthquake.
- Discuss how speed might affect the number of earthquakes. Would you expect more earthquakes each year if 2 plates collide at high speed or at low speed?
- Discuss whether earthquakes would be expected where plates are separating. (Not so much.)
- Put it together. So, the Earth’s surface is made up of plates riding along on convection currents in the mantle. The plates are moving in all different directions. In some places the will collide head on. In some places they will rub against one another going in opposite directions. Where they collide or rub, you’ll get lots of earthquakes. In some places they will separate from one another. Where the separate, you’ll get much fewer earthquakes.
- Ask students to get out their color coded World Earthquake Maps.
- Tell them that their job, in teams of 2 or 3, is to see if they can figure out which direction each plate is moving using the information on their maps (earthquake epicenter locations, volcano zones, mid-ocean ridge locations). Their job is to draw an arrow in pencil on each plate showing the direction that plate is moving. If they believe a plate is standing still, then don’t draw anything at all. If you want to give a hint, tell them to start with the Pacific plate or the Nazca plate and go from there.
- Give students 10-20 min to discuss their maps and make hypotheses. Not that the Eurasian and African Plates don’t fit the pattern as well because they
- Different groups will figure out the patterns at different rates. For those groups that finish early, have them modify the length of their arrows to show the speed of the plates. For instance, a fast moving plate should have a very long arrow while a slow moving plate should have a short arrow.
- When most groups have finished, bring the focus of the class up to the large map at the front of the classroom. One by one, go through the plates (starting with the Pacific and Nazca plates) discussing what direction they inferred the plate would move and why. Students should correct their personal maps if there are any errors.
- Optional: when all the direction information has been added, see if students can figure out how fast each plate is moving. Use the length of the arrow to represent speed.
Seafloor spreading model
- On the students’ labeled World Earthquake Maps have them focus on a diagonal rectangular strip starting at the South American plate, crossing west over the Nazca and Pacific Plates, past Hawaii, and ending in the earthquake/volcano zone on the right edge of the Phillippine Plate and Korea.
- Tell students that they will be building a model of this region today.
- Pass out the Seafloor Spreading Model pieces, the scissors, staplers, and colored pencils.
- Begin with the page with the diagram cut-through view of the mantle showing convection currents. Fold the paper lengthwise along the fold line “hot dog style”. Focus on the large diagram for now.
- Label the diagram (mantle, mid-ocean ridge, lithosphere, oceanic crust, continental crust, island arc, volcanoes, ocean, convection currents, etc.)
- Color the diagram in.
- Use blue to represent the ocean.
- Use one color to represent the left-most oceanic crust with the volcanic island arc.
- Use another color to represent the two slabs of oceanic crust in the middle spreading outward from the mid-ocean ridge.
- Use another color to represent the continental crust on the far right.
- Use a bright color like orange or red to represent that magma in the mantle and welling up through the volcanoes.
- Now describe that the big diagram is the side view while the other part on the other side of the fold is the top view. Drape the model over the edge of the table with the large diagram hanging over the side to get everyone oriented.
- Next label and color the top of the model so that they match the colors on the cut-through view.
- Use the scissors to cut a slit along each of the dark black lines on the top of the model. Do not cut all the way to the edge. It helps to make a light fold perpendicular to the line and make a small snip with the scissors to get the cut started. Then you can open the paper up to cut the rest of the line.
- Now take the other piece of the model (the piece of paper with all the grey stripes). Cut out the grid along the outer lines and throw the edges away.
- Cut the two strips apart lengthwise down the middle “hot dog style”.
- Put the two strips together, one on top of the other with the writing on the inside, facing one another.
- Glue or staple the two strips at the end that says “Glue this end to other strip”.
- Feed the stapled/glued end into the slit labeled “mid-ocean ridge”.
- Feed the free end of the left hand strip into the subduction zone on the left.
- Feed the free end of the right hand strip into the subduction zone on the right.
- Hold a free end in each hand and slowly pull the strips of paper through the subduction zones. “New” ocean crust should appear at the mid-ocean ridge. Pull down at the mid-ocean ridge to reset the model.
- Point out the divergent and convergent boundaries on the model.
- Once the models have been built, discuss what the white/grey stripes represent – magnetic reversals recorded in the rock on the ocean floor in the form of tiny pieces of iron, aligned with the Earth’s magnetic field.
- Discuss what the age markers mean – the ocean floor nearest the mid-ocean ridges are youngest and the ones nearest the subduction zones are oldest.
- Point out the mirror-image symmetry on each side of the mid-ocean ridge and how that provided the first strong evidence to scientists that seafloor spreading actually takes place. (Now GPS (global positioning systems) can measure the creep of the plates in millimeters per year, corroborating the evidence trapped in rocks on the ocean floor.)
- Refer back to the labeled World Earthquake Map and the Pacific/Nazca plate boundary area, pointing out the relationship between the map and the model. Notice how subduction zones tend to have lots of earthquakes and volcanoes. If the subduction zone is in the ocean (like the Alaskan Aleutian Islands, the Philippines, the South Pacific Islands, and the Caribbean Islands), a chain of volcanic islands forms (a volcanic island arc). If the subduction zone is on the edge of a continent (like Central America and South America), a volcanic mountain range is pushed up on the continent.
- Finally, discuss the 2 plate boundaries that are not shown in this model: transform boundaries and continent-continent convergent boundaries. Use 2 pieces of paper to have each student model these boundaries.
- For a transform boundary, slide the 2 pieces of paper past each other, one moving up, and one moving down. Explain how this is similar to what is happening at the edge of California along the San Andreas fault. Notice on the World Earthquake map how transform boundaries tend to have lots of earthquakes but few volcanoes.
- For a continent-continent convergent boundary, slide the two pieces of paper into one another on the table top but don’t let one piece slide below the other like subduction. Instead, the 2 pieces of paper should collide and then buckle, each piece wrinkling up off the table. Explain how this is similar to what is happening at the top of the Indo-Australian plate where India has rammed into the Eurasian Plate. Notice on the World Earthquake Map how continent-continent convergent boundaries have lots of earthquakes and huge mountain ranges.
Submitted by irene on Fri, 2006-08-11 10:38