Volcanoes form inland from the subduction zone where seafloor meets continental crust.
Explaining the hypothesis of seafloor spread -- itself a rather complex topic -- is not an easy undertaking, but can be done successfully with a bit of research and pre-planning. The theory of plate tectonics underlies any explanation of how the sea floor spreads from mid ocean ridges to subduct, or slip under, a continental plate. The two major lines of evidence rely on the potassium-argon dating of the sea floor and the magnetic alignment of the crystals and molecules in the basalt sea floor. Understanding both is essential to any explanation.
Instructions
Sea Floor Age
1. Outline the basic working of a radioactive "clock" as a means of explaining the origins of the seafloor spread hypothesis. A radioactive element like Potassium 40 (K-40) decays at a steady rate into Argon 40 (Ar-40). When a radioactive beta particle hits K-40, one proton is turned into a neutron and Ar-40 is formed. The ratio of remaining K-40 to the accumulated Ar-40 is then used to calculate the age of the rock, generally in millions of years.
2. Discuss how molten magma wells up along the mid-oceanic ridge and pushes the already solidified lava sea floor further away. When the lava is hot any existing Ar-40 is able to escape, since it is a gas. As the magma cools and crystallizes into basalt, the K-40 is sealed. As the K-40 decays the Ar-40 is trapped in the basalt and available for analysis.
3. Explain that rock core sampling across both the Atlantic and Pacific basins has turned up evidence of a banding effect in the basaltic crust, with the youngest crust closest to the separating ridge and the oldest right beside the continental margins.
4. Indicate that oceanic crust older than about 200 million years is simply never found. This is the major support for the theory of subduction whereby the oceanic crust is pushing down and underneath the continental crust that it runs into because it is so much heavier. The oceanic crust is pushed down into the mantle until it melts and rejoins the convection current in the mantle and starts a return trip to the mid oceanic ridge.
Sea Floor Magnetic Strips
5. Remind students that the new magma that comes up to create the mid-oceanic ridge is essentially lava. It is a hot, viscous fluid that takes some time to cool. As it cools, the magnetite and other iron based ores or crystals will align themselves with the current magnetic field of the earth, much as the needle of a compass would. Once cooled, that alignment is "frozen" in place, and will never change. It can be closely examined from core samples.
6. Explain that for reasons that are not entirely clear, the Earth's magnetic field reverses and the South Pole becomes the North Pole about every 300,000 years. The most recently laid down lavas will line up with the new pole, and will be pointing in the opposite direction to the immediately adjacent older sea floor.
7. Share scientists' belief that this "striping" pattern was almost exactly the same when measured either by the age of the rocks or by magnetic reversals. The two lines of evidence reinforced the concept of Plate Tectonics in general, and confirmed that the ocean floors were indeed spreading at a relatively constant rate.
8. Summarize the evidence with illustrations of the spreading that is taking place in Iceland where active volcanism is nearly continuous. In addition, the examination of both the Atlantic and Pacific Ocean plates shows the same patterns.
9. Demonstrate the patterns of seafloor spreading using simple chalkboard or white board drawings. Use different colors to indicate heavier sea floor and lighter continental crust. Discuss the trench that is created at a subduction zone, and describe it as still more evidence of the rock cycle.
Tags: continental crust, oceanic crust, Atlantic Pacific, both Atlantic, both Atlantic Pacific, Explain that
