Question 3 - 20 points

This is the other half of the paired questions 2 and 3. Here, you were asked to give a short list of possible driving forces, not energy sources, for plate tectonics. Your textbook discusses five possible driving forces for plate tectonics:

1. ``Slab Pull''. This model says that the weight of the plate sinking into the mantle at subduction zones drags the rest of the plate along with it. Mid-Ocean Ridges are just passive features in this model -- all the real action happens at subduction zones.

2. ``Ridge Push''. This model says that the force of magma rising from the mantle shoulders aside the plates at Mid-Ocean Ridges, allowing the magma to rise and form new ocean crust. In this model, the action takes place at the ridges, where the plates are being forced apart.

3. Plates Dragged by Convection. This model says that the mantle is in a state of convection, with hot rock rising from the bottom of the mantle, cooling as it reaches the top, and sinking back down into the lower mantle. In this model, the plates are just passively riding along -- they are not driving flow, they are just driven by it.

4. Plates are the Cooled Top of Convection Cells. This model says that the plates are, in fact, the upper part of convection cells, where the molten rock cools and hardens. In this model, the plates are an integral part of the convection in the mantle, not just passive features. The other interesting feature of this model is that all the convection takes place in the upper mantle, not the entire mantle as in the previous model. (Just as an aside -- whole-mantle vs. layered-mantle convection is one of the most hotly argued debates in geoscience today.)

5. Thermal Plumes. The idea here is that hot thermal plumes rise from deep within the mantle, with hot rock positively rocketing to the surface. Some of the material erupts at the surface in the form of a hot spot, while the rest hangs around under the plates, slowly cooling until it gets cold enough to sink back down into the mantle.

   A good way of visualizing this idea is to imagine holding a garden hose under a window screen, with the open end of the hose pointing at the screen from below. Turn on the water and watch what happens. Your hose is the equivalent of a thermal plume, and the screen takes the place of a plate. Some of the water will make it through the holes in the screen -- this water is the equivalent of the magma at hot spots. Most of the water, however, will flow along the bottom of the screen until it gets to the point where gravity makes it pull away from the screen and get your feet wet. That water is the equivalent of the ``return flow'' from the top of the mantle in the Thermal Plume model.

Anyone who gave me at least one of these models got full credit. Anyone who just gave me ``subduction'' or ``divergence'' got 8 points.

A number of people said that the driving force was seafloor spreading. While this may appear identical to the ``ridge push'' model above, it is in fact different. Seafloor spreading is a model which explains the existence of the magnetic stripes discussed in Question 1 (among other things) -- it is an integral part of plate tectonics. However, seafloor spreading only happens because something is forcing the plates to spread apart, either by pushing them apart, pulling them apart, or whatever. It is that force, whatever it may be, which causes seafloor spreading. Answering the question with ``seafloor spreading'' got you 10 points.

I thought you might find it interesting to hear what geoscientists think about this problem. I think most geoscientists would agree that the driving forces for plate tectonics are not completely understood. Just 10-15 years ago, the most accepted model was that the magma rising to the mid-ocean ridges pushed the plates apart, and that subducting plates had nothing to do with the picture (the ``ridge push'' model). Today, I think it's safe to say that most geoscientists think that the most important driving force is actually not ``ridge push'', but ``slab pull'' -- the weight of the cold subducting plate pulling all the rest of the plate behind it.

My own personal suspicion is that we will find out that it is a combination of ``ridge push'', ``slab pull'', and convection dragging the plates. You'd have to ask Lisa what she thinks.

Your book discusses this problem in Chapters 19 and 20, and you can look at figure 20.27 particularly.

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