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Next: Question 2 - 20 Up: ES 10 HW Previous: Short-Answer Questions

Question 1 - 20 points

Here is a list of possible answers (in no particular order):

  1. Seafloor Bathymetry. Plate tectonics states that new seafloor is made at mid-ocean ridges. Thermal models state that as the rock cools, it sinks at a particular rate, related to its age. Actual seafloor depth data match the predictions of plate tectonics.

  2. Magnetic Stripes. A pattern of alternating magnetic ``stripes'' is seen on the seafloor. Plate tectonics gives a simple explanation for them -- as rock spreads away from mid-ocean ridges and cools, it becomes magnetized in the direction of the Earth's magnetic field at the time of its formation. As the magnetic field flips back and forth in direction and the seafloor spreads apart at the mid-ocean ridges, stripes of alternating magnetization direction are formed on the seafloor.

  3. Seafloor Ages. This ties in with the item above, but is measured differently. The ages of seafloor rocks, as measured by looking at drilling cores from the Ocean Drilling Project, can be explained nicely with plate tectonics.

  4. The Locations of Earthquakes and Volcanoes. Plate tectonics does a good job of explaining the locations of most earthquakes and volcanoes. Also, plate tectonics correctly predicts the sense of motion along transform faults, which can be verified by examining the actual movements as seen in earthquakes which occur along those faults.

  5. SLR and VLBI. SLR is ``Satellite Laser Ranging'', which is a fancy way of saying ``shoot a laser beam at a satellite and figure out how far away you are from it.'' If you do repeat measurements using SLR, and you have very accurate estimates of the orbits of the satellites, you can figure out how your position on Earth is changing.

    VLBI is ``Very-Long Baseline Interferometry.'' In this technique, you use radio telescopes to look at objects very far away in deep space, and from observing those objects, you can figure out how the positions of points on Earth are changing.

    Both of these high-tech techniques (which I have not even scratched the surface of) can be used to actually observe the motions of plates as they happen today. You can actually ``see'' plate tectonics happening -- if you wait long enough!

  6. Matching Continental Shelves. If you look at a map of the Atlantic Ocean floor, and look at the shapes of the continental shelves, you will see that they look as though they once fit together pretty nicely. But if they were once together, how did they get so far apart? Plate Tectonics!

  7. Apparent Polar Wander Paths. This is that fancy palaeomagnetism thing that Lisa was discussing, with the dots connected by lines, all on a globe. Basically, if you figure out where the North Magnetic Pole was over time, and plot those on a globe, it appears to move around. Of course, the North Pole didn't move around - it was the plates moving around. If you take measurements from various continents which were once part of Pangaea, but are now widely separated, the polar paths fit nicely on top of each other -- if you put the continents back together. Plate Tectonics!

  8. Fossil Distribution and Evolution. Very similar fossils of creatures which could not swim across large bodies of water are found on continents which are separated now by oceans. Your textbook uses the example of Mesosaurus, but there are many others as well. If these animals couldn't swim from continent to continent, and the continents are now far apart, the continents must have once been much nearer to each other, if not actually joined.

    Also, certain fossil lineages show very similar evolutionary paths in currently widely separated geographic areas, up until a certain point in time -- then the evolutionary paths diverge radically. The ages of the sharp divergences appear to correlate with the proposed age for the breakup of Pangaea.

  9. Locations of Ancient Mountain Belts. In several widely separated parts of the world are mountain ranges with very similar ages. If one assembles the continents into a single supercontinent using the continental shelves (Pangaea again!), these mountain ranges appear to line up and form large belts.

  10. Distribution of Rock Sequences. Just like with fossils, there are very similar rock formations on continents which are now widely separated. If one assembles the continents into Pangaea, these ``divorced'' rock sequences unite into larger sequences.

  11. Distribution of Glacial Deposits. Glacial deposits can be used to determine flow directions in a glacier (see Chapter 15 in your textbook). Such deposits are found in many locations on Earth, several of which are currently in low or middle latitudes, not near the poles. The direction of flow indicated by these deposits makes no sensible pattern, but if the continents are squished back together into Pangaea, the patterns make sense. The glaciers appear to be radiating from a single point near the South Pole.
Anyone who gave me five or more of the above pieces of evidence, with a little explanation (like I have above), got full credit for this problem.

Several people had answers which were wrong, primarily about earthquakes and volcanoes, continental drift, and Pangaea.

The mere existence of earthquakes and volcanoes is not strong evidence for plate tectonics. It is certainly possible to think of mechanisms which could generate earthquakes and volcanic activity without plate tectonics. For example, there are moonquakes -- mostly due to tidal pulls from Earth -- and the Moon does not have plate tectonics going on. Also, there are volcanoes on other planets and moons, notably Jupiter's moon Io. Io's volcanoes are not caused by plate tectonics -- again, they are largely driven by tidal heating, this time from Jupiter. Instead, plate tectonics does a good job of explaining why earthquakes and volcanoes happen where they do -- and that's the important part.

Also, Pangaea is not evidence for plate tectonics. You must keep in mind that Pangaea was never directly observed by anyone or anything alive today. It is only a theoretical result of the ideas of plate tectonics. If you take today's plate tectonics and try to reconstruct the past, you can end up with Pangaea. But you cannot turn that around -- start with Pangaea, break it up, and end up with today's plate positions -- without assuming something about plate tectonics and how it works. So Pangaea is a theoretical result of running plate tectonics backwards, not actually a piece of evidence for plate tectonics.

Continental drift is also not evidence for plate tectonics. Continental drift is a theory which was devised to explain some of the evidence mentioned above -- the fit of the continents, the fossil evidence, mountain belts, glacial signatures, and rock sequences. Those individual pieces of evidence are evidence for plate tectonics, but continental drift itself is not. Continental drift is just another theory which can explain those pieces of evidence.

You can read a lot about this in Chapter 20 of your textbook, as well as in the lecture notes from Lecture 4 and Lecture 10. Also, this web page has some really good information on this problem.

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Greg Anderson
Mon Feb 17 15:24:05 PST 1997