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Here is a list of possible answers (in no particular order):
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.
- Seafloor Bathymetry. Plate tectonics states that new seafloor
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.
- 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.
- 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.
- 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.
- 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!
- 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!
- 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.
- 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
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.
- 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.
- 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.
- 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
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
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
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
this web page has some really good information on this problem.
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Up: ES 10 HW
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Mon Feb 17 15:24:05 PST 1997