4
$\begingroup$

This is a question that has bothered me for many years. As a new teacher I was asked this question by a 12 year old in our plate tectonics class. I said I would go away and find out. I would really appreciate some help!

I have some understanding of plate tectonics but I struggle with understanding how the plate boundaries have changed in terms of their actual spatial positions over long periods of time. For example, what would the 'jigsaw pattern' of plates at the time of Pangea have looked like? At plate margins, does the pressure of compression or expansion actually change the spatial position of the margin? Anderson comments... "The definition of a plate and a plate boundary is subjective. Things are constantly changing. It is improbable that there is a steady state or equilibrium configuration of plates because both ridges and trenches migrate and only a few of the 16 kinds of triple junctions are stable."

This serves to confuse me further unfortunately. I have no issues understanding how continents move around, it is the potential change to the size, shape, orientation and position of plates that baffles me!

This probably relates to the potential for new plate boundaries to be formed...and existing ones to close up. Is this possible? Sorry if this is an overly simple question. If this is possible it could explain a lot. A geologist friend told me that the East African Rift Valley is an example of a weakness in the crust that could turn into a plate margin. If that's true that could explain a lot. If new plates can form and existing ones disappear this would go a long way to solving my dilemma.

Any views would be much appreciated and I can then give my 12 year old pupil a decent answer! Many thanks all.

PS. Just about every geography text book out there suggests plate tectonics is a result of convection currents in the mantle. Many teachers teach this because that's what they've been taught. I appreciate the science has moved on and now involved slab-pull and several other potential forces! Without making it too complex, what should we be teaching 12+ year olds?

$\endgroup$
  • $\begingroup$ Plate tectonics is part of large scale mantle convection so keep teaching them that. Whilst you're at it remind them that the mantle is solid. It is not made of liquid magma. $\endgroup$ – bon May 12 '18 at 16:27
  • $\begingroup$ Thanks..very helpful and good to know I;m not leading them too far astray! I tend to describe the mantle as being like treacle or 'golden syrup'..am I wrong to do this? Is it more solid than a viscous substance like treacle? $\endgroup$ – nbgeog May 13 '18 at 19:38
  • $\begingroup$ Yes it is solid, not liquid like treacle. If you got a piece it would look just and behave just like any other rock you have ever seen. However, on long timescales it does flow. A good analogy for students is if you heat up chocolate it stays solid but it goes soft as it gets closer to its melting temperature. This is what happens to rocks in the mantle. $\endgroup$ – bon May 14 '18 at 6:58
  • $\begingroup$ Thanks - much appreciated. I'll use your chocolate analogy in future! $\endgroup$ – nbgeog May 15 '18 at 7:49
3
$\begingroup$

I am not sure if you are actually asking about Tectonic Plate boundaries, or the boundaries of continents, which are often described a puzzle pieces when teaching continental drift due to the way continents like South America and Africa fit together. I'm going to try and address both possibilities.

Tectonic Plate boundaries are constantly changing. Many of the edges of tectonic boundaries are thin freshly-formed oceanic crust which isn't going to put up much of a fight. You know about subduction so you understand that oceanic crust is constantly 'recycled'.

The present day amount of continental crust on earth hasn't existed for all of earth's history. Current thinking is that it has been been accumulated overtime (theories differ as to how quickly), through a number of processes which have built up the present level of continental crust that is more buoyant that the mantel and oceanic crust and thus resistant to subduction. As a result the size and shape of the continents have been slowly changing and growing over earth's history as they gain new bits, join and split.

For example, what would the 'jigsaw pattern' of plates at the time of Pangea have looked like?

this video shows continents moving around, growing and joining and how they change over time, at least for the last 1.5 billion years of earth's 4.5 billion history. Pangea was around from 335Ma to 175Ma, you can see it form and break up in the video.

https://www.youtube.com/watch?v=IlnwyAbczog

This probably relates to the potential for new plate boundaries to be formed...and existing ones to close up. Is this possible?

In this video blue and red lines show active tectonic plate boundaries. you can see that they change. They move around, old ones stop and new ones start. They even change from divergent to convergent or consume each other.

At plate margins, does the pressure of compression or expansion actually change the spatial position of the margin?

Absolutely. at the end of the video watch India slam into Asia and create the Himalayas as a result.

I'm not an education specialist, but i have seen videos like these be used to explain the processes to kids.

$\endgroup$
  • $\begingroup$ Thanks so much - that video is exactly what I was looking for but couldn't find! I have used video heavily in my lessons but none of the ones I found included the plate margins, just the movement of the continents (both past and into the future). In my experience, video is a great learning tool especially for situations that are 'in flux'. I had no idea the situation was as fluid and changeable!! Plate margins seem to be dissolving and emerging constantly $\endgroup$ – nbgeog May 11 '18 at 19:43
  • $\begingroup$ @nbgeog, glad Crumpet's answer was helpful. If you are satisfied this is a best answer to your question, there's a check mark button you can use to mark it as your "accepted" answer. $\endgroup$ – JeopardyTempest May 12 '18 at 7:56
0
$\begingroup$

uneven heating and cooling of the earths core cause the plates to move in all directions. this is not unlike the same way the sun alters our weather. however one must think in 3 dimensions to determine what influences what - afterall everything is connected and constantly moving. Example home owner had a roof leak. so he kept replacing the shingles directly above the leak - still kept leaking. contractor had to replace the entire roof and determined that the leak was actually 25 ft away

$\endgroup$
  • $\begingroup$ Many thanks Chuck - I'll try and think in 3-D!! $\endgroup$ – nbgeog May 11 '18 at 19:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.