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?