Back from conference ... and I see we have a new discussion from Mike.

I had a problem with the extra mass (obviously) but the thing that hit me was does the theory require extra mass?

I have seen reports of the earth changing shape (this is way outside my area)

http://news.nationalgeographic.com/news/2002/08/0807_020807_earthgirth.html
http://www.nasa.gov/centers/goddard/earthandsun/earthshape.html

Originally Posted By: Bill S

Would I not be right in thinking that in plate tectonics theory it is the up-welling of magma that initiates the movement of plates by causing a slope down which the plates slip under the influence of gravity?


Correct the process is called ridge pushing and the edges are forced under the continental plates being lighter and they therefore hang down also dragging the plate via slab pull.

Since the advent of GPS we have the movements of the Earth for the last 15 years (http://sideshow.jpl.nasa.gov/mbh/series.html)


Originally Posted By: Bill S

Would the friction involved, plus the heat of the mantle material, not ensure that the subducting plate margin was neither cold, nor particularly hard?


Again I am no expert in this field but the current view is the subducting crust is deflected down at quite an angle in the Benioff zone it is not just being forced underneath so to speak.
(http://myweb.cwpost.liu.edu/vdivener/notes/subd_zone.htm)

Perhaps someone can tell me if we have any empirical data or proof of the Benioff zone that I don't know sorry Bill S.

I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.