The literature on the topics covered in this chapter is large, so that the treatment of the subject matter in this chapter is of necessity brief and incomplete. However, we suggest that the discussion has been sufficient to demonstrate that there is no general acceptance for any of the various mechanisms cited. There are obvious incorrect conclusions regarding the various models proposed for splitting of continents by domical uplifts or by tensile stresses. Moreover, none of the models set out address the problem presented by Lithgow-Bertelloni and Richards cited above, namely that the models used in analysing plate movements are currently imperfect, for they do not have a model which explains 'sudden plate motion changes that define stage boundaries'. (By 'sudden' we suggest that these authors mean that the period of change in plate motion is accomplished in a few tens of thousands of years.)

Even though the various concepts and studies discussed in this chapter have possibly advanced our understanding of some mantle phenomena, after decades of research no clear-cut and completely irrefutable mechanism has come to light that will satisfy the requirements set out by all authors cited above. Clearly, large volumes of basalt must reach the surface of the Earth. However, the uprising of hypothetical mantle plumes and the melting phase that takes place below the lithosphere requires several millions of years, followed by another million years or so in which basalt erupts on the surface of continents and the floor of oceans. None of the mechanisms cited above can explain the development of 'sudden plate motion changes that define stage boundaries'. Indeed, such sudden changes in direction and/or speed of plate motions simply do not come into the parameters considered by the various plume models.

Indeed, geoscientists have been ignoring a vital parameter. We have argued that the assumption of instantaneous lateral stretching is not valid. Nevertheless, we agree that the very large volumes of melt required to form LIPs can only be generated by dramatic thinning of the lithosphere. We are at odds with the authors cited earlier because they have only considered thinning of the lithosphere to be generated from the mantle upward. This has resulted in them requiring an impossible, instantaneous, stretching mechanism. However, in the following chapters, we shall demonstrate that the thinning of the lithosphere and the production of large volumes of melt can be generated by substantial thinning, or even complete removal of the lithosphere, from the surface of the crust downward, by the impact of large cometary bodies. In Chapter 5 we will introduce factual evidence regarding important experimental work which reproduced 'Moon craters on Earth', as well as presenting the pertinent theoretical concepts regarding 'impact tectonics'.

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