In this book, we have adduced a significant amount of evidence that correlates known geological features. We suggest that these correlations are so conclusive that we can now, with little danger of error, assume that every major abrupt change in track, even though it cannot initially be linked to a specific event, must of necessity be attributed to a major impact. For convenience, it has long been tacitly assumed that impacts are random events, which nevertheless result in a near-linear, logarithmic, accumulation curve. However, in the last decade or so, many workers have been studying and correlating a variety of geological phenomena, which include extinctions, continental flood basalts, sea-level lows, ocean-anoxic and black shale events, and abrupt changes in rates of sea-floor spreading. It transpires that these varied events appear to be cyclical.
It was suggested by Kearey and Vine (1990) that current plate tectonic concepts permit one to infer that all major aspects of the Earth's long-term regimes should be related. For example, the global carbonate-silica cycle, which is taken to be driven by plate tectonic processes, gives a direct link between plate motion and the composition of the oceans and the atmosphere and also of the climate. It has also been shown that there have been abrupt changes of sea-floor spreading which may be correlatable to other events. Let us now briefly consider some of these aspects.
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