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These can be summarised as: 1. A pulsating Earth, where cyclic expansion of the Earth is said to have opened the oceans and contractions are said to have caused orogenesis (mountain-building). This proposal fails to satisfy exponential expansion, as shown by modern oceanic mapping, and the late Professor Carey considered the theme to have arisen from the misconception that mountain-building implies crustal contraction. In addition, Carey saw no compelling evidence for intermittent contractions of the Earth. 2. Meteoric and asteroidal accretion. This is currently a popular theory, also proposed to explain some of the various extinction events that have plagued the Earth. It basically says that expansion is caused by an accumulation of extraterrestrial debris over time. This theme was rejected by Carey as the primary cause of Earth expansion, since expansion should then decrease exponentially with time, not increase as shown by the oceanic mapping. Nor does it explain ocean-floor spreading or the distribution of oceanic crust or covering sediments. 3. Constant Earth mass, with phase changes of an originally super-dense core. This was again rejected by Carey as the main cause of Earth expansion because the theme implied too large a surface gravity throughout the Precambrian Superaeon to the Late Palaeozoic Era. A large Precambrian surface gravity was not evident from studies carried out during the 1970s. Fora constant Earth mass, density would have also been unacceptably high during the Precambrian. 4. Secular reduction of the universal gravitation constant, G. Such a decline of G was said to cause expansion through the release of elastic compressional energy throughout the Earth and phase changes to lower densities in the mantle. Carey rejected this proposal as the main cause of expansion for three reasons: (a) formerly, the surface gravity would have been unacceptably high; (b) the magnitude of expansion is probably too small; and (c) the arguments for such a reduction in G were considered not to indicate an exponential rate of increase in radius. 5. A cosmological cause involving a secular increase in the mass of the Earth. This suggestion remains the most popular theme. Where the required excess mass came from was considered at length by Professor Carey. Einstein's equation E = mc’ suggested to Carey (and others) that matter and energy are interconvertible. Matter is therefore the antithesis of energy, where matter is created from energy and vice-versa. Carey further considered that new matter added to the Earth must appear deep within the core. But, he also considered that the ultimate cause of Earth expansion must be sought in the cosmological expansion and . complementary generation of new- ~ matter processes within the entire Universe. The proposed causal model for Expansion Tectonics (figure 6), while still largely speculative, involves an increase in mass by segregation of new matter from energy within the Earth's core. This new matter accumulates at the core-mantle interface, and the increase in volume results in swelling of the mantle. This proposal is borne out by seismic and tomographic evidence, whereby the core-mantle interface is shown to be the most active part of the Earth's interior. In this proposal, mantle swell is then manifested in the outer crust as crustal extension and is currently occurring as extension along the mid-ocean-rift zones. Matter segregation within the Earth's core is seen as an ongoing reaction, which, if the reaction decays with time, may ultimately reverse the present exponential Figure 6: Cross-section through the present-day Earth showing crustal extension, sea-floor spreading and opening of the modern oceans in relation to mantle swell. 36 * NEXUS JUNE - JULY 2010 www.nexusmagazine.com