426 EARTH STRUCTURE AND ORIGINS islands become progressively younger towards the south-east Oceanic lithosphere is hypothesized to move over a static hotspot Sceptics believe that the 670 km barrier cannot be penetrated by a rising plume (see Mantle Plumes and Hot Spots) There is also no agreement on the mechanism of plate movement Ridge push, subducting-slab pull, and a combination of both have all been favoured, as has a mechanism of gravity gliding The plate movement must start with spreading away from the ridge before subduction begins to operate (no slab yet to pull), so slab pull can never provide the entire answer It seems fair to say that, whereas the plate-tectonics paradigm must reasonably be accepted as fact, the mechanism remains unexplained, and mathematical calculations not really explain how the forces needed to move the plates are derived (see Plate Tectonics) Figure The widely accepted configuration of plate movement away from a mid ocean ridge and down subduction zones be neath continents (reproduced from Van Andel TJ (1994) New Views on an Old Planet, 2nd edn Cambridge: Cambridge University Press, with permission from the author and publisher) Figure 10 The tectonic plates of the Earth, Ar Arabia; Phil Philippines (reproduced from Van Andel TJ (1994) New Views on an Old Planet, 2nd edn Cambridge: Cambridge University Press, with permission from the author and publisher) yielded conflicting results and suggested differences in movements at margins and within plates There are conflicting opinions as to whether convection operates down into the lower mantle (the more orthodox view) or only above the 670 km boundary, which forms a barrier The latter view (‘closed upper-mantle circulation’) is held by D L Anderson and W B Hamilton and considers the reality or otherwise of deep plumes, rising from the base of the mantle Plumes were originally invoked to explain chains of seamounts and/or islands such as the Emperor Seamount and Hawaiian Islands in the Pacific Ocean, where the ages of the individual Geochronological Comparisons – Earth and Other Solar System Bodies It is early days yet in Space exploration, so, whereas we can derive a very detailed and accurate chronology for our own planet, based on the radioactive decay of isotopes, palaeontology, palaeomagnetism etc., we have very little chronological information about other bodies There is no palaeontological data for other bodies (there may have been no life there) We have isotope-based data for the Moon, but there is really no geochronological data from there for the last 3000 Ma or so Palaeomagnetism cannot be applied to bodies other than the Earth There is a set of dates for the formation of the rocks comprising the SNC meteorites (shergottites, Nakhlites and chassignites), which are supposed to have come from Mars, and these have contributed to a Martian geochronology (which may yet prove to be wildly incorrect) Isotopic dates of around 4500 Ma for the formation of the asteroids have been derived from chondritic meteorites, and slightly younger dates, recording a slightly later melting process in the asteroidal parent bodies, have been derived for achondritic meteorites (see Solar System: Meteorites) For Venus and Mercury we have no yardsticks at all, although surmises have been made from the radar-derived images of Venus Mercury resembles the Moon in its surface cratering and appears, like the Moon, to be a ‘dead’ multicratered body with no or negligible eruptivity since the deduced bombardment that formed the craters (see Solar System: Mercury; Venus) Sequential relationships without any benchmarks have been derived from the superposition orders of crater populations and volcanic structures The comparison diagram (Figure 11) utilizes the very limited evidence from outside the Earth to make this comparison