of secondary crust where it undergoes further fractional crystallisation. Two stages of evolution produce an increased proportion of incompatible elements. Initiation of plate tectonics Plume induced subduction The formation and development of plumes in the early mantle contributed to triggering the lateral movement of crust across the Earth's surface. The effect of upwelling mantle plumes on the lithosphere can be seen today through local depressions around hotspots such as Hawaii. The scale of this impact is much less than that exhibited in the Archean eon where mantle temperatures were much greater. Localised areas of hot mantle rose to the surface through a central plume wedge, weakening the damaged and already thin lithosphere. Once the plume head breaks the surface, crust either side of the head is forced downwards through the conservation of mass, initiating subduction. Numerical modelling shows only strongly energetic plumes are capable of weakening the lithosphere enough to rupture it, such plumes would have been present in the hot Archean mantle. Pre-tectonic subduction can also be inferred from the internal volcanism on Venus. Artemis Corona is a large plume formed by the upwelling of mantle derived magma and is on a scale potentially comparable to that in the Archean mantle. Models using its known characteristics showed that continued magmatism from conductive heat through the plume caused gravitational collapse. The weight of collapse caused the spreading of the surrounding crust outwards and subsequent subduction around the margins. The anhydrous nature of the crust on Venus prevents it from sliding past each other, whereas through the study of oxygen isotopes, the presence of water on Earth can be confirmed from 4.3 Ga. Thus, this model helps provide a mechanism for how plate tectonics could have been triggered on Earth, although it does not demonstrate that subduction was initiated at the earliest confirmed presence of water on Earth. Based on these models, the onset of subduction and plate tectonics is dated at 3.6 Ga. Late Heavy Bombardment Impact cratering also had consequences for both the development of plume-induced subduction and the establishment of global plate tectonics. The steepening of geothermal gradients could have directly enhanced convective mantle transport which now beneath an increasingly fractured lithosphere could have created stresses great enough to cause rifting and the separation of crust into plates. Crustal growth rates Lithological dating Crustal growth rates can be used to calculate estimates for the age of the continental