was further supported by apparent metamorphic gradients in the same part of the belt, that are similar to those observed in modern subduction zones. However, this interpretation has been strongly contested on the basis that rock types and strain are extremely consistent across the various faults in the proposed accretionary wedge, and that peak metamorphic grades are consistent across the entire belt. Non-plate tectonic models Non-plate tectonic models include heat-pipe and mantle plume models, both of which suggest that the volcanic sequences at Isua formed through eruption of mantle derived magmas with minimal crustal input. In a heat-pipe model, rapid eruption of volcanic rocks and the corresponding removal of melt from the mantle below causes downward movement of the lithosphere and burial of mafic rocks. The buried mafic rocks eventually heat up and melt, producing the TTGs associated with the Isua Greenstone Belt. This model can account for the mafic composition of pelitic sediments at Isua, suggesting there was little felsic crust present during its formation, and the relatively simple deformation and uniform metamorphic grade observed across the belt. However, it has been criticised on a number of grounds, including the fact that there is no evidence that the 3.7 Ga volcanic rocks or TTGs ascended through the 3.8 Ga sequence, as would be expected for vertically stacked volcanism in a heat pipe model. Metamorphism Following its formation, the Isua Greenstone belt has undergone two major metamorphic episodes. The first predates the formation of the <3.5 Ga Ameralik dykes and is associated with the Eoarchaean deformation at Isua. Amphibolite-facies conditions were reached across the belt between ~3.7 and 3.6 Ga. Though higher pressure conditions have been suggested locally on the basis of Ti-humite group minerals in peridotites, the reliability of these minerals to document high pressure processes has been questioned. The second event also reached amphibolite-facies conditions, and appears to have been a protracted event between ~2.9 and 2.6 Ga, followed by widespread retrogression of locally varying intensity. The effect of these two metamorphic and deformational events adds significant complexity to interpreting the primary geochemical compositions and geological structures present in the belt (e.g., see 'Early life controversy', below). Early life controversy Because of its age, the Isua Greenbelt has long been the focus of studies seeking to identify signs of early terrestrial life. In 1996, geologist Steve Mojzsis and colleagues hypothesized that isotopically light carbon in the structure's carbon-rich