interpreted, with a high level of uncertainty, to have converted roughly half of the 'continental' crust into terrestrial maria, thereby providing a method for the formation of crustal dichotomy, as seen today. Types of crust Primordial crust The initial crystallisation of minerals from the magma ocean formed the primordial crust. A potential explanation of this process states the resultant solidification of the mantle edge took place approximately 4.43 Ga. This would subsequently produce continents composed of komatiite, an ultramafic rock rich in magnesium with a high melting point and low dynamic viscosity. Another line of research follows up on this, proposing that differences in the densities of newly formed crystals caused separation of crustal rocks; upper crust largely composed of fractionated gabbros and lower crust composed of anorthosites. The overall result of initial crystallisation formed a primordial crust roughly 60 km in depth. The lack of certainty regarding the formation of primordial crust is due to there being no remaining present day examples. This is due to Earth's high erosional rates and the subduction and subsequent destruction of tectonic plates throughout its 4.5 Ga history. Furthermore, during its existence the primordial crust is thought to have been regularly broken and re-formed by impacts involving other planetesimals. This continued for several hundred million years after accretion, which concluded approximately 4.4 Ga. The outcome of this would be the constant alteration in the composition of the primordial crust, increasing the difficulty in determining its nature. Secondary crust Recycling of existing primordial crust contributes to the production of secondary crust. Partial melting of the existing crust increases the mafic content of the melt producing basaltic secondary crust. A further method of formation due to the decay of radioactive elements within the Earth releasing heat energy and eventually causing the partial melting of upper mantle, also producing basaltic lavas. As a result, most secondary crust on Earth is formed at mid ocean ridges forming the oceanic crust. Tertiary crust The present day continental crust is an example of a tertiary crust. Tertiary crust is the most differentiated type of crust and so has a composition vastly different to that of the bulk Earth. The tertiary crust contains over 20% of the abundance of incompatible elements, which are elements with a size or charge that prevent them from being included in mineral structure. This a result of its generation from the subduction and partial melting