In geology, a supercontinent is the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", which leaves room for interpretation and is easier to apply to Precambrian times. To separate supercontinents from other groupings, a limit has been proposed in which a continent must include at least about 75% of the continental crust then in existence in order to qualify as a supercontinent. Supercontinents have assembled and dispersed multiple times in the geologic past (see table). According to modern definitions, a supercontinent does not exist today; the closest in existence to a supercontinent is the current Afro-Eurasian landmass, which covers approx. 57% of Earth's total land area. The last time the continental landmasses were near to one another was 336 to 175 million years ago as the supercontinent, Pangaea. The positions of continents have been accurately determined back to the early Jurassic, shortly before the breakup of Pangaea. The earlier continent Gondwana is not considered a supercontinent under the first definition since the landmasses of Baltica, Laurentia and Siberia were separate at the time. Supercontinents throughout geologic history The following table names reconstructed ancient supercontinents, using Bradley's 2011 looser definition, with an approximate timescale of millions of years ago (Ma). General chronology There are two contrasting models for supercontinent evolution through geological time. The first model theorizes that at least two separate supercontinents existed comprising Vaalbara (from ~3636 to ) and Kenorland (from ~2720 to ). The Neoarchean supercontinent consisted of Superia and Sclavia. These parts of Neoarchean age broke off at ~2480 and and portions of them later collided to form Nuna (Northern Europe North America) (). Nuna continued to develop during the Mesoproterozoic, primarily by lateral accretion of juvenile arcs, and in Nuna collided with other land masses, forming Rodinia. Between ~825 and Rodinia broke apart. However, before completely breaking up, some fragments of Rodinia had already come together to form Gondwana (also known as Gondwanaland) by . Pangaea formed by through the collision of Gondwana, Laurasia (Laurentia and Baltica), and Siberia. The second model (Kenorland-Arctica) is based on both palaeomagnetic and geological evidence and proposes that the continental crust comprised a single supercontinent from until break-up during the Ediacaran Period after . The reconstruction is derived from the observation that palaeomagnetic poles converge to quasi-static positions