<P> 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, the definition of a supercontinent can be ambiguous . Many earth scientists use the term supercontinent to mean "a clustering of nearly all continents". This definition leaves room for interpretation when labeling a continental body and is easier to apply to Precambrian times . Using the first definition provided here, Gondwana is not considered a supercontinent, because the landmasses of Baltica, Laurentia and Siberia also existed at the same time but physically separate from each other . The landmass of Pangaea is the collective name describing all of these continental masses when they were most recently near to one another . This would classify Pangaea as a supercontinent . According to the modern definitions, a supercontinent does not exist today . Supercontinents have assembled and dispersed multiple times in the geologic past (see table). The positions of continents have been accurately determined back to the early Jurassic . </P> <P> The following table displays historical supercontinents, using a general definition . </P> <Table> <Tr> <Th> Supercontinent name </Th> <Th> Age (Mya: millions years ago) </Th> </Tr> <Tr> <Td> Vaalbara </Td> <Td> ~ 3,636--2,803 </Td> </Tr> <Tr> <Td> Ur </Td> <Td> ~ 2,803--2,408 </Td> </Tr> <Tr> <Td> Kenorland </Td> <Td> ~ 2,720--2,114 </Td> </Tr> <Tr> <Td> Arctica </Td> <Td> ~ 2,114--1,995 </Td> </Tr> <Tr> <Td> Atlantica </Td> <Td> ~ 1,991 - 1,124 </Td> </Tr> <Tr> <Td> Columbia (Nuna) </Td> <Td> ~ 1,820--1,350 </Td> </Tr> <Tr> <Td> Rodinia </Td> <Td> ~ 1,130--750 </Td> </Tr> <Tr> <Td> Pannotia </Td> <Td> ~ 633 - 573 </Td> </Tr> <Tr> <Td> Gondwana </Td> <Td> ~ 596 - 578 </Td> </Tr> <Tr> <Td> Laurasia and Gondwana </Td> <Td> ~ 472 - 451 </Td> </Tr> <Tr> <Td> Pangaea </Td> <Td> ~ 336 - 173 </Td> </Tr> </Table> <Tr> <Th> Supercontinent name </Th> <Th> Age (Mya: millions years ago) </Th> </Tr>

How many supercontinents have there been in earth's history
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