By 700 million years ago the supercontinent was leveling off, and the giant insulating blanket of sediment and other rocks caused mantle material to rise and parts of the crust to melt This upwelling magma forced the supercontinent to rift, forming the Iapetus Ocean .
(the supercontinent; was leveling off; T:By 700 million years ago)
the supercontinent
was leveling off
T:By 700 million years ago