Complex sentence: Tamu Massif was formed about 145 million years ago during the Late Jurassic to Early Cretaceous period over a relatively short period of time (a few million years) and then became extinct.
Split into simpler sentences: Tamu Massif was formed about 145 million years ago during Late Jurassic to Early Cretaceous period. It was formed over a relatively short period of time (a few million years) then became extinct.