October 31. India takes off

The end of the Jurassic is marked by a relatively minor extinction, perhaps only regional in extent rather than global. I talked a little about it a few days ago. It did significantly impact the distribution and numbers of Sauropods and stegosaurids as well as ammonites and other marine animals, but no major groups were wiped out completely as far as I can tell.

Tectonically, Pangaea’s break-up was increasing. By the end of the Jurassic, East Gondwana, one of the two big pieces of Gondwana that had stayed pretty much intact for hundreds of millions of years, was beginning to fragment. We talked earlier this month, on October 5, about the Karoo Volcanics that resulted from the initial rifting though parts of East Gondwana. The rifts, cracks, that allowed the volcanics to erupt were followed by a period of crustal stretching throughout much of the Jurassic period. By about 150 to 155 million years ago, within 10 million years of the end of the Jurassic, East and West Gondwana were probably fully separated from each other, albeit by a narrow, strait-like ocean. And at the same time, India plus Madagascar, which was part of East Gondwana rather than West Gondwana and Africa, began to separate from Australia.

Gondwana when it was intact

Also in the late Jurassic, the eastern side of what is now Australia was also fragmenting, with continental blocks now known as the Lord Howe Rise and New Caledonia pulling apart from the Australia portion of Gondwana.

Pretty close to the end of the Jurassic – it might have been a bit into the Cretaceous, but this whole process is millions of years, so it’s a little silly to point to an instant of geologic time as the time when continents separated – so close enough, at the end of the Jurassic, rifting between India and Antarctica and Australia and Antarctica were also well underway. Those separations continue into the Cretaceous (and in fact to the present day) so we may discuss them again, but they got started about the end of the Jurassic.

The maps of the world at the start and end of the Jurassic Period were remarkably different from each other. From Pangaea – admittedly with some cracks, but still pretty much one supercontinent – to a global distribution of continents that was becoming almost recognizable as the continents of today.

Tomorrow, the Cretaceous begins.

—Richard I. Gibson

Australia separates from Antarctica


Gondwana map based on original by Petter Bockman, public domain, via Wikipedia.