Travel Reference
In-Depth Information
million years, while soft-bodied life in the oceans experimented with the idea of greater
progression.
The
sedimentaryshelf
began to form early in the Palaeozoic (500mya), as macroscopic
organisms with external skeletons evolved and diversified explosively. The old mountain
belt had been eroded down near to sea level, and the waning of an ice age caused flooding
of the land. Across the world, life evolved dramatically towards a climax of plant pro-
ductivity in the Carboniferous (320mya) and then shivered through its most severe ice age
and most catastrophic extinction only 30 million years later (96% of marine species van-
ished). That ice age scraped all evidence of the preceding sedimentation from Madagascar
and the geological record there is reset, beginning with gravel and boulder deposits laid
down as the glaciers retreated. In the middle Permian (270mya) the southern continents
werestillassembledasonesupercontinent-Gondwana-inwhichMadagascarwasacent-
ral part without identity,boundonthe west bypresent-day Africa, the east byIndia andthe
south by Antarctica.
Gondwana began to split apart soon after, towards the modern continental distribution.
As a continent divides, rifts form, gradually becoming seas, then oceans. The rocks of the
sedimentary shelf record 100 million years of deposition spanning that cycle for the rift
separating Madagascar from Africa. The earliest sediments are mixed glacial, river and
lake deposits that contain a fossil flora common to all the modern southern continents.
Later, the land was inundated and marine carbonates were deposited in this new shallow
sea preserving the remains of some primitive fish. During the Triassic (240-210mya) the
landscape was uplifted and terrestrial gravels and sands were deposited, including those
exposed in Isalo National Park. A major rise in sea level followed, and from this time until
after the demise of the dinosaurs (63mya) fossil-bearing limestone sedimentation domin-
ated in the growing Mozambique Channel. These sediments today make up the Bemaraha
Plateau and the
tsingy
landscapes of western Madagascar. The shallow marine ecosystems
were characterised by a great abundance of ammonites, and today the sedimentary carbon-
ates of west Madagascar are an incredible repository of these fossils. Uplifting occurred
again 50-30mya as a major eastern rift developed between India and Madagascar, ending
sedimentation on the shelf.
The
volcanicedifices
ofMadagascararelessobviousthanthevolcanoesresponsiblefor
the islands of the Comoros, Réunion and Mauritius. They are widespread in the north and
along the east coast, and inland they make up the Ankaratra Massif, Itasy highlands and
Montagne d'Ambre. Underwater volcanic activity began during the Cretaceous (120mya)
and has persisted off the north coast up to the present day. The lavas and intrusive rocks
produced have a bizarre and unique chemistry. The most recent major volcanic activity
occurred less than 2mya, giving rise to Nosy Be, where modern hot springs testify to the
island's relative youth. There are suggestions that the volcanic focus is moving southeast
towards Madagascar; the volcanic record in the island is potentially far from over.
