Some days ago, North Island enjoyed a visit from Dr. Gregory Shellnutt, geologist and professor at the National Taiwan Normal University. Earlier this year he came to our neighbouring island, Silhouette, to obtain some rock samples for his research, but the uniqueness of North Island geology brought him back to Seychelles.
During his stay, he sampled rocks in the three main hills of North Island, trained our staff and even gave a talk to our guests. So why is North Island so special geologically speaking?
As Greg was able to explain us into detail, the islands of the Seychelles are a unique geological feature of the Indian Ocean and one of the few ocean island groups composed entirely of granitic rocks. The main islands of the Seychelles Archipelago are located on the northern end of the Mascarene Plateau of the western Indian Ocean. The Mascarene Plateau is a submerged fragment of ancient continental crust and extends for 2000km from north to south and covers an area of 115 000km2. The maximum water depth on the plateau is 150m, nearly 4km above the surrounding abyssal plain of the Indian Ocean.
Within the Seychelles Archipelago there are two islands that are even more geologically special: Silhouette and North Island. They are located 20-30km north west of Mahé and are the only two islands which are composed of syenite; but perhaps most important is the fact that these islands were created 65 million years ago when the Mascarene Plateau was immediately adjacent to the shoreline of western India.
On top of that, North Island has a special geological feature in one of its hills. As we have mentioned just previously, although syenite (a buff grey rock) is the principal rock found on North Island, there is a comparatively small exposure of darker olivine-biotite-bearing gabbro/diorite to the southwest which forms the congoment promontory, also known among North Islanders as Spa Hill.
So many fascinating reasons why North Island is quite unique!