Geology
The Kerguelen islands form an emerged part of the submerged Kerguelen Plateau, which has a total area nearing 2.2 million square kilometres. The plateau was built by volcanic eruptions associated with the Kerguelen hotspot, and now lies on the Antarctic plate.
The major part of the volcanic formations visible on the islands are characteristic of an effusive volcanism, which caused a trap rock formation to start emerging above the level of the ocean 35 million years ago. The accumulation is of a considerable amount; basalt flows, each with a thickness of three to ten metres, stack on top of each other, sometimes up to a depth of 1,200 metres. This form of volcanism creates a monumental relief shaped as stairs of pyramids.
Other forms of volcanism are present locally, such as the strombolian volcano Mont Ross, and the volcano-plutonic complex on the Rallier du Baty peninsula. Various veins and extrusions of lava such as trachytes, trachyphonolites and phonolites are common all over the islands.
No eruptive activity has been recorded in historic times, but some fumaroles are still active in the South-West of the Grande-Terre island.
A few lignite strata, trapped in basalt flows, reveal fossilised araucarian fragments, dated at about 14 million years of age.
Glaciation caused the depression and tipping phenomena which created the gulfs at the north and east of the archipelago. Erosion caused by the glacial and fluvial activity carved out the valleys and fjords; erosion also created conglomerate detrital complexes, and the plain of the Courbet Peninsula.
The islands are part of a submerged microcontinent called the Kerguelen sub-continent. The microcontinent emerged substantially above sea level for three periods between 100 million years ago and 20 million years ago. The so-called Kerguelen sub-continent may have had tropical flora and fauna about 50 million years ago. The Kerguelen sub-continent finally sank 20 million years ago and is now one to two kilometers below sea level. Kerguelen's sedimentary rocks are similar to ones found in Australia and India, indicating they were all once connected. Scientists hope that studying the Kerguelen sub-continent will help them discover how Australia, India, and Antarctica broke apart.
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