A group of islands in the southern Indian Ocean. It is a territory of France. They are composed primarily of Tertiary flood basalts and a complex of plutonic rocks. The trachybasaltic-to-trachytic Mount Ross stratovolcano at the southern end was active during the late Pleistocene. The Rallier du Baty Peninsula on the SW tip of the island contains two youthful subglacial eruptive centers, Mont St. Allouarn and Mont Henri Rallier du Baty. An active fumarole field is related to a series of Holocene trachytic lava flows and lahars that extend beyond the icecap.