Marion Island lies at the SW end of a submarine plateau immediately south of the SW Indian Ocean Ridge, opposite Prince Edward Island. The low profile of the 24-km-wide dominantly basaltic and trachybasaltic volcano is formed by two young shields that rise above a flat-topped submarine platform. The island includes about 150 cinder cones, smaller scoria cones, and coastal tuff cones. The earliest dated eruptions took place about 450,000 years ago, but much of the island is covered by Holocene aa and pahoehoe lava flows, and more than 130 scoria cones formed during the Holocene. Many of these appear younger than the 4,020 BP peat layer overlying one of the flows (Verwoerd, 1981). Unvegetated lava flows appear to be only a few hundred years old (Verwoerd, 1967). An eruption in 1980 produced explosive activity and lava flows from a 5-km-long fissure that extended from the summit to the west coast.