The Amsterdam St Paul hotspot is one of the weaker hotspots around. It has created the St Paul and Amsterdam Islands, the now active Boomerang Seamount (last known eruption 1995), and an elongated chain of seamounts called the Chain of Dead Poets. These are remnants of the eruptive wake of the Amsterdam St Paul hotspot as the plates move on over it. The hotspot has had 2 episodes of increased activity after it became active. The first period lasted from 10 million years ago to six million years ago. The second period started 3 million years ago and lasts up until today. Amsterdam, St Paul and the Boomerang Seamount have all been produced during this second period of activity.
The hotspot is associated with the South East Indian Ridge and its rift system, and the chains volcanoes show evidence of changing in its chemical composition as the hotspot moved into the SEIR.
The Island is the northernmost of the Antarctic sub-aerial volcanoes. It has had two eruptive centers down the line. Both with visible craters, the younger of the craters are far more visible on the image. Both of the craters are from periods of heightened activity, but later volcanism on the Island has primarily been of the flanking fissure type. Even though no eruption has been witnessed lava samples taken from the flanks of the younger crater shows that the volcano has indeed erupted during the last 100 years.
St Paul Island
The island had a large eruption a few years before 1780 in which the predominant caldera formed. Even though the caldera is small for being a caldera it was probably formed by a Krakatoa style eruption starting with a for the volcanic system unusually large eruption with a subsequent magma chamber roof failure that let the ocean water down into the chamber. The ensuing steam explosion gutted the chamber. In 1780 the vestigial remnants of the caldera wall facing the ocean crumbled and the ocean has during the following years carved out a fairly broad, but shallow canal that is open for smaller sailing ships due to its limited depth of around 3 to 5 meters.
The Island is together with Isle du Kerguelen the best harbor in the southern ocean, and many trans-globe sailors make a port of call for repairs, or just general relaxation and landfall.
Not much is known about Boomerang that lies 18 kilometers north of Amsterdam Island. It rises 1 100 meters above the sea floor, but is still 650 meters below the ocean surface. During an expedition in 1996 they dredged up a lava sample and tested its Uranium/Thorium content. It showed that the lava had been erupted only 5 months prior to the visit.
The Seamount has a 2km caldera showing that the volcano has had at least one substantial eruption and probably have been a bit closer to the surface before.