Marion Island is a youthful intraplate shield volcano in the Southern Ocean, generally
considered to represent the location of a mantle plume. New K–Ar age determinations on 37 whole
rock samples of so-called older grey basalt from the island range from 450±10 to ∼30 ka. The lavas
are remarkably fresh and holocrystalline, appearing to be ideal for dating. The age results from two
almost vertical sections, where relative age relationships are not in doubt, are in accord with the
stratigraphy. A third section gave aberrant ages that indicate the presence of inherited or excess
radiogenic argon. This is highly unusual for subaerial flows on an oceanic island. We therefore urge
caution in accepting K–Ar ages on samples without stratigraphic control. Volcanic activity on Marion
could have been essentially continuous during the last half million years but on the basis of local
unconformities and gaps between the measured ages it is considered more likely to have been episodic.
Eight periods of effusive activity at approximately 450, 350, 240, 170, 110, 85, 50 and <10 ka ago can
be distinguished with varying degrees of confidence. Glacigene deposits that are intercalated with the
lavas can be correlated with glacial stages 2, 4, 6, 8 and 12 of the Pleistocene time scale based on the
marine oxygen isotopic record. Indications are that at least some of the earlier volcanic activity coincided
with glacial stages but the two most recent episodes, including the Holocene one extending up to
the present, are clearly interglacial.