AbstractThe Havre 2012 deep-sea rhyolite eruption went unobserved and was initially recognised from a massive pumice raft at the sea surface. Havre pumices are predominantly white or grey, however pink pumice is common in the raft. In subaerial explosive eruptions, pink pumice is understood to result from high-temperature atmospheric iron-oxidation. The presence of pink pumice questions the effusive eruption model for the Havre raft. Here we report results from X-ray Absorption Near Edge Structure spectroscopy, magnetic measurements, TEM imaging and glass chemistry that collectively show the colour results from increasing amounts of magnetite nanolites in the raft pumice glass oxidizing to hematite. This suggests a short-lived but powerful explosive eruption phase penetrated the water column allowing hot pyroclasts to oxidise in air. Our results therefore challenge the known depth limits for explosive eruptions in the marine realm and suggest pink pumice can be an indicator of magnetite nanolite-driven explosive eruptions.