The Kermadec section of the Pacific Ring of Fire is mostly submarine, extending over 1,400 kilometres between New Zealand and Tonga. Only the Kermadec Islands occur above sea level. Even these are the emergent caps of larger submarine volcanoes.
Seafloor mapping techniques are now routinely discovering the location and structure of these volcanoes. Scientists can interpret the type of volcanoes by dredging seafloor samples. Since about 2000, these studies have discovered over 40 volcanoes larger than 5 kilometres in diameter, with some as large as Ruapehu, and some as shallow as 60–80 metres below sea level. It is almost certain that other volcanoes have yet to be discovered along the Kermadec Ridge.
Submarine cone volcanoes
Submarine cone volcanoes form classic steep-sided seamount volcanoes, built with layers of lava flows and volcanic sediments. These volcanic sediments are typically formed by various explosive interactions of hot lava with cold sea water during eruptions or by subsequent collapse at some time after the eruption. Most of the cone volcanoes are basalt or basalt–andesite in composition.
A critical factor in the evolution of submarine stratovolcanoes is water depth. The pressure of the overlying water reduces the quantity and rate at which bubbles can form in the magma rising beneath the sea floor. In deep water, bubble growth is suppressed and the magma generally erupts effusively as pillow lavas and sheet flows. In shallow water, bubble growth is more rapid and vigorous, potentially leading to explosive and fragmenting eruptions that produce volcanic sediments. Many of the Kermadec stratovolcanoes, built between water depths between 500 and 2,000 metres, show this transition between effusive and explosive eruptions: pillow lavas form on the deeper flanks and volcanic sediments at the peak. This change is gradual over a range of depths.
Submarine caldera volcanoes
Seafloor mapping has also shown a greater number and size of rhyolitic caldera volcanoes than previously supposed. The Kermadec islands of Macauley and Raoul were known to have phases of caldera volcanism, with the associated formation and dispersal of pumice. However, new mapping suggests that caldera volcanism is not only a common eruption style north of Raoul, but also occurs to the south. Typically the calderas are 3–10 kilometres in diameter (up to the size of Wellington Harbour) with the caldera walls having relief of up to 700–800 metres. Some calderas ‘overprint’ each other and hence show multiple eruptions. Pumice has been dredged from many of these calderas, and in a few cases entirely mantles the edifice.
How pumice forms beneath the ocean is an intriguing question. Many of these eruptions, especially in shallow water, were explosive with a column of hot pumice, ash and gas sheathed by steam from the surrounding sea water. The hot pumice is suddenly chilled, with steam condensing to water in the pumice holes, and the pumice then sinks to the sea floor. These pyroclastic columns may also collapse to produce submarine flows, cascading down the outer flanks of the newly formed caldera.
Observing submarine eruptions is very difficult. Eruptions have been recorded from Raoul Island since 1800, including eruption sequences in 1814, 1870, 1964, and 2006. Small temporary islands have also been recorded close to Raoul and Macauley islands within the last 150 years, representing small cones that have subsequently been washed away by waves. Aircraft and fishing boats have also reported sea-surface disturbances, including rising steam, above shallow volcanoes.
Ship-towed hydrophones and seismometers on Pacific islands have recorded eruption sequences from Monowai volcano since the 1970s. More recently, repeat multibeam surveys of Monowai volcano in 1998 and 2004 revealed collapse of the volcano crest by about 100 metres, followed by rapid cone reconstruction. Seismometers also recorded a large explosive eruption in this period in May 2002.