Kermadec Arc: Havre volcanic eruptions change seafloor landscape

August 09, 2015

New Zealand (Bernd F. Laeschke – October 2012): Undersea volcanic activity is constantly changing the topography of the world’s ocean floors. One of the known major events took place on July 19, 2012, when the Kermadec volcano that erupted 497 miles (800 kilometers) north east of New Zealand’s Bay of Plenty. New images reveal the impacts of an eruption on the seafloor that produced an enormous pumice raft. To better understand the impact such eruptions have, researchers study the Kermadecs volcanic chain that stretches for 621 miles (1000 kilometers).

"When we mapped the area yesterday, we found a new volcanic cone which has formed on the edge of the volcano, towering [787 feet] 240 meters above the crater rim,” says Dr. Joshu Mountjoy, an ocean geology scientist with New Zealand's National Institute of Water and Atmospheric Research (NIWA). "It is fantastic to be able to record the change on the seafloor following these kinds of events."

The Havre volcano eruption was strong enough to breach the ocean surface from a depth of 3,609 feet (1.100 meters). It produced clouds of ash visible by satellite, and a pumice raft that covered an area of 22,000 square kilometers.

"One of the most exciting aspects of the cruise is the chance to map Havre volcano which, we have now confirmed, erupted in July,” says NIWA's volcanologist Dr. Richard Wysoczanski. "We know the shape of the volcano from previous research. Using the multibeam echosounder, we made a before and after comparison of the volcano to determine the size of the eruption and the change it has made to the seafloor."

NIWA had previously mapped Havre volcano in 2002, showing a 3,281 feet (1 kilometer) high undersea mountain with a 3.11 mile (5 kilometer) wide and 0.5 mile (800 meter) deep central crater. This central steep-walled crater is a caldera, which is a type of volcano, like Lake Taupo, known to produce large and violent eruptions. Over the last decade there has been one significant undersea eruption a year on average across this whole area.

"One side of the caldera wall is bulging in towards the volcano's center," says Mountjoy. "The bulging may indicate where an eruption may occur in the future, or it might lead to an undersea avalanche."

In addition to the material erupted out into the sea and atmosphere, several cubic kilometers of new material has been added to the volcano. Large volumes of freshly erupted pumice have accumulated on the caldera floor, raising the seafloor by up to 33 feet (10 meters). Glassy volcanic rocks were sampled from the fresh crater wall, typical of newly erupted material.

"We found fresh volcanic rocks related to the eruption all over the area, and there are new volcanic cones in one area," says Wysoczanski. "We have collected volcanic rocks, up to beach ball size, and analysis will take place on these when we are back at NIWA. The rocks vary in color from black glassy material to pumice. Round pebbles of pure sulfur were also retrieved."

Scientists want to know how the volcanoes are changing over time, and how the Kermadec and Colville ridges have drifted apart by more than 62 miles (100 kilometers).

"We want to understand, on a larger scale, how the whole area is stretching apart, and what new material is building the rift," Wysoczanski says. "The key thing, we think, is that the original ridge is the Colville ridge, and that Kermadec ridge is deposits from that. When the team deployed rock sampling equipment on Colville ridge they were surprised to recover large volumes of very fresh pumice. The pumice is interpreted as being from the July Havre eruption, though this will be confirmed with lab analysis."