A new Herald Insights map has laid out the eruptive behaviour of the volcanoes that lie beneath Auckland.

The Auckland Volcanic Field (AVF) is home to 52 volcanoes that have erupted in the last 250,000 years.

These tend to erupt in a short time — spanning days to weeks — and can produce a wide range of eruption styles, said Dr Gabor Kereszturi, of Massey University’s Institute of Agriculture and Environment.

The most violent form — a phreatomagmatic eruption — occurred when rising magma interacted with groundwater, turning the water into steam that expanded and caused a volcanic blast.

A series of such blasts often creates craters with a kilometre-wide diameter, as could be seen at Auckland’s Panmure Basin and Pupuke.

In other places in Auckland, volcanoes were formed from milder eruptions, triggered by escaping gas bubbles from the rising magma.

“This is often similar to eruptive processes going on at the Strombolian volcano in Italy, hence it is called Strombolian eruptions,” Kereszturi said.

In Auckland, this volcanic activity formed small conical volcanoes, such as Mt Eden and Rangitoto.

“This eruption often happens with effusive activity, forming lava flows with various sizes.”

These different eruption styles were explored in a recent paper by Kereszturi that drew on geological evidence preserved on the field in volcanic rocks and sediments.

Kereszturi and his colleagues also used airborne Light Detection ad Ranging Technology to create a digital representation of the topography.

“This data supports geological mapping and we were able to calculate the volume of the volcanoes more accurately than ever before.”

By developing a volcano catalogue, the researchers could come up with the most important environmental and geological conditions that influenced the eruption styles, and which thus were responsible for the wide range of eruption styles observed in the Auckland area.

“By combining spatial layers of the identified factors such as underlying geology, faults and elevation, we could produce a preliminary map that can highlight areas in Auckland that are susceptible for phreatomagmatic — areas with high numbers — and Strombolian eruptions — areas with low numbers.”

The map could help to pinpoint the likely eruption scenarios.

“For example, areas with “high” mean that a future eruption will produce a volcano similar to Panmure Basin, formed through phreatomagmatic eruptions.

“On the other hand, areas with “low” mean that a future eruption would form a small scoria cone with a lava flow, like Mt Eden.”

Since the location of the next eruption was unknown, the technique could help identifying controls on the expected eruption styles.

Kereszturi said the map did not distinguish between hazardous and less hazardous areas, but it highlighted areas that could expect different types of volcanic eruptions.

“This data can be used to develop eruption scenarios and eruption response exercise for different suburbs of the city.”