Causes

The earliest signs of an eruption or geological disaster came around June 1st, 1886, ten days prior. There was a reported increase in local hot spring activity. Locals had noticed the Lake Tarawera water levels rising, as well as waves up to 30cm high and a change in its colour. These were indications that seismic activity had already begun underneath Tarawera, but there were otherwise no signs that suggested its three peaks were to explode.

Tarawera sits above a part of a series of fault lines bordering the Pacific and Australian tectonic plates and is in line with the Pacific ‘Ring of Fire’. Shortly before the volcano exploded, a series of increasingly violent earthquakes shook the area. This suggests that the cause of the Mount Tarawera eruption was directly related to plate subduction. This is when a tectonic plate shifts underneath another. The Pacific plate subducts underneath the Australian plate due to the weight of the Pacific Ocean. The fault lines that Tarawera lies above are part of the plate boundaries of these two plates. Plate subduction often is the cause of earthquakes and volcanic activity or eruptions, and it is common for volcanoes that erupt because of plate subduction to be explosive.

As a tectonic plate subducts and descends further towards the hotter core of the earth, it releases trapped fluids, like water. These hot fluids cause minerals and rock above to melt, and this process produces magma. This magma moves upwards as it is less dense than the rock around it, until it builds up in a magma chamber. This magma is thick and sticky. When pressure builds up and a volcano above finally erupts, this thick magma bursts out at once. Thinner magma will flow down a volcano when it erupts, but thicker magma from a volcano will most likely explode. Mount Tarawera’s magma was thick and its eruption was explosive. Plate subduction, which causes a lot of friction, also causes earthquakes, which would explain the violent earthquakes that shortly preceded the eruption. Information about the earthquakes that night, the type of eruption, the magma, and Tarawera’s location indicates the eruption was caused by plate subduction.