Microscopic deposits tell of ancient eruptions

Masters degree student Madison Clarke Picture supplied

Masters degree student Madison Clarke has uncovered evidence of multiple volcanic eruptions over the last one million years, using deposits of volcanic material so small they are invisible to the naked eye.

In 2018, the IODP (International Ocean Drilling Programme) retrieved a 500-metre-long core from the Hikurangi Subduction Zone. The core represents nearly one million years of history. Clarke is part of a larger team, led by Dr Lorna Strachan at the University of Auckland, utilising the core.

The core, taken from Aotearoa’s largest and most active fault, will allow Dr Strachan’s team to investigate a long-held, but as-yet untested, hypothesis that climate and sea level can impact the frequency of earthquakes and volcanic eruptions.

The core sample was taken from 110km offshore to the east of the Mahia Peninsula.

Clarke’s work searching for microscopic volcanic material deposits, called “cryptotephra”, will help to answer part of this question by improving the record of volcanic activity for the past one million years.

These elusive deposits are sometimes the only evidence of volcanic activity from the distant past and searching for them is no simple task.

“Volcanic eruptions can be so explosive that they destroy on-land deposits of ash from previous events,” said Clarke. “Using an offshore core, we can search for the tiny deposits of volcanic material that was blown offshore by wind.

“There was a chance I wouldn’t find anything, but what I found was anything but a failure. Of the 13 targeted areas, 11 cryptotephra deposits were identified.”

Clarke examined key characteristics, such as shape and concentration, of microscopic glass shards, and the major element geochemistry of the deposits to determine that what she had found was evidence of an ancient volcanic eruption.

By using this technique, Clarke’s research went a step beyond identifying deposits, to making interpretations about them, that can now be used to further develop the record of volcanism from the Taupō Volcanic Zone.

The identified cryptotephra deposits were found to represent several past volcanic eruptions, the latest of which was from an eruption 5500 years ago, and the earliest deposit identified was from 977,000 years ago.

“This shows that this method can be applied on long cores over large time scales.” said Clarke.

Once complete, Dr Strachan’s team will have established Aotearoa’s longest record of earthquake and volcanic history and determine whether there is a causal link between climate and volcanic and seismic activity.