Foote, April1, Németh, Károly2, Handley, Heather1
1Department of Earth and Planetary Sciences, Macquarie University, Sydney, Australia, 2Institute of Agriculture and Environment, Massey University, Palmerston North, New Zealand
Dispersed volcanic fields (commonly labelled as monogenetic volcanic fields) are of great interest in volcanology as they provide a relatively simple volcanic architecture to study regarding magma source to surface processes, edifice growth and subsequent destructions. The Auckland Volcanic Field (AVF) in New Zealand is among the few hundred documented dispersed volcanic fields worldwide that were active through the Holocene and can broadly be defined as a mafic intraplate monogenetic volcanic field. Recent research has highlighted that the transition between monogenetic and polygenetic volcanism is far more continuous than was originally thought, where volcanoes traditionally viewed as monogenetic are commonly found to have had multiple eruptions and complex magmatic plumbing systems.
This study focuses on the last remaining outcrops of Matukutūruru, or Wiri Mountain, one of the southernmost volcanic centres in the AVF. Wiri Mountain presents a unique situation where despite the large extent to which deposits have been removed, the remaining spectacular outcrops allow a clear picture to be formed, including analysis of stratigraphy and facies, vent location and extent of deposits, fragmentation depth, eruptive styles and their transitions and the eruption history of the volcanic centre.
Wiri Mountain has had a complex eruption history, beginning with a pre-existing tuff ring/maar landscape. An initial basal tuff ring was deposited by predominantly pyroclastic density currents with ballistic curtain deposits and some pyroclastic fall, through a debris filled vent that widened mostly at depth through the course of the eruptions. At least two smaller, satellite tuff rings were then deposited on the outer flanks of the first by a combination of pyroclastic density currents and pyroclastic fall, with a transition from phreatomagmatic to Strombolian eruptive style. A central scoria cone was then deposited within the initial tuff ring, followed by lava spatter and lava flows that covered the tuff rings, the scoria cone, and the surrounding area. This complex eruption history highlights the range and transition of eruptive styles leading to the production of multiple types of eruptive products and deposits that can be typical for the AVF.
The small magma volumes typical of monogenetic volcanism allow for significant influence of fragmentation and eruptive products by external water, resulting in a wide variety of volcanic landforms. However, based on the results of this study, volcanic activity at Wiri Mountain and the surrounding area of the southern end of the AVF was potentially more complex than would typically be expected from the textbook definition of monogenetic activity, with a large enough eruptive volume to allow a complex eruptive evolution over time, multiple satellite cones, and potential connections to nearby centres; highlighting the grey area on the concept boundary of monogenetic volcanism.
April Foote is a PhD candidate at Macquarie University. Her studies are based on the volcanology of the southern end of the Auckland Volcanic Field.