Lessons to be learned from human modifications that have left maunga vulnerable to slips
Maungakiekie/One Tree Hill volcanic cone suffered 15 unprecedented landslips during the 27 January 2023 deluge. Although some of other maunga had a small number of slips, many had none at all.
Leading geologist Bruce Hayward* analysed the Maungakiekie slips and concluded that maunga (and also Ōhinerau Mt Hobson and Te Kōpuke Mt St John) are particularly susceptible to slips due to:
* A combination of their particular geologies (their original volcanic rock and scoria are all topped by a layer of impermeable tuff from the Three Kings eruptions that in turn is overlaid by soil that built up over thousands of years when forests were on those maunga)
* The way the pre-European terraces were constructed
* The lack of trees on those parts of the slip-affected slopes
That’s why nearly all of the slips on Maungakiekie (and also at Ōhinerau Mt Hobson) happened on bare ground on the outer edges of terracing. These slips resulted in significant damage to the terraces themselves but also to the midden material contained within. Refer to photos below for examples.
Hayward’s paper doesn’t touch on this, but we note the big Ōwairaka slip was different in that previous quarrying by Pākeha on that slope will likely have affected its stability. As was the case with the other maunga, the potential for the Ōwairaka slip was made worse by there being no trees there. Furthermore, a lack of guttering on the paved road above resulted in vast amounts of water shooting off the edge at a bend and hitting the ground below, thus further destablising the soil.
Pākeha quarrying significantly damaged many of the maunga in the early part of the 20th century - some into oblivion - but pre-European modifications have not been without consequences either.
The post is not designed to point fingers in relation to those maunga modifications because the past cannot be changed, However, the future can. Our take on Bruce Hayward’s report and our observations on other maunga is that extensive modifications (no matter who they were done by) have made many of the maunga vulnerable to slips.
As can is graphically evidenced at Maungakiekie and Ōhinerau in particular, it very much calls into question Tūpuna Maunga Authority’s proposed programme of removing hundreds of non-native trees on the grounds that doing so will “protect” maunga archaeological features.
Read Bruce Hayward’s full report here.
Read our letter to Tūpuna Maunga Authority and Auckland Council here.
*Bruce Hayward is not a HTM member and is not involved with our organisation.
Nearly all of the Maungakiekie One Tree Hill slips, including these four, occurred on bare ground and started at the outer lip of pre-European terracing, Photo: Tom Ang
This big slip at Ōhinerau Mt Hobson occurred on an untreed slope at the outer edge of pre-European terracing.
Looking down at the Ōwairaka slip, with the stumps of two long since felled trees marked on the slip’s upper edge.
The close up of the righthand slip in the adjacent photo gives an idea of its scale, and also shows some of the large amount of midden material exposed as a result.
The upper edge of the Ōhinerau Mt Hobson slip pictured to the right. Some exposed midden material can be seen on top of the soil.
The Ōwairaka slip from a distance. Note the three tree stumps highlighted to the right. Photo: Sharon Jones