These two vent alignments are associated with current subsidence in the last 20 years of about 1.5 cm/year (0.59 in/year) which is assumed to be because of mainly cooling and contraction of previous magma melt.

[6] Other volcanoes connected with the caldera include Putauaki (Mount Edgecumbe) [7] and the maar crater of Lake Rotokawau which is most likely to have formed from a basaltic dike extrusion associated with the common magma mush body.

[9] The underlying arc volcanism is driven initially by large inputs of basaltic melt from the subducted Pacific Plate.

These basaltic melts often never reach the surface due to a relatively high density of the magma compared to the surrounding Australian Plate crust, but may trigger earthquake swarms.

In the case of the Ōkataina Caldera the sub-surface architecture is known to be made up of discrete melt-mush pockets, and with one dacite exception already mentioned, these are rhyolitic.

[2] The composition is related to heat and volatiles transferred between the parent basalts and such rhyolites over the time the sub pocket has been maturing.

Basaltic-rhyolitic magma interaction definitely happens from local and world wide studies, and will also be a factor in the many different eruption styles that have occurred.

[2] The most significant collapse event, with an eruptive volume of 150 cubic kilometres (36 cu mi) was 280,000 years ago.

[1] The second major phase Matahina sub caldera is to the south east and its basement is also about 5 km (3.1 mi) below present ground level.

[2] The original shape of the Matahina caldera has been modified (and buried/destroyed) by various events including at least eight smaller eruptions between 70,000 and 24,000 years ago.

One of these events can be assigned to the Kawerau ignimbrite eruption of 33,000 years ago, with its location within the central part of the Matahina Caldera at level of the Puhipuhi Basin.

[1] An area of low gravity on gravimetric studies is consistent with the fourth phase Kawerau Caldera being here and its basement being about 2 km (1.2 mi) below present ground level.

This fault is just to the east of Lake Rotoma at the boundary between the tectonic Whakatāne Graben and the magmatic Ōkataina segments of the Taupō Rift.