Compositional and textural characteristics of the strombolian and surtseyan K‐Trig basalts, Taupo Volcanic Centre, New Zealand: Implications for eruption dynamics

Abstract

K‐Trig basalts form a NNE lineament of four monogenetic centres, along the eastern side of the Kaiapo Fault, 4 km west of Taupo, New Zealand. Exposures of the northernmost centres (Punatekahi and K‐Trig) are dominated by strombolian volcanism, whereas the southern centres (Kaiapo and Mine Bay) are products of surtseyan activity.

The Kaiapo deposit represents the faulted, eroded remnant of a once larger phreatomagmatic tuff cone. Inner‐wall stratigraphy exposed in the Kaiapo Fault scarp shows the deposit built up during three phases of phreatomagmatic volcanism, each phase comprising a lapilli‐rich lower facies A and a contrasting lapilli‐poor upper facies B. Both are interpreted as the product of multiple “wet” surges, but facies A beds are classic surtseyan products, whereas facies B beds are interpreted as having been deposited during periods of lower magma flux, when decoupled magmatic volatiles streamed through a fine‐grained vent slurry, facilitating weak steam explosions.

At Punatekahi, two coalescing scoria cones were formed by “dry” (strombolian) eruptions, generated when large gas bubbles rose and burst through a lava pond formed within the cone. Grainsize and vesicularity data from one of the cones indicates that discharge rate was generally low, allowing a dense crust to form on the surface of the ponded lava. Occasionally the crust was removed by explosive fragmentation allowing eruption of only actively vesiculating magma.

At K‐Trig, between T‐Trig and Punatekahi, volcanism progressed from initial phreatomagmatic activity to strombolian activity as vent conditions changed from “wet” to “dry”. This site probably represents the northern margin of a once more extensive proto‐Lake Taupo.

Keywords:

• K‐Trig
• Punatekahi
• Kaiapo
• basalt
• surtseyan
• strombolian
• phreatomagmatic
• tuff cone
• scoria cone
• Taupo