Understanding the timing and nature of effusion of lava in the summit regions of stratovolcanoes is important, because these volcanoes generate deadly and destructive block-and-ash flows (BAFs).

The most recent eruption of Mt. Taranaki in New Zealand left a portion of a lava dome which now forms its 2518 m summit. In this study we demonstrate the value of applying paleomagnetic techniques to understand recent effusive episodes.

By matching the direction of remanent magnetization of the dome lava with the regional paleomagnetic secular variation curve NZPSV1k, we estimate that this eruption occurred between AD1780 and 1800.

The paleomagnetic signal and geochemistry show that the summit dome was part of a large lava coulee that spilled from a NW-breach in the crater, with other parts of this structure still remaining down slope (the “Turtle”).

Paleomagnetic analyses were also used to reconstruct the thermal history of deposits formed by the dome/coulee collapse.

Emplacement temperature estimates and groundmass textural data from the BAF deposit clasts show that the lava dome collapsed in multiple stages.

A small, cooler portion of lower dome coulee collapsed first, before the progressively thicker and hotter parts of the central dome.

The collapses generated BAFs which travelled up to 5 km from the source and maintained temperatures > 500 °C more than 2 km from the source.