Products of the complex, long duration, AD 1305 rhyolitic eruption of Tarawera, New Zealand

The youngest rhyolite eruption in New Zealand is the ~4 km3 (DRE) Kaharoa eruption episode from Tarawera volcano at ~1305AD. The complex eruption sequence occurred from seven magmatic vents which define an 8-km linear fissure across the volcano. The main events were: 1) phreatic blasts; 2) sub-plinian/plinian eruptions dispersing fallout over the northeastern North Island; 3) a rhyolite dome extrusion; 4) more sub-plinian fall units and pyroclastic flows, including at least two plinian fall units widespread to the NW over the North Island; 5)a final effusive phase producing three large lava domes. The total eruption duration is unknown, but the time required to extrude the lava domes indicates that it may have spanned several years to perhaps a decade. Collapse of the domes generated extensive block-and-ash flow deposits (BAFD). Four BAF fans were constructed during this time. The two largest fans are 1x108 m3 (N) and 5x107 m3 (SE), with the NW and W fans contributing 1x107 m3 and 4x106 m3 respectively. Unconfined BAFs within the SE fan reached a distance of 7.6 km from the dome margin. The N fan comprises a region of unconfined BAFD which reach 5.4 km from the source, and two valley-confined deposits that reach 9.2 km from source. Despite a complex internal stratigraphy, due in part to the irregular underlying topography, only three block and ash flow depositional units and associated ash cloud deposits may be identified in each of the two major fans. The block and ash flows of the Kaharoa eruptive episode are significantly larger and more widespread than many documented in the literature (Unzen, Japan; Soufriere Hills, Montserrat; Merapi, Indonesia; Mount St Helens, USA). Enhanced mobility of the Kaharoa block and ash flows is interpreted to be principally due to the large volumes of each collapse, involving substantial amounts of hot dome lava. BAF-related ash clouds and their deposits, which were so devastating at Unzen, may extend for up to 20 km from source. The potential for such voluminous and far-reaching BAFs and ash cloud deposits at Tarawera has clear implication for volcanic hazard assessment for future eruptions of this type.