Nathan Ribyat
March 26, 2014
Rhyolitic and Basaltic Eruptions from Tarawera Volcano, Okataina Volcanic Center, New Zealand
Tarawera volcano is located on the North Island of New Zealand in the Taupo Volcanic Zone (TVZ). It lies along the convergent plate boundary between the oceanic Pacific plate and the continental Australian Plate. Within the TVZ is the Okataina Volcanic Center (OVC), seen in figure 1, which has been volcanically active for least 350 ka. Two calderas were formed during the Matahina and Rotoiti eruptions approximately 280 and 60 ka, respectively. Within the past 25 ka, intra-caldera volcanism has been present, forming the Tarawera and Haroharo volcanos. There have been four major rhyolitic eruptions at Tarawera as well as one basaltic fissure eruption in 1886. All of the episodes were plinian style eruptions in which ash and gas are blasted high into the atmosphere and are often accompanied by pyroclastic flows and large amounts of magma erupted. The episodes occurred at multiple vents running southwest to northeast (Lindsay et al., 2011).
The first of the four rhyolytic eruptions was the Okareka episode. Radiocarbon dating of the Okareka tephra produced an age of 18,420 ± 149 years BP; however, after calibrating this to account for inconsistencies in radiocarbon ages, the Okareka episode likely happened 21,900 years BP. Most of the Okareka deposits have been covered by more recent eruptions leaving only a few areas where the layers of tephra are exposed. Of these, the upper portions are often missing from erosion so there are very few whole deposits readily available to study.
The first deposit associated with the Okareka episode is a distinct black layer, 1-15 cm thick, consisting primarily of basaltic scoria (figure 2). There are basalt-pumice clasts, which are basaltic scoria rocks coated by a felsic pumice rock as well as pumices with intermediate compositions between basalt and rhyolite. As the majority of Tarawera magma is rhyolitic, the presence of basalt in the base layer of the Okareka sequence could indicate that the eruption was set off by the interaction between the rhyolitic and basaltic magmas. The lack of paleosols or erosional features between the layer of basalt and the next layer, which is predominately rhyolite, confirms that they were both from the same continuous eruption.
The overlaying rhyolitic layers are generally well sorted ash and lapilli that formed as fall deposits after they were ejected from the volcano. There are small layers of finer, poorly sorted ash that could be the result of a pyroclastic flow. It could also represent phreatomagmatic activity which is generally explosive in nature and is a result of the magma interacting with water. This would likely have been caused by a brief period of rainfall during the eruption. This same layer has been identified in cores taken near Auckland to the northeast, suggesting that there was a southeast wind at the time of the eruption.
A sharp contact between this well sorted layer and the next poorly sorted, layer is interpreted to be a sudden change in the eruption conditions. If the eruption slowly died down, there would likely be a gradual transition into the next layer with a fining upward sequence. Instead, the eruption was quickly quenched and the plinian column of ash collapsed, likely causing a sudden surge of pyroclastic flows. The rest of the deposit is a mixture of ash, crystal poor pumice, and occasionally glass or obsidian. After the collapse of the ash column, there were probably intervals of explosive eruptions until the Okareka episode came to an end. Unfortunately most of the upper layers have been eroded away so exactly how long the episode lasted is hard to say (Cole et al., 2006).
Overlaying the Okareka deposit is the Rerewhakaaitu deposit which has an age of 17,600 years BP. It consists mostly of two types of pumice; crystal poor (3%) containing hypersthene and crystal rich (19%)