Kapenga Caldera

The Kapenda Caldera, just south of the Rotorua Caldera and between this and the Maroa Caldera, is believed to have been buried by subsequent eruptions including those of the Tarawera volcanic complex. The eastern area of the postulated caldera has had, relative to much of the rest of the central Taupō Volcanic Zone, many smaller eruptions after caldera formation.[3] Problematically to date no definite assignment of the up to seven ignimbrites attributed to it can be related to a definite caldera-forming event or documented collapse despite geology suggestive that such may have occurred.[4] It had several very large eruptive events during the ignimbrite flare-up of the Taupō Volcanic Zone between 350,000 to 240,000 years ago.[1] The boundary between the Kapenda Caldera and the Ōkataina Caldera is debated which particularly affects more recent Earthquake Flat activity. The Kapenda Caldera occupies the northern part of the Paeroa Garben between the uplift caused by the now fairly inactive Horohoro Fault and the uplift associated with the still very active Paeroa Fault. To the south of the caldera is the Ngakuru Graben. For an unknown period between the Mamaku Ignimbrite eruption of the Rotorua of 240,000 years ago and about 60,000 years ago Lake Rotorua drained via the Hemo Gorge into the Kapenga Caldera which likely contained a lake, and through the Ngakuru Graben, onwards to the Waikato River as it existed then.[2]^{: 358–360} Subsequent volcanic activity has buried much evidence of this phase of the caldera's history.

A large number of intra-rift faults called the Taupo Fault Belt and associated with the modern Taupō Rift exist in the Paeroa Garben and Kapenga Caldera so it is quite a tectonically active area presently.

The paired in time Rotoiti eruption and Earthquake Flat Breccia eruptions, had the later assigned to the Kapenga Caldera as it was within the old caldera's margins, but with greater understanding is assigned to the magma mush body that underlays the magma bodies of the Ōkataina Volcanic Centre and thus to this volcanic centre.

There are two rhyolite domes of note. These are the Horohoro Cliffs at 817 m (2,680 ft), a rim-fracture dome associated with the Horohoro Fault that marks the northwestern edge of the Kapenda Caldera, and Haparangi a late-stage intra-caldera dome that rises above the Ngakuru Graben to 688 m (2,257 ft).[5] Haparangi Rhyolite has been a term used since 1937 to any rhyolite flows/lavas of Pliocene-Pleistocene origin in the Taupo Volcanic Zone and does not imply any origin from the Kapenda Caldera.[5]

The northern part of the caldera has amongst the highest rate of subsidence in the Taupō Rift.[6]^: 4672 This is believed to be predominantly driven by cooling and subsequent contraction of an underlying magma body at about 6 km (3.7 mi) depth.[6]^{: 4667, 4677}

Its known eruptions were:

• 890,000 - 680,000 years ago
  • Tikorangi Ignimbrite (sometime known as Pukerimu Ignimbrite). Erupted at 0.89 ± 0.04 Ma[7]
  • Rahopaka Ignimbrite (0.77 ± 0.03 Ma)[5]
  • Waiotapu Ignimbrite (0.71 ± 0.06 Ma)[5]
  • Matahana Ignimbrite (0.68 ± 0.04 Ma) [5]
• 310,000 - 275,000 years ago
  • The caldera is not a source of some of the Mamaku ignimbrite as previously speculated - this all originates from Rotorua Caldera.[8]
  • Neither is the caldera the source of the Ohakuri Ignimbrite as previously speculated.[5] This comes from the Ohakuri Caldera[9]
  • Pokai ignimbrite (0.275 ± 0.01 Ma)[1] (previously assigned 0.23 - 0.22 Ma)[5]
    • Eruptive volume 100 cubic kilometres (24 cu mi)[1]
    • Welded hard deposit almost reached Tokoroa in west and about Tauriko in north east.[1]
    • The vent still has some uncertainity and may be in the Ōkataina Caldera[1]
  • Waihou (Chimp, Chimpanzee) Ignimbrite (0.31 ± 0.1 on stratographic grounds)[1] (previously assigned 0.26 - 0.25 Ma) [5]
    • Eruptive volume 50 cubic kilometres (12 cu mi)[1]
    • Loose unwelded deposit reached Tauranga in north east and almost to Tokoroa in west[1]
    • The vent still has some uncertainity and may be in the Ōkataina Caldera[1]
• (felt misassigned)[5] 47,400 ± 1500 years ago (Earthquake Flat Breccia)[10]
  • The historical ages in the literature were close to those now accepted.[11][12] Some composition[5] does not match well Waihou (Chimp) Ignimbrite or earlier. Current ages are based on 20 years revision work.[1]