Mt. Karioi

The Te Toto amphitheatre provides a rare view into the heart of a large basaltic volcano.

From this site we can see that Karioi began its fiery life as a collection of tuff (volcanic ash) cones that exploded through the sediments of a shallow sea floor. These siltstones, complete with shells and carbonised logs, are exposed on the lowermost coastal terrace above the boulder beach. Eventually the tuff rings were accompanied by lava flows of fine-grained, alkali basalt. Both the tuffs and lava contain striking green fragments of mantle material (rich in olivine) that were brought up during the violent eruptions.

The next phase of Karioi, which began not long after deposition of the tuff rings, began with explosive eruptions that rained down ash with large pyroxene crystals up to 2 cm wide (!). This new injection of magma produced a shield of ankaramite (a type of very crystal rich basalt) erupted from a central vent or fissure.

Another radical change in the magma supply of the volcano led to closure of the ankaramite vent and development of another larger vent towards the centre of the mountain. A sub-plinian blast, marked by a 1 m thick red-ash layer (called the main marker horizon), draped the landscape. This was followed immediately by the eruption of 15 or so lavas of alkali basalt, which spread over the entire western margin of the volcano between Whale Bay and Papanui Point.

Geochemically, the Te Toto section provides clear evidence for the intercalation of “intraplate” (alkali basalt) and “subduction” (ankaramite) type lavas, which indicates that Karioi’s magmas came from two very different depths in the Earth’s mantle. So far, this phenomenon has been recorded at only two other volcanoes on Earth (Turrialba, Costa Rica & Mt. St Helens, USA).

From the view point, have a look towards the northern cliff of the amphitheater. From here you can see the three main volcanic formations, which are the tuff ring (base), ankaramite lavas (mid-lower), and thick sequence of shield lavas (mid to top of cliff).
Note also the presence of reddish ash layers separating the ankaramites and shield lavas. These ash deposits mark important changes in the eruption intensity of the volcano.