The large, deep craters known as calderas have extremely violent origins. They form when a vast amount of rhyolite magma, bubbling with gas, erupts explosively from a magma chamber that may be only a few kilometres beneath the ground. During these eruptions, so much magma is erupted that the chamber empties, leaving the ground above it unsupported. This area collapses, dropping like a piston, to form a wide, deep depression.
In places the caldera walls can be seen as steep cliffs, but many are difficult to observe in the landscape because they may be filled in with erupted material or covered by water.
Rhyolite calderas may be active for several hundred thousand years, but large eruptions are rare, often with thousands of years between events. Caldera collapse is not the only effect on the landscape from these large explosive eruptions. Huge quantities of pumice, ash and gas are pumped into the atmosphere, and through a combination of heat and momentum, a seething column of this material may rise to over 50 kilometres above the caldera. From this height, ash and especially aerosols – gases and tiny drops of acid – can spread around the globe, affecting the world’s climate for several years.
Closer to the caldera the landscape may be buried by metres of pumice. The most devastating process, however, occurs when this column of material falls back to earth like a fountain, then surges out in all directions from the caldera as a hurricane-like billowing flow of hot pumice, ash and gas. These pyroclastic flows or ‘density currents’ can travel over 100 kilometres at the speed of a racing car, leaving behind a layer of volcanic (pyroclastic) debris that might be more than 100 metres deep. Some flows are so hot (600–700°C) and thick that the ash and pumice fragments fuse back together, forming solid rock known as welded ignimbrite. Cooler and thinner pyroclastic flows form loose, comparatively soft (non-welded) ignimbrites.
Dome building after explosions
Eruptions from rhyolite volcanoes are not always so catastrophic. A small amount of rhyolite magma may remain after a caldera eruption, which is exhausted of all gas and so can only ooze from the volcano slowly, often along the faults and fissures opened up by earlier caldera collapse.
The very high viscosity of rhyolite lava means that it will not flow far, and instead it piles up around the vent to form steep-sided domes. These domes are prominent landscape features. For example, Mokoia Island and Mt Ngongotahā are rhyolite lava domes erupted within Rotorua caldera, and Mt Tarawera is a collection of lava domes that erupted around 1314 AD within Okataina caldera.
Calderas in the Taupō Volcanic Zone
There are two active calderas in the Taupō Volcanic Zone which have erupted frequently in the last 10,000 years:
- Taupō – 22 eruptions in the last 10,000 years
- Okataina – 6 eruptions in the last 10,000 years
The Okataina caldera includes the Tarawera volcano which erupted most recently in 1886 and about 1314 AD. There are also at least six older calderas, including Mangakino, Kapenga, Whakamaru, Reporoa, Rotorua and Maroa. Large explosive eruptions over the last two million years from this nested collection of rhyolite volcanoes has produced a huge volume of pyroclastic rock which has buried older vents. The products of these big rhyolite volcanoes form the extensive flat ignimbrite plateaus flanking the eastern and western sides of the volcanic zone.