A violent volcanic eruption on Takapuna Beach with little warning. The forest close to the cafe was destroyed.
Auckland is the only city in the world built on a live volcanic field and the Takapuna eruption opened the show.
Large masses of partially molten lava under Auckland City rise relentlessly to the surface. They began their journey 80 kilometers below in the upper mantle. Approximately 50 basalt melts have completed their journey during the last 100,000 years. Volcanic pockmarks litter the Auckland landscape.
Never pour water on a burning chip pan and stand clear when molten rock at 1,200 ºC encounters water or wet materials near the surface. Under these conditions, water flashes explosively, forming steam under very high pressure. The result is a phreatomagmatic explosion.
Olivine makes up more than 50 per cent of the earth’s upper mantle and crystallises in environments with high magnesium ion and low silicate contents. Crystal faces in the original basalt melt exchange atoms by diffusion on their upward journey. Analysis of diffusion rims indicates magma can travel to the surface within a month. Once the shakes begin, there is little time to prepare. A massive explosion is imminent, valuable real estate will be blasted high into the atmosphere. Molten basalt will escape the 2-kilometer-wide crater and devastate the surrounding suburbs.
The trees in a forest close to the Takapuna boat ramp felt mild shakes in the months, leading to cataclysmically destructive eruptions over 100,000 years ago.
A rapidly expanding cloud of steam, magmatic gas, fragmented magma and other rock fragments was hurled upwards and outwards. leaving a shallow depression currently occupied by lake Pupuke.
Dendrochronolgy, or tree growth ring dating.
Basalt rings record the death of a tree that grew on this spot 100,000 years ago. The initial water-driven explosion (phreatomagmatic) blasted ash and mud high into the air. It rained down on the surrounding forest near the cafe. Lava fountaining within the crater built up several small scoria cones, lost as the volcanic activity progressed.
Runny lava at 1000 °C flowed through the forest. It chilled and thickened against the trees. The trees were consumed by fire leaving hollow basalt stumps.
Gas holes (vesicles) in the lava.
Bubbles form in the molten lava when trees combust and release volatiles. They are trapped near the surface of the flow when it solidifies.
Arches linking adjacent stumps.
The arch marks the “high tide” of the lava flow. The hard crust that formed on the surface presumably collapsed and crumbled as the runny lava beneath drained away.
Trees and branches were incinerated in the fast flowing lava.
Cylindrical holes are molds of trees or large branches, carried in the flowing lava. This outcrop is along O’Neil Bay. The frothy lava surrounding the light-coloured hole was probably caused by a tree branch cooking in its molten basalt oven. Oval-shaped holes may be large gas blisters, formed by expanding gas.
Inside a large gas bubble.
“Droplets” appear to cover the roof and walls of the “bubble cave”. Burning peat and leaf mould may have provided air at a temperature sufficient to remelt the cave wall and roof. A miniature blast furnace.
Tuff deposits, look like rough brown concrete.
Rock smashed into tiny pieces by the force of the phreatomagmatic explosion, has settled to form Tuff.
Sedimentary rocks and molten basalt were smashed into tiny fragments by the force of the phreatomagmatic explosion. Some of the ash clouds fell directly back into the crater forming a low Tuff ring around the perimeter. The remainder travelled outwards before drowning the forest in scalding hot dust. The weight of airborne material snapped tree branches to be carried away in the runny lava.
Tuff.
Dark basalt fragments in a matrix of ash from the destruction of wet sedimentary rocks close to the surface.
Gas bubbles concentrated near the surface.
The large crack near the bottom of the lava flow has been filled with dark-coloured lava.
Gas makes lava more runny.
The dark lava filling this crack is filled with gas holes.
Analysis of gas bubbles in widespread Jurassic basalts shows that carbon dioxide coming out of solution (exsolution) is a major force driving magma up through the crust from the mantle. Basalts are carbonated just like fizzy drinks and they behave like them too.
Volcanic Eruptions around Auckland followed a similar pattern.
Three eruption styles dominate the Auckland landscape. The style at any particular time depends on the gas content of the magma.
A series of explosions caused by rising magma hitting wet sedimentary rocks near the surface announces the start of deadly volcanic activity. Superheated steam and gas, released from the magma, create a giant mushroom cloud of ash and rock fragments that rises high into the atmosphere. Some material falls back, forming a low tuff ring around the exposed crater. The rest surges outwards, scorching and burying everything in its path. Shallow explosion craters, two kilometres in diameter, remain after the dust settles. The volcano has been “uncorked”, and magma can spew over the landscape. The serious business of constructing an Auckland volcano starts in earnest.
What happens next depends on the gas content of the basaltic magma reaching the surface.
When gas-charged magma flows through the uncorked vent, it froths on the surface. Shaking a bottle with a fizzy drink and then giving it to someone else to open has a similar effect. A bottle of carbonated drink shaken fizzes and makes a mess.
Molten basalt is carbonated. When molten basalt with a high gas content froths it’s called lava fountaining. Carbon dioxide is responsible for between 10 and 40 per cent of the gas content in molten basalt.
While the lava fountaining part of an Auckland volcano is in progress, a steady stream of red hot magma lumps are sent flying into the air. They cool and fall as brown-coloured scoria, riddled with holes left by escaping gases. Whether relatively short, a few weeks or months, or longer the end result is a steep-sided scoria cone.
Drinks lose their fizz and become “flat”. The same thing happens to basalt. When the magma becomes depleted in carbon dioxide, it stops lava fountaining and flows freely on the surface as lava.
Phreatomagnetic explosions, scoria cone formation and streams of free-flowing basalt will occur at a postcode too close for comfort if you live in Auckland.
Scoria blocks in the wall by the football club, Te Pane o Mataoho / Te Ara Pueru / Māngere Mountain.
At 100 metres high this is Auckland’s best-preserved scoria cone, it erupted 70,000 years ago
Māngere Lagoon from the summit of Te Pane o Mataoho.
Mangere Lagoon is an explosion crater with a tiny 100-metre-wide scoria cone in the middle. It is surrounded by a low tuff ring. This eruption hardly got past the first stage of an Auckland volcano sequence. The eruption predates Te Pane o Mataoho and was a kind of early warning for the main event.
References.
(2) Māngere Mountain wikipedia.
(3) A Field Guide to Auckland. Ewan Cameron, Bruce Hayward, and Graeme Murdoch. ISBN 1 86962 014 3
(4) Deep CO2 in the end-Triassic Central Atlantic Magmatic Province
(5) Lava and Strata. A guide to the volcanoes and rock formations of Auckland. Lloyd Homer, Phil Moore and Les Kermode
ISBN 0-908800-02-9