Before delving into the eruption scenario, I'd like to dispel a few lay-person derived misconceptions around the whole set of circumstances.
To British resident Neil Love (reported here) no, Mt Cook (the tallest mountain in New Zealand) is NOT a volcano - it is simply part of an uplifted mountain zone formed by the collision of two tectonic plates - exactly the same as the Himalayas or the European Alps. A couple of reasons for this conclusion - the rock composition is not volcanic - the mountain slopes are too steep and finally, there's neither a subduction zone nor a magma hot spot under the Southern Alps; the primary tectonic activity is derived from two land masses pushing against and sliding past each other, it's called the Great Alpine Fault.
Next, the Banks Peninsula complex is generally regarded to be a Shield Volcano which erupts over a relatively long period, but is mostly given to display lava flows rather than explosive eruptions. Shield volcanoes are generally not connected with crustal interactions, which means they are neither derived in the same fashion nor expected to behave in the same way as the various North Island structures. And indeed the Banks Peninsula
Returning to Neil Love, he also offers us a suggestion of some kind of impending super-volcano which seems to be based on some kind of fanciful idea of a circular pattern in the ocean to the east of Christchurch. Never mind that (if it existed) it would be some hundreds of times larger than the currently largest-known super volcano (Lake Taupo in New Zealand's North Island).
Scientists at the local Institute of Geological and Nuclear Sciences (GNS) have attempted to dispel the volcano theory but in this writer's opinion they neglect to include many of the salient points in their dismissal. For instance, they avow that there is no available pool of magma with which to cause an eruption within 500km, yet fail to consider the origins of eruptions around Dunedin (200km south) and the more recent volcanoes immediately south of Christchurch. With this in mind, there is a recent study which shows that seemingly dormant volcanoes can reawaken in just a few weeks.
Well-known examples of shield volcanoes include the Galapagos and Hawaiian Islands, the east African Rift volcanoes and many (but not all) of the Icelandic volcanoes.
We are all aware of the endless Hawaiian volcanic events - significant lava flows with limited eruptive activity. In addition, shield volcanoes don't (as a rule) "blow their top" in the manner of Krakatoa or Mt St Helens (the recent eruption in north western USA).
The difference is that volcanoes linked to subduction zones (as most "ring of fire" volcanoes are) are the result of sea floor crust being thrust under continental crust, carrying with it a large amount of moisture. Of course when all this water gets hot (which it must do as the sea floor material is pushed tens or hundreds of kilometres deep into the crust) it is pressurised and can only go up; rapidly. Explosively, perhaps.
Shield volcanoes, on the other hand, are derived from hot magma (without the explosive pressure of stored gasses) relentlessly pushing its way to the surface.
In order to graphically compare the two, readers might like to perform a simple experiment. Take two stubbies of beer; one recently opened with the contents intact (no drinking!), the other with the beer replaced by hot water (empty the bottle by any suitable means). Forcibly tap (or bash) the top of each prepared stubby with the bottom of a third bottle (empty is good!). What happens?
If your bottles behave the same as mine, the hot water-filled bottle will do absolutely nothing whereas the beer-filled one will erupt and flow all over the table, potentially expelling over half the contents of the stubby. The difference? The beer-filled stubby contains an excess of dissolved gasses - the strong tap simulated an earthquake, and the bottle (simulating a volcano) erupted.
The beer-filled stubby can easily be compared with a subduction volcano, the hot water-filled one with a shield volcano.
So, if previous suggestions end up being true and we are indeed on the verge of a major eruption, what will Christchurch (and the wider Canterbury region) experience?
At some point, a larger tear will open up (this could be anywhere on the dome - at the top or on any flank) and lava will begin to flow down hill, towards the sea.
As the pressure drops the lava flow will eventually stop. For a while. Then the whole process will repeat. Over and over again.
The best outcome in this scenario is that the eruptions occur to the south of the two existing volcanic craters; the worst (and judging by the earthquake pattern, most likely) is that they will occur either under the existing structure or immediately to the north - right where Christchurch is situated.
No matter how this eventuates, there will be sufficient time to evacuate the city - the actions of a shield volcano eruption are relatively slow-moving, but very long-term. Does anyone remember a significant period of time (I mean decades here) when the Hawaiian volcanoes were NOT erupting?
There is however, an alternate theory that doesn't include volcanoes, but is probably more catastrophic; stay tuned, as they say - more to come soon.
Note: the author holds a 1st Class Honours Degree in Geology from Victoria University of Wellington, one of New Zealand's leading geoscience institutions.