What would a Christchurch volcano eruption look like?

David Heath
Tuesday, 17 January 2012 21:25

Opinion and Analysis

I've been asked to speculate on the appearance and effect of an eruption (should it ever occur) in the volcanoes immediately south and east of Christchurch, New Zealand.

A few days ago, I wrote of a theory that the recent sequence of earthquakes around Christchurch (on the South Island of New Zealand) over the past 18 months may be the precursor of an eruption of the two (perhaps more) volcanoes that form Banks Peninsula.

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.