These mountains owe their existence to the fault line in the Earth's crust where the Pacific plate meets the Australian plate. Being much older, colder and denser, the Pacific plate dives under the lighter Australian Plate and, in doing so, drags 100 million years' worth of sedimentary rocks from the Pacific seabed down with it.
Layers of clay in these sediments contain a lot of water.
At a certain depth (according to Professor Tim Stern, at an earlier talk, this is between 50 and 100km), the conditions of pressure and temperature are such that the water is driven off from the clay causing partial melting and forming magma which can then rise to the surface.
Okataina and Taupo volcano groups in the North Island erupt rhyolitic tephra, meaning silica-rich rock fragments from eruptions. These areas also have the potential to dump volcanic ash on the Whanganui region.
Rock dating indicates an average time between eruptions from these areas as about 1700 years.
Statisticians are yet to pin down this time span accurately but the last major eruption from Taupo was 1800 years ago.
To put things in context, Taupo had one eruption 26,000 years ago which ejected about 10,000 times the volume of material ejected in the 1995-96 Ruapehu eruptions.
This deposited an ash layer two metres thick over most of Hawke's Bay and dumped lesser amounts as far away as the Chathams.
This type of eruption also gives rise to large, hot gaseous flows. The Pinatubo eruption in 1991, perhaps 500 times larger than the 1995-96 Ruapehu eruptions, caused such flows to run down river valleys carrying huge amounts of pumice.
In the case of the 26,000-year-old Taupo eruption, this flow can be seen in the terracing at Te Maire and most of Aramoho sits on about one metre of pumice brought down the Whanganui valley by this hot flow.
The problem does not end with the settling down of the eruption as the hot flow wipes out most vegetation in its path meaning the surface is not consolidated.
Subsequent rainfall results in large pumice floods and there is evidence of similar happenings in the Rangitikei rivers.
Taranaki is a mountain which has collapsed and been rebuilt many times in geological time scales.
However, surface flows have never got closer to Whanganui than the north of Hawera.
The real threat to our region from Taranaki is from airborne ash carried by the prevailing wind, which is strongly from the west and northwest.
Research shows that in the past 8000 years, ash from Taranaki has blown at least as far as Hawke's Bay.
Analysis of mud layers laid in the Auckland region over a period of 250,000 years shows traces of Taranaki ash at an average interval of about 400 years.
Investigations in the Taranaki ring plain show 76 ash eruptions in the past 28,000 years giving a frequency of about every 330 years for events that were large enough to show outside the present National Park boundary.
Many other studies have been carried out around Taranaki, resulting in professor of geo-statistics Mark Bebbington, of Massey University, giving a figure of about 60 per cent probability of Taranaki erupting in the next 50 years.
Ruapehu is different to the other volcanoes in that it has a crater lake.
It is the breaching of this lake that causes the lahars that can cause so much damage.
After the Tangiwai disaster and other events, more care is put into the structure and placing of at-risk buildings and bridges along with warning systems.
The main threat now presented by lahars is acidic water and grit getting into town water systems and hydro dams.
Boulders and sand cause serious damage to hydro generator turbines, and the highly acidic nature of the water from the crater lake decimates fish stocks.
Prompt action in closing off water systems and replacing vulnerable open channels with covered concrete pipes have now removed much of these risks.
It would be easy to regard volcanic action as all doom and gloom, but this would ignore important ecological and economic aspects.
There were significant (but temporary) increases in extractable sulphate and selenium levels in soils receiving '95-'96 Ruapehu ash.
Extractable sulphates briefly approached toxic levels, demanding a rethink in fertiliser recipes for the area. Pasture in this area is naturally selenium-deficient so the ash was a positive.
The volcanic soils of the Taranaki and Waimarino regions are ideal for food production.
The Westmere soils to the north of Whanganui have a large volcanic ash content, and their free-draining and friable properties make them among the premier soils of New Zealand.
In summing up, Professor Neall indicated the most probable volcanic effect on the Whanganui District is a sprinkling of ash from Ruapehu and, apart from cleaning it from roofs and gutters, it is unlikely to be a major problem.
It is not one to be ignored, however, and should be considered when designing local infrastructure.
■Frank Gibson is a semi-retired teacher of mathematics and physics who has lived in the Whanganui region since 1989.
