Nature at Work: the Silt Volcanoes

One of the wonders of the Canterbury earthquake has been the mini  ‘silt volcanoes’ that have formed on the flatlands  adjacent to the Port Hills. In my last post I suggested it was due to a pressure wave transmitted through the aquifers that lie beneath the Canterbury Plains. On reaching the Port Hills, which are comprised of  ancient volcanic rock, there was nowhere else for the water to  go but upwards. The water spouted and bubbled up, carrying the disturbed silt with it, and then deposited this silt in the form  of mini volcanoes, typically about 30cm high, and each with its individual vents and craters. The water then caused considerable surface flooding. 

 These mini volcanoes are to be found spread across paddocks, in back yards, and also on the roads, where they have broken through the asphalt.  Apart from on the farms, they are rapidly being lost in the clean-up.

Some people are calling them ‘sand volcanoes’,  but what I have been seeing is silt and not sand. I think the distinction is important.  I have also  seen one media report referring to them as liquefaction, but that too seems likely to be a mis-interpretation. I think we are seeing semething unusual, perhaps even unique, and linked to the specific geology of the Port Hills combined with the extenive aquifers within the shingle fans that characterise the plains.  I also suspect that the shock wave passing through the aquifer can  provide explanation as to why and where some of the infrastructure damage has occurred.   As I have travelled around there seemed to be good correlation between the presence of silt volcanoes in the paddocks and nearby road and building deformation.

The photos below were mainly taken in the Tai Tapu and Ladbrooks area, which is a few kilometres west and south west of my home on the outskirts of Halswell. The photos  were taken about 60 hours after the quake on Monday 6 September. 

These next two photos are of some damage in the same area on Old Tai Tapu Rd

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About Keith Woodford

Keith Woodford is an independent consultant, based in New Zealand, who works internationally on agri-food systems and rural development projects. He holds honorary positions as Professor of Agri-Food Systems at Lincoln University, New Zealand, and as Senior Research Fellow at the Contemporary China Research Centre at Victoria University, Wellington.
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2 Responses to Nature at Work: the Silt Volcanoes

  1. Dr Andreas Antoniou. Geotechnical Engineer. says:

    From the above photos it is obvious that the subsoil was liquefied. The water table in those areas should be few meters below ground surface. During earthquake and usually after few cycles the subsoil under water table looses its bearing capacity due to excess water pressure and behaves as a liquid. Then human activities on the top of those subsoils submerge and tilt and usually are not suffer from the shock of the earthquake (collapses, or significant cracks on the concrete frames).
    This phenomenon is observed to loose silty sands of loose fine-grained sands under water table and the result is those “mini silty volcanoes”.
    Extensive liquefaction was observed at Kobe’s earthquake in Japan, and for the first time was observed in Niigata, Japan, in 1969.

    • Keith Woodford says:

      Hello Andreas
      Yes, in the months ,since last September here in Christchurcsh we have learned a lot more about liquefaction. The Feb 22 earthquake actually produced a lot more than the September earthquake (from which these photos came), and then again on June 13. However, the liquefaction in the Halswell Catchment in September was particularly interesting in relation to the pressure at which the water was emitted, and volume of water which contnued to flow at pressure for quite some hours. These characteristics were different than in the low lying eastern suburbs which have high water tables but do not appear to have the same connection to the aquifers via springs, and where the ratio of sand to water was much higher. So I think the concept of a pressure wave traansmitted through the big underground aquifers beneath the Canterbury Plains is valid, although I would now write this post diffferently based on what we have learned thereafter. The way in which large underground oil tanks in Halswell were pushed upwards would also suggest there was considerable pressure from below.
      The liquefaction in the eastern suburbs from the February earthquake was enormous, and the damage was further exacerbated in the June 13 quakes. Decisions,are now being made to abandon whole suburbs in the east of the city. So far the zones to be totally abandoned comprise 5100 houses, but there will be many thousands more houses in other zones that will have to be pulled down and rebuilt. As for the central business district, that is still totally off limits to ordinary citizens as a consequence of the February 22 quake, and almost everything including modern buildiings are to be demolished. Although both the Febraury 22 and June 13 earthquakes were only M6.3, (cf 7.1 in September) their shallowness and proximity to the city led to extremely high accelerations and consequent damage..
      KeithW

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