Today (Tuesday 7 September) I have been trying to increase my understanding of where the silt volcanoes have been forming, and perhaps some reasons why. My first insight for the day came when I realised that the surface water in the paddocks below my house was still not draining fully. I realised that what I thought were ponds of water were in fact more silt volcanoes. Clearly, these were lying in a pattern which looked like a drainage line. So in this post I am including both a photo from above, and some close-up photos from within the paddock.
Travelling more widely around the district has confirmed to me that there is a clear relationship between their siting and the tongues of ancient volcanic rock that extend out and under the shingle alluvium. This alluvium has been laid down in the last 16,000 years, whereas the Banks Peninsula volcanic rock was laid down some millions of years ago. And it is when the underground water comes up against the underlying rock that it is forced up to the surface.
Underlying volcanic rock will not be the only reason that silt volcanoes form. Wherever there are artesian springs, with water welling-up on account of an obstruction of one type or another, then the conditions are going to be right for these silt volcanoes to form.
From Halswell I drove to TaiTapu and then to the low-lying Greenpark, which is renowned for its wetness. But Greenpark is away from Banks Peninsula (and the associated Port Hills) and there were no signs that I saw of the silt volcanoes. But Greenpark did undoubtedly experience a strong force, as at one point my way was blocked by a bridge that had reared into the air at one end by 50 cm.
Coming back to Halswell has confirmed the distinctive patterns, with the silt volcanoes being broadly in lines which are obvious in the paddocks, but much less obvious in the suburbs, where buildings and driveways serve to block the visual pattern. The volcanoes are particularly apparent adjacent to the Kennedy’s Bush volcanic tongue, including Larsen’s Road, Glover’s Road, Kennedy’s Bush Road and including the grounds of the Halswell Primary School. They also extend down Sparks Road, and on the south side thereof, and in the Hoon Hay district. There is a very close association in these areas with road deformation, and also I believe with building damage. (Those of you who saw TV footage tonight of All Black coach Steve Hansen’s modern but munted Tai Tapu home would have also seen the remnants of some nearby silt volcanoes.)
Talking to one of the Halswell locals who has artesian bores confirms that there has been considerably increased water pressure following the quake.
I am told by an earthquake expert (whom I will acknowledge once I get permission to use his name) that silt volcanoes are common with earthquakes, and that it is caused by “high temporary pore water pressures and weakening of soil due to shaking”. I am comfortable with that explanation but also quite confident that there are special geophysical conditions that are relevant here. The expert has also supplied me with a photo of a ‘dry volcano’ that he took out at Sandy Knolls about one km from the fault line (which I will post if permission is given). Clearly there are some of the same physics at work, although differences are also obvious.
See below for more photos of the silt volcanoes, this time all relating to the paddocks below my house.