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.
Posts from Keith Woodford 
Hi Keith, sounds like I live around the corner from you (Samuel Street).
Thank you for your research on the phenomena. Seems you have been better prepared than the earthquake commission.
Ever since the earthquake I have had a stream pop up from under my house’s foundation and am able to hear gurgling sounds coming from under ground. An initial dig revealed a crater about 2 cube meters about a meter under ground level.
Have you been able to find any information relevant to newly formed streams, and more importantly; do you by chance have an idea of the of the outcome re bulldozing the house given the building code vs insurance companies trying to save pennies?
Thank you,
George
Hi George
I am trying to understand from observation, and also information that people such as you give me, something of the complexity of the situation. There have definitely been changes in the underground springs. My understanding is that where we are in here in Halswell, in what on the soils maps are called the ‘lowland fringes’, the water is under pressure in the aquifer. This is because the water which is flowing from higher up the plains comes up against the volcanic rocks of the Port Hills, which extend out underneath us. The water in the aquifer is lying in sand and shingle, and cannot easily come up through the silt and clay to the surface. Hence, when it does break though (such as under your house), then it tends to keep flowing up through the new channel that it has made. I have heard of one woman on the rural outskirts of Halswell who had a sand boil (sand volcano) come up through her pantry, and I have seen others where they have broken through concrete driveways. Water under pressure can do amazing things.
I have no idea as to issues re the building code vs insurance companies.
KeithW
Hi George
I just saw your article and you had mentioned about the no silt volcanoes in Greenpark there are plenty at my brother’s he lives in that area paddocks are a sea of slit.
Thanks Rose
Yes, I subsequently realised that there were indeed parts of Greenpark with lots of silt volcanoes. In general they seem to have followed the River, all the way down from Tai-Tapu as the river wends its way around. However, there are also lots and lots of them on Hudson’s Road as that road approaches towards the Greenpark Church. I presume that Greeenpark was so named because of the surrounding springs that kept it green. So I still think that my theory of the volcanoes being closely associated with springs is correct, and I still think that these springs tend to be where the aquifer comes up against the underlying volcanic rock which forces the water to the surface. But I agree that underlying rock may not be the only reason for the springs to occur. Actually it would be very interesting to know what is the underlying rock at Greenpark, i.e whether it is volcanic and extending out from the Pensinsula or whether there are deep underlying gravels. I suspect the former but have no evidence. Does your brother live in that part of Greenpark I referred to or somewhere else?
KeithW
Hi again, and sorry I got your name wrong, yes my brother dose live in that area it seems to be worse along the old railway line! by looking at the picutres they have sent me as I am in Australia.