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