Even when you know that the area in which you live is prone to earthquakes, perhaps the last thing you might expect is a volcano in the back yard. Yet these are not the volcanoes we see in Hollywood movies. Neither are they the mud volcanoes which occur near geysers. These are something quite different. These are sand volcanoes.
A sand volcano usually occurs after an earthquake. It is formed when sand is thrown up to the surface from a central position. As the sand is ejected it builds up in to a cone, taking on the appearance of a volcano as the sand comes to rest on its sides. A crater forms which can range in size from the tiny, just a few milimeters, to one which can extend to several meters.
So why do sand volcanoes form? All of the pictures you can see here are from the Christchurch, New Zealand earthquake of last year. The process is usually associated with something known as earthquake liquefaction – when the earth behaves just like a liquid.
In order for a sand volcano (also known as a sand blow) to form, stress must be applied to the earth in the locality. The soil loses its strength and although it does not actually become a liquid, it certainly behaves like one. It is most likely to happen when the soil is saturated, loose and sandy. Soil is often saturated in areas which are below sea level.
The water fills the spaces between the grains of soil, known as pore spaces. When an earthquake happens pressure is exerted upon the soil. This load makes the sand compress. The water responds to this compression by attempting to flow from this high pressure area. This invariably means upwards towards the surface.
Yet something more happens when an earthquake strikes. As the ground trembles the water does not have enough time to escape before the next time it is shaken. When this pressure becomes great enough for the grains of soil to lose the contact stress which holds them together, liquefaction occurs. You could say that the sand volcano is a combination of both the water heading towards the ground and the loss of contact stress in the soil.
Similar processes caused sink holes to appear and cars to sink in to the streets of Christchurch.
Once the initial shock of the earthquake had passed and casualties looked after, the attention of many suburban Christchurch residents turned to the sand volcanoes that had appeared in their back yards, let alone their streets. Someone even jokingly put up a sign offering tours.
Even hours after the earthquake, a gentle shake of the the surface of the now dormant sand volcano and it quickly re-liquifies.
The original earthquake was followed by strong after-shocks. As these too affected the sand volcanoes, miniature fault lines began to appear in their structure.
People were understandably too busy at the time to witness the initial explosion of sand on to the surface. After, however, the community, young and old, came together to help with the clear up. Yet this is probably the most bizarre side effect of the 7.1 magnitude earthquake which killed almost two hundred people. Nevertheless, just the cost of cleaning up the sand volcanoes ran in to millions of dollars.
Although the residents of the Christchurch suburbs cleared their own sand volcanoes within a short time (albeit at great expense), those left undisturbed leave behind a record of their presence. Sand volcanoes are thought to be clear signs of prehistoric earthquakes when they are discovered in buried sediment segments. Christchurch’s sand volcanoes would appear to agree with this theory.