In a study published last month in Nature Communications, Dr. Buchner and his collaborators proposed a recipe for how sand spikes form. The necessary ingredients, the researchers suggest, include powerful ground shaking, loose sand, water and calcite, which functions like a glue.
The process starts when the fastest-moving seismic waves, or primary waves, pass through sand and compress it, the researchers suggest. That sends most, but not all, of the moisture within it to the surface, Dr. Buchner said. “The sediments are basically dry but there are still a few pockets of water.”
A few seconds later, slower seismic waves, called secondary waves, ripple through the sand again. Those waves trigger dramatic increases in pressure, which cause remaining reservoirs of water to rapidly heat and then vaporize explosively. Each explosion excavates a hollow structure that sprouts a tail pointing away from the source of the pressure. The surrounding sand rapidly flows back into the void, and calcite, the primary mineral in limestone, cements the structure together.
Sand spikes probably form in a matter of seconds, the researchers suggest. That’s unlike most other geological structures, said Franek Hasiuk, a geologist at the Kansas Geological Survey who was not involved in the research. “A lot of our rocks take at least millions to tens of millions of years to form. This happens in a geologic blink of an eye.”
Besides Germany and California, sand spikes have been spotted only in a few other places. Given their relative scarcity, particularly powerful ground shaking must be necessary to form them, Dr. Buchner and his colleagues propose.
Because sand spikes are markers of intense, potentially destructive ground shaking, they’re valuable indicators of hazardous conditions, Dr. Hasiuk said. It’d be a bad idea to build a school or a nuclear power plant where sand spikes have been found, he said.
“Understanding ancient seismicity can really help us more intelligently plan our infrastructure.”