River of flowing ROCKS in New Zealand after cyclone Gita strikesDaily Mail
February 25, 2018
This incredible video shows a river of rocks flowing down a gorge in New Zealand after they were dislodged by Cyclone Gita.
The phenomenon, known as 'granular flow' was filmed at Terrible Gully near Canterbury in the aftermath of the tropical storm.
It happens when rocks of a similar size, in this case greywacke, are thrown together in large numbers down an incline, causing them to act like a liquid.
Granular flow is actually quite common, according to geologist Dave Petley of the University of Sheffield.
He explained in a blog post: 'In essence the pebbles behave as particles, allowing behaviour that is akin to that of a fluid.
'Of course the density of the fluid is somewhat higher than that of a more familiar liquid, such as water, and the particles are many times larger, but the river of rock is still able to generate the sorts of flow structures that we see in water.'
The video was filmed by Donna Field of Cleardale Station, who told Stuff.com: 'It was really loud [and] sounded like when a gravel truck tips all its load, but if you were right underneath it.'
Greywacke, a form of sandstone, is easily eroded in extreme weather events.
Cyclones are common in the Pacific at this time of year and Category Five systems have proved catastrophic when they make landfall.
Cyclone Winston killed 44 people in Fiji in 2016, and Cyclone Pam claimed 11 lives and damaged 65,000 homes in Vanuatu in 2015.