A Hawai'ian man has been found dead after apparently falling into a sinkhole on his property. Police were called to the home of 71-year-old Robert Hisashi Nishimoto of Hilo on Big Island, on Monday 4 November 2019, after neighbours failed to see him for several days. a search of the property revealed a hole approximately 60 cm wide and eight metres deep, into which Mr Nishimoto had apparently fallen while trimming branches in his garden.
The approximate location of the Hilo sinkhole. Google Maps.
Sinkholes are generally caused by water eroding soft limestone or unconsolidated deposits from beneath, causing a hole that works its way upwards and eventually opening spectacularly at the surface. Where there are unconsolidated deposits at the surface they can infill from the sides, apparently swallowing objects at the surface, including people, without trace.
On this occasion the sinkhole is believed to have been caused by the collapse of the roof of a lava tube beneath the surface. A lava tube is a form of cave formed not by erosion, as in the case of most limestone caves, but by the way in which the lava from which the rock is formed cools and solidifies. As lava flows on the surface its outermost layers, which are in contact with the atmosphere, cool and harden more rapidly than layers lower down. This means that a solid crust can form over the lava flow, slowly solidifying from the outside, while the lave beneath is still flowing. Eventually a cavern is formed through which lava is flowing surrounded by solid rock. If the supply of lava is then cut off (by an eruption ending or the lava finding a different course) before the route through which the lava is draining is blocked, then a void (cave) will be left running through the rock, which we call a lava tube.
The formation of a lava tube. Caves of Lanzarote.
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