Underground fire causing ground to collapse

Common in areas where trees are large enough to have large roots, eg the tall forests of SE Australia. The heartwood of a large tree (and eucalypts can be very large) is essentially dead wood (trunks, branches and roots) and can become hollowed out. A fire in a branch or trunk can become a chimney and accelarate burning material quite high up; it's a major part of the blacking out to ensure no chimneys are still burning. When the fire penetrates into the root system it can go unnoticed for many days. Some roots can be quite large and extend in a "plate" for some distance from large trees.

In the 1962 fires in the Dandenongs a bloke was walking through a forested area a few days after the fires had been extinguished (to all intents and purposes) when his foot went through the topsoil and into a bed of coals still burning in the root system; he had proper gear on but his leg was still burned almost up to the hip.

When training those who spend much of their time in grasslands I make sure they understand this particular aspect of forest fires.

On a similar matter, I remember seeing the skeleton of a tree perched on top of a 40' high tor in the Picton Range is southern Tasmania. It transpired that the Tasmanian equivalent of Victoria's Black Friday fires (started 13 January 1939), before February 1967, had been in 1932 and that fire on Mt Picton had been left to burn and had burned through 40' deep peaty organic Cool Temperate Rainforest soil over several months. Similar fires in similar (but tropical) soils are still burning in parts of Indonesia.

Cheers, Rowan

Cheers, Rowan