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Remains of a Brisbane volcano hidden in plain sight

 

Brisbane’s 226-million-year-old tuff rock has been used throughout the city’s various construction projects, but it also hides a destructive past.

The ancient volcanic rock is commonly found at the Kangaroo Point cliffs, where it was extracted for use in the construction of some of Brisbane’s oldest buildings, including St Mary’s Anglican Church.

Tuff rock is a type of rock that forms because of a volcanic eruption. Brisbane’s variant is unique to the state and includes colours such as pink, green and purple due to oxidation processes.

Queensland museum geologist Andrew Rozefelds told the ABC the rock includes evidence of pyroclastic flows.

“Tuff is a type of rock formed by the compaction of volcanic ash and the Brisbane tuff also includes evidence of pyroclastic flows, as well,” he said.

According to Rozefelds, the rock fragments were deposited at the same time volcanic ash from an ancient volcano fell.

“These ash falls and flows can build up many metres of sediment and the Brisbane tuff is over 50 metres in thickness in some areas, and sometimes much more,” he said.

“These volcanic eruptions were incredibly destructive events, and branches and tree trunks have been found buried in these compacted ash falls.”

When tuff rock was extracted at Kangaroo Point, the modern day cliffs formed the back wall of an old quarry.

Builders in the 1930s tried to recreate the tuff into “Benedict Stone”, according to former Geological Survey of Queensland worker Ian Withnall.

“They actually crushed Brisbane tuff with granite from The Gap and mixed it with oxide to give it the right colour and moulded many of the building stones including the Tattersalls Club and The Manor Apartments with it,” Withnall told the ABC.

There is little that remains of Brisbane tuff to be extracted as suburbs and land development in Brisbane have gradually covered it up.

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