by Tas Walker
South of the Mapleton Plateau near Brisbane, Australia, some remarkable volcanic plugs known as the Glass House Mountains dominate the Sunshine Coast hinterland, a popular tourist destination.
Just 60 km (40 miles) north of Brisbane, the area is known for pineapples, strawberries, papayas, passion fruit, avocados and other tropical fruit. Australia Zoo, owned and made famous by the Crocodile Hunter, the late Steve Irwin, is situated in the area.1
Captain Cook named these mountains on 17 May 1770, as he sailed up the east coast of Australia in his ship Endeavour. He wrote in his journal, “These hills lie but a little way inland, and not far from each other: they are remarkable for the singular form of their elevation, which very much resembles a glass house, and for this reason I called them the Glass Houses.”
There are at least a dozen plugs, the remains of volcanoes that pushed their way up through thick almost-horizontal layers of sandstone. It is hard to imagine what it would have been like as the volcanoes were erupting, spewing red-hot ash and lava from their vents and darkening the atmosphere with ash and fumes.
The lava was rich in silica and formed rhyolite and trachyte, volcanic rock that is typically yellow, pink, pale grey and other light colours. Silica-rich lavas are thick like porridge and leave characteristic steep-sided volcanic plugs.
Many of the plugs have well-developed columnar jointing. The northern face of Mt Beerwah is one prominent example, and the exposed columns are dubbed “The Pipe Organ”. Such long columns suggest the lava was extruded rapidly, generating a large volume that solidified and cracked as one extrusion.
At around the same time, volcanic eruptions of a different kind were active to the north and west. In this case the lava was basaltic in composition, rich in iron and magnesium, and it was hotter and more fluid than the lava that formed the plugs. This red-hot molten rock ran like water across the sandstone plateaus and solidified to form thick basalt caps, the Mapleton plateau being one example. Basalt lava forms rich red and brown soils ideal for farming and for the development of rainforest.
The countryside around the Glass House Mountains is made of sandstone, geologically called the Landsborough Sandstone. The thick strata are visible in steep road cuttings along the highway. The Landsborough Sandstone is part of an interconnected series of sedimentary basins called the Great Australian Basin, which extends from the east coast of Australia for thousands of kilometres south and west into New South Wales, west into South Australia and the Northern Territory and north to Cape York in northern Queensland….
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