Geology of the Glass House Mountains

Visitors and locals alike are intrigued by the majestic Glass House Mountains and the question is often asked ‘are the mountains the remnants of old volcanoes?‘ The short answer is no, as they are actually magma intrusions that were formed and matured underground. Volcanoes are formed above ground and tend to disintegrate some years after they become extinct. This is primarily because the erupted lava cools quickly, with a resultant smaller crystal size that is prone to decomposition, and this is what happened with the Maleny plateau.

Interestingly, It was approximately 31 million years ago in the late tertiary period that a magma plume initiated a hotspot resulting in a massive volcano south-west of Maleny. Amazingly this volcano continued to erupt spasmodically for another four million years. These eruptions resulted in several layers of basalt eventually forming the Maleny and Buderim plateaux.

Some two million years after the final eruption, magma intruded into underground crevices and weaknesses between layers of rock to slowly cool and harden to form sills, dykes and laccoliths. Geologists suggest that at the time the land surface was some 300 to 400 metres above what it is today. This difference was primarily Landsborough sandstone which initially insulated the cooling magma resulting in a larger crystal size of the rock and its subsequent hardness. Over the ensuing 25 million years the sandstone gradually eroded to expose the aesthetic and iconic landscape we enjoy today. The relationship between the Maleny volcano and Magma intrusions is purely speculative, however it is feasible that the continuing eruptions did result in sub-surface fissures or weaknesses in the surrounding landscape.

Another theory is that the hotspot that caused the Maleny volcano was also the source of the Magma intrusions. For this to be so, we would need to reassess the northward movement of the Indian/Australian tectonic plate, currently estimated at 5.5 centimetres/year. Whatever the reason for the “fissures” they did exist and were filled with magma that had separated by a chemical reaction called differentiation into different types of rock called trachyte and rhyolite which became the embryo of the Glass House Mountains.

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