Within the beautiful landscape of Australia are a series of hills which jet out of the
ground, showcasing beautiful layers of rock which contrast with the green vegetation which
blankets the region.
Known as the Glass House Mountains, these monoliths of rock are volcanic in origin,
and originated during a period approxiamtely 26 million years ago when the climate was
colder, and more temperate rainforests covered much of the continent.
Ancient Australia had split from Antarctica only 10 million years prior, but more importantly
the continental landmass began to pass over three unusually hot regions of the Earth's
mantle.
Known as hotspots, as the Australian continent passed over them, it resulted in significant
volumes of magma to intrude into the crust, occasionally erupting onto the surface and
forming a chain of volcanoes.
These volcanoes trended in a south-southwestward direction as the mantle hotspots largely did
not move but the overlying crust did as it moved slowly to the north-northeast.
One of these hotspots known as the Comboyne hotspot first appeared at Fraser Island where
it erupted basaltic lava.
However, over time the basalt magma chambers present began to melt surrounding rock which
mixed with it, causing the molten rock to turn to andesite, then dacite, and finally
rhyolite.
Occasional rhyolite volcanic eruptions occurred over the next 3 million years until it reached
an area just east of Yandina.
As the silica rich magma breached the surface, it erupted in an explosive manner, producing
many 10 to 20 kilometer high eruption plumes.
Smaller eruptions also occurred, and in addition to ejecting large amounts of ash which coated
the surrounding landscape, volumes of silica rich rhyolite and trachyte magma erupted.
Due to their high viscosities, these lavas did not travel very far and instead built
up into what are termed a lava dome complex volcano which were up to 3 kilometers wide
and a kilometer in height, much like the modern Lassen Peak volcano in California.
However, since the hotspot derived magmas involved a path of least resistance to the
surface, the time that these volcanoes existed was limited.
Thus, over a timespan of 200,000 years, the volcanoes explosively erupted, grew in size,
and then became extinct.
Yet, when they were active, a large magma chamber grew underneath the active volcano
in addition to nearby intrusions of magma which never reached the surface.
After the volcanoes stopped erupting, erosion kicked in.
Over the following 27 million years, the overlying layers of more easily eroded ash and sandstone
eroded away the level of the ground by several hundred feet until it reached a section of
hardened rhyolite and trachyte rock which was more resistant to erosion.
Since these former failed magmatic intrusions and magma chambers had a large vertical component
to them, as the ground surrounding them was eroded, near vertical monoliths of ancient
intrusive rock was revealed.
And this is how the first two of the 16 so-called Glass Mountain peaks formed which were Mount
Coolum and Mount Ninderry.
There was subsequently a decrease in the number of volcanic eruptions for the following 500,000
years until the hotspot started producing volcanism around the town of Beerwah.
Over the following 1.5 million years, this area saw active volcanism as multiple gray
colored volcanoes existed with large volume magma chambers.
These volcanoes also occasionally produced basaltic lava in what were less viscous and
more runny lava flows.
The hotspot would later continue to the south before abruptly ceasing to cause eruptions
on the surface 16 million years ago as the last feature which formed was the Comboyne
shield volcano.
Around the town of Beerwah, slightly more than 20 million years of erosion exposed more
than a dozen volcanic plugs and failed magmatic intrusions, creating beautiful towering volcanic
monoliths.
I want to note that not all of these monoliths were centered underneath a volcano at one
point as it is quite likely that the majority instead were isolated and failed magmatic
intrusions of varying sizes.
Today, all of the 16 million to 31 million year old volcanoes this hotspot created are
long extinct and will never erupt again and can be appreciated as towering features in
the modern Australian landscape.
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