One of the most well known and recognized geologic features in the Australian state
of Victoria is a monolith which rises from the ground known as Hanging Rock.
This rock formation was even the site of a famous book turned into a movie and is now
visited by a little less than 1 million people a year.
Its most striking features are a series of hard pillars which jet out from the ground,
with gaps wide enough between many of them for a person to walk through.
It might come as a surprise, but this feature is volcanic and originated from a mantle plume,
somewhat similar to the one in the Hawaiian Island Chain.
Mantle plumes, or hotspots, consist of fairly uncommon regions of the upper mantle which
are warmer than surrounding regions.
As a result, it is much easier for molten rock to intrude in the crust in comparison
to average temperature regions.
Mantle plumes largely remain stationary but appear to move not because they move at high
rates but because the tectonic plates overlying them move.
Thus, over millions of years, a spotty record of volcanoes are created along its path, with
features getting progressively older the further you go from the mantle plume's current location.
This means that volcanoes the hotspot creates are only active for a brief period when they
are close to the hotspot, before becoming permanantly extinct.
The mantle plume responsible for forming Hanging Rock is known as the Cosgrove Hotspot, which
over 33 million years traveled across Queensland, New South Wales, and Victoria, forming volcanic
features along the way, before reaching its suspected present location today 100 kilometers
west of the island of Tasmania, deep beneath the ocean.
Hanging Rock, which towers 104 meters or 341 feet above the surrounding landscape formed
during a single volcanic eruption 6.25 million years ago.
This eruption ended up creating a lava dome out of a type of viscous lava known as trachyte.
Before this eruption began a little over 6 million years ago, the ground repeatedly shook
due to increasingly shallow earthquakes that were triggered by the movement of magma.
The ground eventually began to slowly rise above the eventual eruption site by as much
as 20 centimeters.
Then, with a boom, a patch of ground exploded upwards as magma interacted with the area's
water table, causing the water to flash to steam and explode.
However, from within this newly formed crater emerged brownish gray trachyte lava.
Unlike the eruption witnessed on the island of La Palma in 2021, or Kilauea in Hawaii
in 2023, this lava was not highly fluid as in contrast it had a high viscosity.
Thus, instead of forming long and thin flows of runny lava, the lava practically stayed
in its same location, being only able to move in small amounts due to the sheer viscosity
of the lava.
As the eruption continued, a white gas plume rose into the air, and the orange glow the
lava produced would have been visible from a large radius.
Over the course of what was likely several months, a larger and large lava dome developed,
likely reaching a height of 150 meters and a width of 750 meters.
Eventually, this eruption ended, but this is when the crucial bit occurred relating
to the formation of Hanging Rock.
Right after the eruption ended, the magma conduit which supplied molten rock to the
surface which contained a vertical column of rock began to solidify, starting at the
top.
As the top fully soldified, the rock contracted, causing cracks and then gaps between part
of the trachyte to appear.
The cooling trachyte began solidifying at deeper and deeper depths and the cracks continued
to propagate at deeper depths, until what remained in the conduit were numerous pillars
of rock that were closely spaced.
Over the next approximately 6 million years, erosion removed the softer surrounding lava
dome rock while keeping much of the more erosion resistant interior.
This eventually resulted in Hanging Rock's remarkable modern appearance.
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