In Asia, Hong Kong is well known for its modern cityscape, but few people are aware that it
sits on a volcano.
The volcano in question is no ordinary volcano, as it produced one of the planet's largest
explosive eruptions.
In fact, Hong Kong is built on top of volcanic deposits which measure hundreds of meters
thick that originated from the High Island Caldera supervolcano.
These deposits largely originated from a single eruption which formed an 18 kilometer or 11
mile wide depression in the ground referred to as a caldera.
Although this volcano has long been extinct, remnants of the area's explosive past can
be found through a series of tens of thousands of hexagon shaped columns, which represent
thick beds of rhyolite ash.
The High Island supervolcano is located in far southeastern China where its complex encompasses
much of the special administrative region of Hong Kong.
Its outline is shown on screen, and does not represent the original caldera rim.
Instead, this region represents where its thick volcanic products are located, within
which the ancient caldera lies.
Due to extreme tilting over millions of years, what used to be the top of the caldera is
located on the eastern edge of the complex and the bottom of the caldera is located on
the western edge.
The High Island Supervolcano formed during an era which began 148 million years ago when
the region experienced numerous catastrophic eruptions that formed similarly large albeit
smaller calderas in the ground which may have been as much as a kilometer deep.
These calderas formed via rhyolite plinian eruptions that ejected dozens to hundreds
of cubic kilometers of material, forming 4 large adjacent calderas.
The reason why such highly explosive volcanism occurred relates to the area's ancient geologic
setting.
At the time, a tectonic plate collision was ongoing several hundred kilometers east of
the mainland.
One plate was being subducted underneath another, from which melted material migrated upwards,
forming a chain of volcanoes.
However, the intruding magma slowly began thinning the overlying crust over millions
of years, causing it to eventually begin spreading apart and forming a back arc basin.
Material in the mantle then rushed upwards to fill the empty space left by the thinning
crust, resulting in successive supermassive eruptions.
Over tens of thousands of years, beginning 141 million years ago, a massive body of magma
intruded into the crust.
During this timespan, it generated a few eruptions on the surface, forming lava domes, but the
majority of magma stayed at depth.
This magma chamber eventually reached a volume of several hundred cubic kilometers, after
which something triggered a large volume of material to rush towards the surface.
As the magma erupted from a lengthy fissure, powerful explosions occurred due to the region's
abundant groundwater.
As viscous rhyolite lava flows poured out of these vents, the eruption became more energetic,
releasing a sustained column of material at a rate of several million cubic meters of
rock per second.
As a plume of ash shot 50 kilometers or 164,000 feet into the atmosphere, part of the eruption
column collapsed, producing highly energetic gas currents referred to as pyroclastic flows.
These flows were up to a kilometer in height and seemingly defied local topography by travelling
over mountains, and reaching more than 100 kilometers distant.
After a several day long intensive eruption, the underlying magma chamber had been drained
to a point that the rock overlying it collapsed downwards by 1850 meters, forming an 18 kilometer
wide caldera.
In total, 1,300 cubic kilometers of rock was ejected in an eruption 30% larger than the
most recent supereruption of Yellowstone.
Some areas were buried in more than 400 meters thick of ash in the course of less than a
week.
This ash was thickest within the caldera and took many years to cool.
As it cooled, the material contracted towards a series of evenly spaced centers.
Due to the overall structure of the material, this occurred from 6 different sides, creating
hexagon shaped fractures.
These hexagon fractures then propagated downwards as the rest of the material cooled, forming
beautiful rhyolite ash hexagonal columns.
More than 20,000 of these can be found around Hong Kong which measure up to 30 meters tall.
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