How was Hong Kong made? By a really big volcanic eruption

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South China Morning Post

Geologists say the origins of the city can be traced back to the lava and ash left behind after a supervolcano erupted 140 years ago in the area

South China Morning Post |
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Geologists say Hong Kong was formed after a supervolcano in the area erupted 140 million years ago.

It may be shocking to some, but geologists have confirmed that Hong Kong exists today thanks to an extraordinary volcanic eruption that happened 140 million years ago.
It was also one of only about 50 supervolcanoes to have erupted since man walked the planet, none of which have occurred in recorded history. Its existence was only unveiled in 2012 when geologists realised what they were looking at during the course of their daily survey work.

“It is correct to say that the main city sits on a very old volcano. Though most of the remnants are submerged, the granite in Kowloon and the rock pillars in Ninepin Islands can be traced back to this one source,” said Denise Tang Lai-kwan, Civil Engineering and Development Department geotechnical engineer.

“Though most of the remnants are submerged, the granite in Kowloon and the rock pillars in Ninepin Islands can be traced back to this one source.”

Her colleague, Dr Roderick Sewell and the man behind the discovery of Hong Kong’s supervolcano remembers the moment he realised he was on to something big - the explanation for the city’s unique landscape.

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“The ‘Ah-ha!’ moment for me was realising that everything pointed to one source – a supervolcano, the one system that could have preserved all the unique geological features of Hong Kong,” said Sewell, who has worked as a geologist in the Civil Engineering and Development Department for more than 25 years.

Sewell recalled suddenly comprehending: “We’re seeing something much bigger than what we thought.” He added: “There are only around 50 supervolcanoes in the history of mankind. We suspect more will be added to the list following our discovery.”

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Geologists have mapped out its entire anatomy running from eastern Sai Kung to Kowloon and Hong Kong Island itself, formed by the lava and ash left behind. It has been named the High Island Supervolcano after the Sai Kung island, which marks the volcano’s top edge.

The 18-kilometre-wide caldera (a kind of crater) of the supervolcano was formed as high pressure and heat at the earth’s core caused the surface to collapse.

The deepest part of the supervolcano is in the middle of the city, in Kowloon and Hong Kong Island, where leftover pockets of magma are marked out in the form of granite.
“From the granite in Kowloon, to the large hexagonal columns in the Ninepin Group [of islands], we can finally explain the link and source of these landscape features,” Tang said.

The hexagonal volcanic pillars covering a large area of eastern Sai Kung were formed as the thick layer of volcanic ash cooled and contracted. The pattern is nature’s most efficient arrangement to release the tension.

But what is unique about the rock pillars is that they tilted to the east by 30 degrees. It suggested that the whole system had tilted due to tectonic forces.

“The tilt of the system made a lot of difference,” Sewell said. “Erosion and weathering presents a cross-section of the caldera that gives geologists a window into the anatomy of a volcano in exquisite detail.”

Edited by Doris Wai

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