Geotech engineer realised it all pointed to a supervolcano

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.

"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 Dr Roderick Sewell, who has worked as a geologist in the Civil Engineering and Development Department for more than 22 years.

Sewell recalled suddenly comprehending: "We're seeing something much bigger than what we thought."

In the four years since his epiphany, he and his colleagues in the Geotechnical Engineering Office (GEO) have thrown themselves into long periods of survey work and careful calculations that resulted in the unveiling of the High Island Supervolcano yesterday - 140 million years after it erupted.

In the mid-1990s, GEO geologists' work on precise dating of rocks and geological mapping revealed that 80 per cent of Hong Kong's landscape could be attributed to volcanic eruptions. They put it down to a series of eruptions, however, rather than one supersized event.

As their calculations took form, Denise Tang Lai-kwan, a fellow geotechnical engineer from GEO, was shocked by the scale of what was unfolding.

"We were not prepared for such huge numbers. We went over the numbers again and again, and made sure to use the minimum numbers to avoid overestimating the scale," said Tang, who co-wrote a scientific paper with Sewell on it.

The paper was published in the journal Geochemistry, Geophysics, Geosystems after evaluations from international experts, and acknowledgement that they had proved the presence of the supervolcano.

"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."

Tang added: "It is rare in the world to see a complete volcanic system. This will be especially significant to academia."