Hong Kong geologists unveiled the whole picture of an ancient supervolcano in the territory while carrying out daily survey work, which is the first discovery of the kind in southeastern China.

The Hong Kong Civil Engineering and Development Department made an official announcement of the discovery here Thursday.

Located in the southeastern part of Hong Kong, the ancient supervolcano, known as the High Island Supervolcano, was tilted on its side at about 30 degrees and has an original diameter of about 18 km.

It was thought to be the same type of modern-day collapse caldera that formed Taal crater in the Philippines, Tambora and Krakatau volcanoes in Indonesia, but on a much larger scale.

"The most important (thing) is that the source (of the volcano) was found," Denise Tang, geotechnical engineer of the department, said at a press briefing held on a vessel near the Ninepin Islands that the whole system of the volcano could be seen when the source and the magma conduit were confirmed belong to the same period.

When a volcano is defined as a "super" one, it erupts more than 1,000 cubic kilometers of ash, Tang explained, adding that there were about 50 supervolcanoes on record in the world.

It didn't happen overnight, Roderick Sewell, a veteran geologist of the department, said, "The last piece of the puzzle was put, I'd say, in December, 2008."

Sewell told the reporters that it took a lot of time and work to confirm the discovery, which was published in the U.S. geophysical journal Geochemistry, Geophysics, Geosystems in Januray, 2012.

He said the discovery was an exciting one and hoped it would arouse the interest of Hong Kong people, especially children, in local geological features.

The High Island supervolcano last erupted 140 million years ago, and due to erosion and weathering, most of the volcanic rocks have been removed and only a remnant remains.

Spectacular large hexagonal rock columns seen across a large area of eastern Sai Kung from High Island to Ninepin Islands within the Hong Kong Global Geopark of China which has been attracting loads of visitors were in fact products of the eruption.

The department said the discovery has opened the way for further detailed analysis of the processes taking place in the large-scale eruptions and enabled better understanding of the origin of the rock columns that formed.

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