Much of Hong Kong and its Victoria Harbour, including this rocky islet, lies atop the remnants of an ancient supervolcano with an 11-mile (18 kilometer) wide caldera, Chinese authorities revealed last week.
But even though the volcano has produced some gigantic eruptions, it poses no risk to today's millions of Hong Kong residents. It's been extinct since it last erupted and largely collapsed into the ocean 140 million years ago, Hong Kong's Geotechnical Engineering and Development Department said.
In fact, geologist Denise Tang said in a state-sponsored broadcast, the biggest risk is to tourists trying to visit the rocky islands by boat.
"There are no facilities here so we cannot recommend trying to land," she said.
(Related: "Ancient Supervolcano Rocked Washington State.")
—Richard A. Lovett
Photograph by Zhao Yusi, Xinhua/ZUMAPRESS.com
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Journalists film Hong Kong's newly identified volcanic islands, pictured, on August 30. The islands are known as the Ninepin Group or the Kwo Chau chain. The volcano as a whole has been dubbed the High Island Supervolcano.
The islands represent only part of the volcano's hardened lava, much of which is either beneath the sea or long since eroded away.
"This discovery is very important, not only because we have discovered huge volumes of materials that have been erupted, but we know the magma source," Tang told the Hong Kong news agency. That source, she added, is preserved as the granite rocks on the northern part of Hong Kong island itself.
The same cooling process that produced hexagonal lava columns (pictured) on the High Island Supervolcano can produce columns with as many as seven sides or as few as three, geologists report.
Individual columns can be as thin as a few inches across or as wide as 10 feet (3 meters) and can be more than 30 feet (9 meters) tall. (Related: "Hawaii's 'Gentle Volcano' More Dangerous Than Thought.")
What sets supervolcanoes apart is their ability to produce at least 240 cubic miles (1,000 cubic kilometers) of lava in a single blast. The Hong Kong supervolcano theoretically could have done so, though its last major eruption, 140 million years ago, appears to have produced 72 cubic miles (300 cubic kilometers) of ash and lava from an 11-mile-wide (18 kilometer-wide) caldera—enough to cover all Hong Kong.
Yellowstone, by contrast, may have produced more than 480 cubic miles (2,000 cubic kilometers) of lava in a single eruption two million years ago, making Yellowstone not only a lot larger, but also a lot more dangerous. (Related: "When Yellowstone Explodes.")
Photograph by Norbert Rosing, National Geographic Stock