The Supervolcano Beneath Hong Kong

Hong Kong stands out as a geologically stable region. Although minor tremors stemming from earthquakes in mainland China are occasionally felt, earthquakes with epicentres within Hong Kong itself are extremely rare and weak. In fact, the strongest tremors ever felt in Hong Kong date back to 1918. That earthquake, centred in Shantou—300 kilometres (185 miles) east-northeast of Hong Kong—registered a Modified Mercalli Intensity of VI to VII locally.

Yet, it may come as a surprise that the most densely populated areas of Hong Kong—Kowloon and the northern edge of Hong Kong Island—are built directly atop the granite remnants of an ancient supervolcano's magma chamber. While the actual structural collapse occurred to the east in Sai Kung, millions of years of erosion on this granite plumbing network carved out the deep valley that eventually became Victoria Harbour.

When this supervolcano erupted 140 million years ago, it ejected 1,300 cubic kilometres (312 cubic miles) of ash and lava. To put that figure into perspective, the most recent supereruption at Yellowstone “only” ejected 1,000 cubic kilometres (240 cubic miles) of material.

The volume of ash buried the entirety of present-day Hong Kong under a 1.3-kilometre-thick layer. This rapid emptying of the underground magma chamber caused the surface to cave in by an estimated 1,850 metres, largely shaping the topography of Hong Kong as we know it today.