Volcanoes: our unpredictable, often dangerous, reminder of a planet still in formation
These dramatic formations are the most vivid reminder that our planet is a work in progress
Roaming across a satellite crater of Tangkuban Perahu - the "Upside Down Boat" volcano in west Java, Indonesia - gives some inkling of the awesome power in the earth beneath. It's a primeval place, an expanse of yellowish and sullen red rock forming a gently sloping depression amid jungle. Steam rises from a perpetually boiling spring, hot sulphurous gases emerge from vents.
The larger, main crater is above. Here, a column of steamy gases surges skywards, thrust from an opening at such pressure that, even from some 200 metres above, it sounds like a powerful waterfall. A road leads to the rim, where a path passes simple cafes and souvenir stalls. Hundreds of visitors arrive during the morning, as if this is as safe as a regular tourist spot.
Yet the volcano is dangerous, subject to phreatic eruptions in which water superheats to become steam that blasts rock apart. Just weeks after my visit, a phreatic eruption created a plume of ash that covered the car park, and closed the volcano to visitors. In May, a more violent phreatic eruption killed five climbers on the Mayon volcano, in the Philippines.
Phreatic eruptions can be hard to predict, but increased ash coupled to seismic activity before the latest one at Tangkuban Perahu did lead to a higher alert level. Monitoring equipment here also checks gas compositions, as scientists have found these can change before an eruption. Other warning signs can include the ground being pushed upwards by magma rising from beneath.
This month, there was a report that Alaska's Mt Redoubt volcano emitted a "seismic scream" just before an eruption, as a flurry of tiny earthquakes built to a crescendo of about 30 per second. These "screams" - perhaps audible to people as humming sounds - could be another signal to check for. But, with just 30 seconds of silence between scream and eruption, they may only provide insight about what's happening deep below ground, rather than enable warnings.
In October 2010, the Indonesian government issued its highest alert level for another Javan volcano, Gunung Merapi, or Fire Mountain. There were frequent earthquakes, and a lava flow, and evacuation orders were issued for 19,000 residents on Merapi's fertile slopes. Mbah Maridjan, the volcano's spiritual guardian, was among the few who refused to leave. He died on October 26, early in a series of eruptions that lasted till the end of November. These resulted in lava flows, pyroclastic flows - clouds of incandescent ash and rocks racing down the slopes - ash columns rising 6 kilometres, and even a fireball that soared 2 kilometres into the sky. Despite the evacuation order, more than 350 people were killed.
I visited Merapi in February this year, and took a jeep ride across a landscape produced by those eruptions. There were ruins of farmhouses, and nearby the driver told us that for six months, the ground was too hot to walk on. Few plants grew among the rubble.
Bamboo stalls had been erected, as simple stores and cafes for visitors. At one, a lady showed a photo of her former home - a bucolic-looking farmhouse set in lush greenery. "Now, my home is seven metres under this place," she said.
During our visit, torrential rain began to fall, turning streams into brown torrents and creating another danger on volcanoes: lahar, or mud flows resulting from fresh ash being easily eroded. As our jeep crossed a narrow bridge, the raging stream ripped a spur of land away. Later, a news report told of a man who had been collecting ash in a streambed who failed to get to safety in time. He was swept to his death in waters so powerful they also carried his truck downstream.
Indonesia has at least 76 active volcanoes, more than any other country. This results from it spanning junctions between four tectonic plates - immense sheets in the earth's crust. One of these, the Indo-Australian Plate, dives beneath the Eurasian Plate. As the rock is pushed deeper, it melts to become magma, which rises through fissures to form volcanoes running east from Sumatra, through Java to Bali and beyond.
The area that is now Hong Kong lay along a plate boundary during the late Jurassic and early Cretaceous, leading to several episodes of volcanic activity.
These culminated in the collapse of the High Island Supervolcano, releasing 1,300 cubic kilometres of ash - roughly 130 times the volume of material erupted by the Philippines' Mount Pinatubo in 1991. Much of the hot ash welded together to form tuff, with impressive columnar joints you can see today at places like the southeast Sai Kung Peninsula, and islands including the Ninepins.
"Supervolcano" is not a rigorous geological term, but is applied to volcanoes capable of massive eruptions. About 50 supereruptions have been identified, with High Island among 11 that ejected more than 1,000 cubic kilometres of material. More recently examples include Mount Toba in Indonesia, which erupted about 70,000 years ago, leaving a layer of ash across South Asia. There is a theory that this killed so many people that it caused a bottleneck in human evolution, though several researchers dispute this.
Yellowstone in the US is surely the world's best-known supervolcano, thanks partly to science fiction tales envisaging a devastating modern eruption, and media articles such as a CNN article describing it as a "geological time bomb". The latter suggested that "after the initial eruption, clouds of gas and rock would burn everything in its [sic] path," and that "ashfall would cover the western US and enter the jet stream with the potential to cripple air transportation and threaten the world's food supply".
But don't get too alarmed: as 2012 approached - and you may remember there were notions this was the last year of the Mayan calendar, so a cataclysm would impact the earth - Nasa pointed out that supervolcanoes erupted only about 1.4 times every million years.
Martin Williams is a Hong Kong-based writer specialising in conservation and the environment, with a PhD in physical chemistry from Cambridge University.