Volcanoes: our unpredictable, often dangerous, reminder of a planet still in formation

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.