People keep moving into volcano areas, so the risk to people keeps going up

Marianne Guffanti

In 1984, lava from Hawaii's Mauna Loa, the world's biggest volcano, came within a mile of the town of Hilo.

Hilo was built on old lava flows, according to vulcanologist Dr Dave Rothery. And sooner or later, Mauna Loa will erupt and lava will flow again.

"It's inevitable," said Dr Rothery, of the Open University's Volcano Dynamics Group in the UK. "Eventually it will happen."

On the west side of the island of Hawaii is Hualalai. The volcano's flanks are covered with resorts, homes and businesses.

Cutting roads

The United States Geological Survey (USGS) describes it as "a potentially dangerous volcano that is likely to erupt again in the next 100 years".

When it last erupted in 1801, the ensuing lava flow reached the west coast in about 24 hours.

The Keahole Airport was built atop the flow and according to Dr Rothery it is in the volcanic firing line.

"There will be a flow going through [that area] and it could go very fast," Dr Rothery said. "It could cut the belt road round the island in a few hours with very little warning."

Mount Nyiragongo in full flow

A third Hawaii island volcano, Kilauea, is widely considered to be the world's most active. It has been erupting continuously since 1983.

Kilauea has already produced lava that cut a through the island's perimeter road affecting hundreds of homes.

In Europe, the greatest threat is posed by Mount Etna.

Villages are scattered across the volcano's flanks, and the town of Catania is only 26km (16 miles) away.

Last July, Etna's lava destroyed a ski lift, car park and cable car station.

Slow moving

Like Goma's Mount Nyiragongo, these volcanoes are known for producing lava alone, without the threat of deadlier, swift-moving pyroclastic flows.

Such eruptions rarely lead to deaths. Lava moves slowly, giving residents enough time to escape on foot.

But Dr Rothery says that the destruction the flows cause to roads and property "raises questions about the wisdom of building on these places".

Marianne Guffanti, a volcanologist with the United States Geological Survey, agrees.

"People keep moving into volcano areas," she said. "The risk to people keeps going up."

Etna flows destroyed tourist buildings in 2001

On Mauna Loa, for example, homes have been erected near the vent area of past eruptions.

"People are very close to it and they won't have a lot of time to get out of the way," she said.

Given such obvious risks, why do people continue to build in the shadow of volcanoes?

Scenic spots

In some countries, there is little choice. Japan, for example, has only a small percentage of land that is inactive. The Philippines and Indonesia are similarly volcanic.

But there are also economic motives for building on and around volcanoes. Eruptions leave behind fertile soils - an irresistible lure to farmers from poor, agriculture-based societies.

In wealthy nations like the United States, the attraction is more recreational than vocational. Volcano slopes offer residents scenic spots for camping and skiing.

Ms Guffanti says people who live near Etna and the Hawaiian volcanoes are well aware of the risks. They have simply decided that the benefits outweigh them.