The ticking of nature's time bomb

Growing populations in the Pacific Northwest mean that any eruptions in the Cascades could have an unprecedented devastating impact.

Mount Rainier looms over Lake Washington, near Seattle. Experts believe it poses much more danger than Mount St Helens.

Residents of the Pacific Northwest have always had to cope with challenges their beautiful but unpredictable environment throws at them, be they tornadoes, earthquakes, landslides or forest fires. Until recently, the region's many volcanoes - with a few notable exceptions - have posed less of a threat. But scientists warn that with growing populations in the states of Washington, Oregon, California and the Canadian province of British Columbia, which are home to 18 potentially active volcanoes, the consequences of an eruption are becoming ever more dire.

The last serious eruption in the area was that of Mount St Helens in Washington state on May 18 1980. The volcano erupted laterally, blasting tonnes of molten rock, gas, steam and ash through its side at 1,080kph (almost twice the speed of sound) and blanketing the surrounding area, killing 57 people, tens of thousands of animals and turning the lush national park black and barren. Mount St Helens reawakened in 2004 and had low-level eruptive activity until last year, but caused no harm to the environment or population.

What concerns geologists today is that many of the Cascade volcanoes, a chain that arcs 1,100km up western North America, have the potential for far more devastating eruptions and in more densely populated areas. The largest eruption in the Cascades is believed to have occurred 7,000 years ago at Mount Mazama in Oregon and is estimated to have been about 40 times bigger than the 1980 eruption of Mount St Helens. The question is, when will an eruption on this scale happen again?

"The point is we do not know at any given moment whether a volcano will reawaken tomorrow or 5,000 years in the future," said Cynthia Gardner, scientist-in-charge at the Cascades Volcano Observatory in Vancouver, Washington. "If you had asked any of the Cascades Volcano Observatory staff on September 22 2004 whether they thought that Mount St Helens would reawaken the next day, I think they would say that they saw no sign that that was likely."

According to the US Geological Survey (USGS), a bureau of the department of interior dedicated to studying the landscape and natural hazards, an average of two eruptions occur in the Cascades each century. In the past, most have either been too small to wreak havoc or the region's population has been out of harm's way. That is no longer the case. Steve Malone, a geophysicist at the University of Washington's department of earth and space sciences who predicted the eruption at Mount St Helens, said Mount Rainier, 70km north-east, was a serious concern.

Eruptive activity was recorded at the volcano several times throughout the 19th century and more than one million people now live in surrounding towns and cities. "Mount Rainier is considered much more dangerous" than Mount St Helens, Mr Malone said. "It is much bigger, has lots of ice and snow on it and many people live in the valleys just below it. Even a small eruption there could have devastating effects."

Mr Malone and other scientists say volcanic mudslides, known as lahars, form one of Mount Rainier's biggest threats. A mixture of mud, water and rock (often molten), lahars at Mount Rainier - at depths of up to 30m - have travelled at speeds of up to 80kph. All of which leaves residents with little time to evacuate. Several towns are built upon the solidified debris of previous lahars. Washington's Emergency Management Division (EMD) has lahar detection sensors and a map detailing slide paths to inform land-use plans and limit the number of people living in the path of volcanic mudflows.

Additionally, said Dave Nelson, an earthquake programme co-ordinator at the EMD, scientists work with public officials and the media continually to "maintain communication and readiness". But, he admitted: "As a result of the long periods of quiescence between eruptions, maintaining public readiness for the next eruption is difficult". Mr Malone was critical of official efforts to reduce the risks to local populations. "More and more people are moving directly into harm's way and the civil authorities are doing little. They have warning systems set up, but with so many people at risk it is highly unlikely that all will be saved when the mudflows come."

The Cascades volcanic arc, running from Vancouver in British Columbia down through Washington and Oregon and into northern California, was formed by the convergence of three tectonic plates with the North American plate and their subduction beneath it. As the plates collided and ground against one another, turning rock to molten lava, massive areas of land on the American west coast were wedged upwards, creating spectacular ridges and mountain ranges with about 4,000 volcanic vents, including 18 major volcanoes.

The ongoing subduction beneath the North American plate also accounts for the frequent earthquakes in the region. Of the 10 most dangerous volcanoes in the US, seven are in the Cascades (nine are in the top 12). The region accommodates a population of 10m, all of whom can conceivably be affected by volcanic activity. Native Americans who inhabited the region before the arrival of European settlers from the 16th century, saw the eruptions, lava flows and blasts of steam and ash as the antics of the gods. The august Mount Shasta, which at 4,300m has recorded awesome eruptions in the past, was believed to be a centrepoint of divine energy and creation; it now attracts thousands of pilgrims - mostly new-age spiritualists - each year.

Mount Hood in Oregon and Mount Adams about 70km north in Washington were believed to be rivalling deities that would throw fire and rock at one another, testament to the volcanoes' eruptive past. The work of gods or not, since the eruption of Mount St Helens in 1980, these volcanoes are treated with absolute seriousness. Mount Shasta, for instance, is believed to have last erupted in 1786 - spewing ash as far as 480km away - and has gone off on average every 300 years for the past 4,000, according to the USGS, making an eruption this century possible. The volume of snow and ice on the mountain would make large lahars and pyroclastic flows likely, threatening the towns and cities of the nearby Sacramento Valley.

For that reason - and 17 others - said Mr Nelson of the Washington EMD, scientists, mainly geologists and seismologists, observe the Cascade volcanoes "24/7". According to Ms Gardner of the Cascades Volcano Observatory, the level of disaster preparation varies from one volcano to another, depending on such factors as the condition of the monitoring network, how recently inter-agency plans have been prepared and practised, and the level of state funding.

Overall, however, she is cautiously optimistic about the Pacific Northwest region's ability to deal with whatever its volcanoes throw at it. "We're concerned about all of [the Cascades], from Mount Baker in northern Washington to Lassen Peak in northern California ? [But] as long as we continue to work together and remember the scale of destruction that volcanoes can pose, I think we are a long way towards being adequately prepared," she said. "So I don't want to say everything is rosy but nothing is entirely dire either."