Mountain in Motion

By Dr. Robert Thorson

Ignorance is bliss … until it kills you.

Lahar. What is it? What does it mean?

It meant death to the Philippine village of Guinsaugon, where 206 children were buried alive in their school last month. It meant death to residents of the Andean nation of Columbia, where 23,000 people were killed in 1985. And unless the residents of Washington state understand its meaning, it will likely mean death to some of them as well.

Lahar is an Indonesian word that describes a swift-moving debris flow involving volcanic soils. They are frequent in valleys throughout the island archipelago comprising the Philippines, Indonesia, Malaysia and New Guinea. Drenching monsoon rains alternate with summer dry seasons, causing the lava, ash and other volcanic ejecta to weather into reddish-brown, greasy, clay-rich soils.

Monsoons during La Nina years are especially dangerous because the already weak soils become even weaker when completely saturated. The result is a sodden mass of rock and mud that barely holds together.

Then the slope squashes under its own weight. A coherent mass gradually begins to break apart and creep downhill. As sliding commences, the water within pore spaces is wrung out as if from a sponge. But because it can’t seep away fast enough, the whole mass liquefies into a dense pudding that picks up rocks and debris as it flows downhill like a galloping valley glacier. The cement-like mush eventually comes to rest, leaving behind a mixture of mud, rock, forest debris, mangled houses and entombed corpses.

As is typical in the wake of any natural disaster, the role of the Philippine government at all levels has met with mixed reviews. Officials have been blamed for allowing the logging that contributed to the problem, praised for recognizing the warning signs and ordering an evacuation, and criticized for lax enforcement of the evacuation order and the resulting calamity.

But as the drenching rains continue, the government has finally gotten deadly serious. After the disappearance of one village, 11 others were re-examined in late February to assess the landslide threat. Five of the 11 villages were found to be in such critical danger that they were evacuated and are being relocated elsewhere. I just hope that residents have enough sense to follow the advice of their own government geologists and engineers. If ever there was a circumstance where government can perform a public service better than the private sector, this is one of them.

The word lahar is also widely used by earth scientists in the United States, but seldom if ever by the public. Decades of work by government scientists at the U.S. Cascade Volcanic Observatory and the Washington State Division of Geology and Earth Resources have demonstrated that killer lahars intermittently descend from Mount Rainier, the massive strato-volcano that looms above Tacoma and Seattle, and upon which I was married. Every 500 to 1,000 years, lahars move down from the mountain to inundate what are now densely populated sectors of the Puyallup Valley up to 60 miles away. Because they will almost certainly come again with deadly force, a lahar-detection system and an early warning system for emergency evacuation are being installed and tested.

The case study on which the Mount Rainier system is based comes from Armero, Columbia. This small city lies in a pleasant Andean valley, 45 miles below the summit of a Mount Rainier clone, Nevado del Ruiz. There, the combination of earth tremors and an eruption that melted glacier ice triggered a 1985 lahar that raced down the valley faster than anyone could run, killing 23,000 people. The geologists had predicted what would happen, though few understood the threat, including the mayor of the town, who died while telling residents not to worry.

I am delighted that the Philippine government is relocating villages built on landslide and lahar deposits. I’ll be equally delighted when the residents living in the shadow of Mount Rainier (and below other wet and glacier-capped volcanoes) pay attention to the evacuation orders that must inevitably come. Public safety must defer to earthly power. And there is no power on earth like that of a volcano, whether blowing up in an eruption or falling down with a lahar.