Lessons learned from the eruption

Lessons learned from the eruption

Monday, May 8, 2000

Brace yourself for the next one:

The mountain's last dome-building eruption was in October 1986, so scientists think there has been enough time for the magma under it to crystallize and form a massive rock plug blocking the conduit below.

That means massive pressure will build in the conduit until the plug blows out with explosive force expected to be as great as, if not greater than, the 1980 eruption.

It's everyone's problem:

An eruption can cause social and economic disruption hundreds of miles from the mountain. Before St. Helens, few people considered the impact of ash and other abrasive particles that blanketed hundreds of square miles east of the mountain so deep it had to be removed with snowplows.

Volcanic ash isn't fun:

It looked like fluffy snow, but it caused eye and respiratory irritation, short-circuited some electric-power transmission lines, damaged machinery, endangered jet aircraft and threatened to overwhelm sewer and water systems.

Keep your shovel handy; don't pray for rain:

The only thing worse than dry ash is wet ash.

When it gets wet, ash becomes two to three times as heavy and it becomes sticky, adhering better to sloped surfaces. Four inches of wet ash is enough to collapse some roofs.

Don't bother running:

Pyroclastic flows of 1,470-degree rock fragments and gases descend a volcano's flanks at speeds of up to 200 mph. They incinerate, asphyxiate, bury or blow apart whatever they hit. Evacuation plans are pointless. It's best just to stay way, way back.

Define way, way back:

Geologists say St. Helens once produced pyroclastic flows that traveled 12 miles from the mountain. Because no two eruptions are the same, no one can say how far the eruption will reach or where it will go.

Not all volcanoes ooze fiery lava:

Stratovolcanoes like Mount St. Helens are symmetrical, steep-sided peaks that erupt in explosive bursts, rather than seep lava like the shield volcanoes that formed Hawaii.

The difference is in the silica content of the molten rock fueling the volcanoes.

Magma in stratovolcanoes is viscous due to its high silica content. Gases build in the thick molten rock, escaping with a blast. In contrast, magma in shield volcanoes has less silica, allowing the easy escape of gas and quieter eruptions.

Lava is flashy, but ash is also a hazard:

The tiny, jagged airborne particles of glass and rock are dangerous to breathe, can cause white-out conditions and make roadways slippery.

Ash causes engines to stall by clogging air filters and damaging moving parts. In airplanes, ash sucked into an engine will melt and make a glassy coating on turbines, causing them to quit.

Wet ash also conducts electricity, causing short circuits and power outages following eruptions. Clouds of ash can disrupt radio communication by scattering or absorbing signals.

And its weight alone can cause damage. Four inches thick, dry ash weighs 120 to 200 pounds per square-yard -- and twice that when wet.

Hummocks happen:

For centuries scientists were mystified by giant, goose-bump-like hills that pepper the base of volcanoes worldwide. They guessed the hummocks were formed by lava breaking through the Earth’s crust, deposits left by glaciers, mudflows or even man-made.

From the pimply hummocks -- some 230 feet tall -- left in the valley below St. Helens, geologists learned that the hills are intact chunks of the volcano, carried away in avalanches, joined with debris from lahars and pyroclastic flows.

Formerly majestic St. Helens had pulled back the curtain and revealed that volcanic peaks are just big piles of crummy, rotten rock.

You can see it coming, but not know when:

Precursors to volcanic activity may continue for weeks, months or even years before an eruption. Or they can simply subside without warning and not be followed by an eruption.

Volcanologists can use various instruments to measure earthquake activity, minute changes in the ground surface and emissions of heat or gas.

But current technology can only provide a general warning that volcanic activity in a specific area is becoming more likely.

Precision is a relative term:

Scientists who studied Mount St. Helens during the 1970s recognized that it had been the most active and explosive volcano in the Cascade Range. On that basis, they forecast that St. Helens might be the next Cascade peak to erupt, possibly before the year 2000.

Time is not on our side:

The 1980 blast left a giant crater that is gradually filling with snow and ice that is kept from melting by the shade of the steep crater walls.

Scientists say it will double in depth by 2010, and the accumulated snow and ice is mixing with rock, dirt and debris from the crater walls.

That's a recipe for a massive lahar -- the churning flow of mud and debris that is a greater threat to life and property than any other volcanic phenomenon.

Did you feel that?

Earthquakes large enough to collapse buildings and roads usually do not accompany volcanic eruptions. In fact, quakes associated with eruptions rarely exceed magnitude 5 -- moderate temblors. The largest earthquakes at St. Helens in 1980 were magnitude 5.1, large enough to sway trees and damage but not destroy buildings.

And sometimes an earthquake is just an earthquake. Four magnitude 6 earthquakes struck Long Valley caldera in California in 1980, but no eruption has yet occurred there.

Ash never really goes away:

St. Helens ash covered 22,000 square miles east of the volcano. Some people used it as pottery glaze; others tried it as a soil amendment or hand cleaner.

Most just wanted to get rid of it.

Disposal was complicated by a lack of places where wind wouldn't whip it up again.

About 250,000 cubic yards of ash were stockpiled, covered with topsoil and seeded with grass, so that it could be retrieved for use should anyone come up with an idea for making it pay.

As far as anyone knows, it's still there.

Bigger isn't always deadlier:

Washington's favorite volcano erupted with the force of 24 megatons of thermal energy, equal to 500 Hiroshima atomic bombs.

But it was relatively puny in volcanic terms, barely a 5 on the Volcanic Explosivity Index, which measures force in various ways.

Yet St. Helens killed 57 people while there were 25,000 casualties from the Index 3 eruption of Colombia's Nevado del Ruiz in 1985 and 30,000 casualties in the Index 4 eruption of Mont Pelee in Martinique in 1902.

The difference: Sparse population around St. Helens and plenty of early warning.