MOUNT ST. HELENS, Wash. – The boiling magma rumbling and rising within Mount St. Helens (search) isn't the only thing scientists fear. When large volcanoes erupt they can unleash an awesome arsenal of natural weapons, devastating communities and landscapes even hundreds of miles from the blast.
First, there's the gritty, glassy ash that travels for miles. That's what scientists consider to be the main hazard from Mount St. Helens during the current volcano alert.
But bigger volcano (search) blasts can produce more frightening scenarios: high-speed mudslides caused by a rush of water carrying house-sized boulders and intensely hot clouds of rock fragments and lava.
You think the hurricanes in Florida have been vicious? Don't try boarding up your windows and riding out a pyroclastic flow, a lahar or a tephra blizzard -- the technical names for all those hazards, which are among Earth's most powerful forces.
In the mountain's historic 1980 eruption, they combined to turn a swath of the Pacific Northwest into a moonscape, killing 57 people.
Scientists believe Mount St. Helens is ready to erupt again, although perhaps not with the kind of power seen when the mountain literally blew its top.
"There is a wide range of explosivity," said geologist Willie Scott of the U.S. Geological Survey (search). "It's that uncertainty that makes us cautious."
But it's more than molten rock that makes a volcano dangerous. It's the dissolved carbon dioxide and other gases in the magma. Fresh, foaming magma contains more gas than old magma. Even the water dissolved in magma expands violently when it reaches the surface and hits the air.
Scott compared it to shaking a can of soda pop.
"If the gas content is high, the explosivity is greater," he said.
Sometimes gases vent from the magma even without an eruption. Leaking carbon dioxide can silently kill trees, or people who venture too close. Sulfur dioxide creates smoggy air pollution that contributes to respiratory disease and generates acid rain that kills forests and aquatic life.
The most likely and widespread danger from any volcano is ash. Initially, ash is blasted 60,000 feet into the atmosphere. Then winds carry it for dozens or even hundreds of miles.
Volcanic ash is not the product of combustion like the fluffy ash from a wood stove or charcoal barbecue. Gritty and abrasive, it is made up of tiny fragments of rock, natural glass and minerals that get pulverized by earthquakes and internal explosions.
In its 1980 eruption, Mount St. Helens belched some 500 million tons of ash over surrounding states, where it fell like a gray, minerally snowfall. It caused most of the $1 billion in property damage attributed to the eruption.
The plume can choke the engines of passing aircraft. Ash can clog and wreck machinery, electronics, cars, air conditioners, furnaces and irrigation systems. Dry ash can collapse roofs and scratch windows. Large clouds disrupt telecommunications signals.
Ash poses real health risks, especially to children, the elderly, people with chronic respiratory illnesses, as well as wildlife and pets. It settles on the leaves of plants, preventing photosynthesis.
In this current explosive phase, researchers expect Mount St. Helens to generate less ash than in 1980. So far, it has sprinkled ash on nearby Vancouver, Wash., and other downwind communities.
"We could see an explosion that throws up a column for an hour or so," Scott said.
Other volcanic forces are deadlier still.
Tephra is a catchall term for fragments of volcanic rock and lava thrown airborne. Some tephra is nearly four feet wide, but most of it is gravel that behaves like shrapnel and shreds whatever is in its path.
A lahar is a catastrophic slurry of water and rock fragments that rushes down the volcano's tall, steep slopes. It looks like wet concrete and can carry house-sized boulders, trees, even bridges. It follows river valleys, often growing and gaining speed as it consumes the water in the channel.
In the 1980 eruption, lahars swept downhill on three sides of the mountain at 70 mph. They left mudflows up to 30 feet deep extending for dozens of miles.
A volcano's other doomsday weapon is a pyroclastic flow. At the bottom of the flow is a layer of coarse rock fragments that burst forth like a shotgun blast. It hugs the ground, splintering entire forests like toothpicks and exploding buildings. The top layer is a turbulent ash cloud.
A pyroclastic cloud is fast -- 50-100 mph. What's worse, it's hot -- up to 1,500 degrees. It destroys everything in its path like a boiling hurricane.
In 1980, Mount St. Helens directed a blast of hot material that reached 300 mph. A "seared zone" of timber extended for 17 miles.
This time around, scientists don't expect the mountain to generate such a show of force.
"We're not anticipating hot flow to any great distance," Scott said.