Volcanic Hazards

The awesome and scenic Pacific Northwest Cascade volcanoes have a dark side: They can be geological hazards. While the threat of a volcanic eruption seems obvious, examining the specific hazards can help us lessen their impact. There are also non-eruptive volcanic hazards. The greatest volcanic hazards in the Pacific Northwest are pyroclastic density currents, ash fall, and lahars. Lava flows usually do not have great impact in our region because the Cascade volcanoes tend to erupt more viscous (thick, sticky) types of lava, which don't travel very far. Eruptions of basaltic lavas, which produce thin, far-traveling lava flows, can occur in the Cascades, particularly in southern Oregon, but are more common in the volcanoes of the Hawaiian Islands and at plate boundary spreading centers. 

This has not always been the case here. East of the Cascade Range, between 17 and 6 million years ago, vast amounts of basalt flowed across the land surface, leaving the many layers of Columbia River Flood Basalts.

Pyroclastic density currents are hot, fast moving "clouds" of gas, ash, and rock debris known as tephra. They can reach temperatures up to 1,000 degrees Celsius and speeds of 700 kilometers per hour and are much denser than the surrounding air. Driven by gravity, they tend to hug the ground as they flow rather than creating a plume in the air. They can have even more energy if initiated by a "lateral blast" of the volcano. Images of the snapped, stripped, and stacked logs after the 1980 Mount St. Helens eruption vividly illustrate the power of a blast-enhanced pyroclastic flow.

Pyroclastic density currents are most hazardous on, and immediately surrounding, the volcanic edifice. The best way to keep safe from these flows is to keep distant from the mountain during periods of unrest, when authorities are concerned about a possible eruption. The Cascades Volcano Observatory is the authoritative source for hazards information for the Cascades Volanoes.

Ashfall is the volcanic hazard likely to affect the greatest number of people in the Pacific Northwest during a volcanic eruption. Though falling ash is not typically life threatening, it can impact our lives in many ways.

Volcanic ash does not have the same properties as ash from a fire. Volcanic ash is composed of fine particles of glass which make it much denser and heavier than one might think. Just 20 centimeters of ash can cause structures to collapse.

The systems from which we get our water supply, particularly to the east of the Cascade Range, are open to the air in the form of rivers and reservoirs. Falling ash can contaminate and clog these systems. It can also smother crops and impact livestock.

Because of its interesting properties, volcanic ash can also cause lightning storms and fires. Small deposits on electrical transmission equipment can short circuit transformers and power lines causing power outages.

Volcanic ash also clogs internal combustion vehicle engines' air filters. Without air filters the fine glass particles could enter the engine and quickly erode and destroy the precision parts. Small amounts of ash in the atmosphere also interferes with aircraft. The susceptibility of aircraft that could fly through an ash cloud is a major driver of real-time monitoring of volcanoes even in regions where little else is at risk. The airspace potentially impacted by an ash cloud from any of the Cascade's nine dangerous volcanoes hosts thousands of guests just passing through on billions of dollars of airplanes each day. Therefore ash hazards have their own special alerting systems.

Lahars are mudflows that contain at least 60% volcanic material (ash and debris from previous eruptions included). They originate high on a volcanic edifice, have the density of wet concrete, and follow stream valleys at speeds up to 30 kilometers per hour. 

Lahars are perhaps the greatest volcanic hazard in the Southern Puget Sound region. This is because the stream channels there have headwaters at the lahar-prone Mt. Rainier. Mt. Rainier is very steep and is prone to landslides which may trigger lahars, even when no eruption is occurring. However, the largest lahars are usually associated with volcanic unrest. 

A similar hazard that may occur in our region are glacial outburst floods, also known by the Icelandic term "jökulhlaup" (pronounced "yo-kul-h-loip"). These are much more fluid and move faster than lahars and most tend to remain within or close to the stream channel. However, larger glacial outbursts are quite dangerous and can destroy bridges, campgrounds, and structures. Because outburst floods are unpredictable, you should be alert when visiting valleys with glacier-fed streams, particularly on unusually hot or rainy days. If you are near a stream and hear a roaring sound coming from upvalley or note a rapid rise in water level, move quickly up the stream embankment away from the stream channel and to higher ground. Do not try to escape by moving downstream; debris flows move faster than you can run.

The Nevado del Ruiz eruption and lahar in 1985 brought the danger from lahars world-wide attention. The lahar proceeded in three major pulses. One of the lahars virtually erased Armero; three-quarters of its 28,700 inhabitants were killed. The first lahar was 30 meters (100 ft) deep, moved at 12 meters per second (39 ft/s), and lasted ten to twenty minutes. Traveling at about 6 meters (20 ft) per second, the second lahar lasted for thirty minutes and was followed by smaller pulses. A third major pulse brought the lahar's overall duration to roughly two hours. By that point, 85 percent of Armero was enveloped in mud.

Pierce County has mapped and installed signs for volcano evacuation routes in case of a lahar from Mount Rainier. A warning system triggered by sensors on the mountain near the Carbon and Puyallup River channels will activate sirens to warn residents downstream. You can see a map of the evacuation routes at the Pierce County Emergency Management website.

This fact sheet from the US Geological Survey contains a map of past lahars and safety information about living with the threat of lahars.