Weird History

What Would Happen If Mount Rainier Erupted?  

Jonathan H. Kantor
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Everyone has some idea of what happens when a volcano erupts, but how much does the average person really know about what might happen were Mount Rainier to erupt? The beautiful volcano may sit in the heart of a national park, and despite being covered in lovely meadows, amazing hiking trails, and wonderful animals, it's still a volcano - and there's a chance it could blow its top just like Mount Saint Helens did in 1980.

The good news: This probably won't happen anytime soon, and even if it does, there are agencies and measures put in place to ensure the number of casualties are low to none. Even when Mt. St. Helens erupted, there was enough warning to limit the casualties to a relatively low 57, and Mt. Rainier is covered with sensors and equipment to offer up a warning long before the volcano erupts.

That being said, the devastation an eruption would cause is relatively well-known. Thanks to other eruptions like the aforementioned, geologists and volcanologists have determined the most likely route lava, lahars, and a pyroclastic flow would take. This knowledge has helped determine the best courses of action to ensure the safety of everyone in Washington should an eruption occur. 

Hot Rock And Volcanic Ash Would Hurtle Down The Mountain At 100 Miles Per Hour
Hot Rock And Volcanic Ash Woul... is listed (or ranked) 1 on the list What Would Happen If Mount Rainier Erupted?
Photo: C.G. Newhall/Wikimedia Commons/Public Domain

One of the first hazards that comes from a volcanic eruption is the presence of ash and rocky debris, which is hurled from the mouth of the volcano at incredible speed. This is coupled with a lava flow, and depending on how steep the slopes are upon which the lava flows down the mountain, it can speed up the debris to as fast as 100 mph. Carolyn Driedger, a geologist who has specialized in studying volcanoes like Rainier, has described this in frightening detail:

The lava flows encounter those very steep slopes and make avalanches of hot rocks and gas that are hurtling down the mountain maybe 100 miles per hour or so.

Fortunately, the large boulders and smaller rocks wouldn't shoot much further than the volcano's base, but their impact upon landing would be incredible.

Pyroclastic Flows Would Produce A Deluge Of Meltwater
Pyroclastic Flows Would Produc... is listed (or ranked) 2 on the list What Would Happen If Mount Rainier Erupted?
Photo:  Walter Siegmund/Wikimedia Commons/CC BY 2.5

Mount Rainier has long been called the most dangerous volcano in the United States, and it's not because of its size or potential explosive force - it's due to the large glacier sitting at its top. The glaciers sitting at the mountain's top are descendants from the Pleistocene age, and while they aren't as large as they were during the last Ice Age, they are still massive chunks of ice, which would almost immediately melt when the volcano finally erupts.

According to the United States Geological Survey (USGS), there are 25 distinct glaciers covering 92 square kilometers (35 square miles) of the volcano. When the volcano is dormant, those glaciers regularly melt to provide the headwater for five major rivers. That's a lot of potential water sitting at the top of five major waterways, many of which contain hydroelectric dams and important bridges.

Once the volcano erupts, those glaciers will melt into a massive deluge of meltwater, which will have no other path than down into those rivers. This will inundate the waterways, which would likely shut down dams, create power outages, devastate roads, and significantly impact the natural landscape for hundreds of miles.

Rushing Meltwater Could Lead To Perilous Lahars Reaching As Far As The Puget Sound
Rushing Meltwater Could Lead T... is listed (or ranked) 3 on the list What Would Happen If Mount Rainier Erupted?
Photo:  Walter Siegmund/Wikimedia Commons/CC BY 2.5

When a volcano like Rainier erupts and melts the glaciers covering it, the flowing water does more than simply go downhill. Most of that water will mix with pyroclastic material and rocky debris to create a lahar. Lahars are incredibly ruinous, and like avalanches composed of snow, they grab anything they pass and incorporate it into the flow. This leads to a great deal of devastation, and depending on the slope, they can flow tens of meters per second, which can amount to a steep 22 mph or more. According to Janine Krippner, a volcanologist at Concord University, “Lahars can lift houses. They can overtake a bridge. They can take the bridge with it.”

Because a lahar can pick up more and more material as it passes (e.g. cars, trees, houses, etc.), they tend to pick up speed and move further than a typical lava flow. Lahars leave a great deal of material as they pass, which makes it easy to determine where a modern lahar might flow. The National Lahar was formed by an eruption some 2,200 years ago, and the deposits suggest the path a new one might take. 

Carolyn Driedger described the National Lahar by pointing out various aspects of what it left behind, “We’re seeing this wall of rocks. Big boulders the size of basketballs, and it looks like something that a human being has constructed.” As a lahar plummets down a slope, giant boulders are picked up and smashed against one another to create smaller, faster debris. Because a lahar will likely flow along the same path into the rivers, it could extend as far as the Puget Sound and directly impact the city of Seattle, WA, via post-lahar sedimentation.

Airborne Volcanic Ash Would Disrupt Aviation
Airborne Volcanic Ash Would Di... is listed (or ranked) 4 on the list What Would Happen If Mount Rainier Erupted?
Photo: USGS/Wikimedia Commons/Public Domain

When a volcano erupts, it typically spews a cloud of toxic ash into the air. Depending on the size and scale of the eruption, this could entail a plume of material, which flows up into the air as high as the upper atmosphere. This can lead to several problems, including a disruption to electronic communication systems, the blanketing of that ash on a large area when it finally drops to the ground, and a large suspension of air traffic.

Aviation shuts down in the immediate area due to a number of factors, including a reduction in visibility, the disruption of communication, and the threat of ash flowing into a jet intake, which would almost certainly lead to a crash.

Volcanic ash is composed of tiny fragments of pulverized rock, volcanic glass, and minerals. It is an incredible risk to flight safety, especially when those flights are conducted at night. When a plane comes into contact with ash, the cinders can scratch up a cockpit, melt inside an engine's combustion chamber, clog fuel nozzles, and lead to a total engine failure. Because this is a real concern to flight safety, aviation is rerouted or canceled in any area considered to be under threat from volcanic ash, and since that could be hundreds of radial miles, the impact can be massive.