Lahars: Past and Future Lahars on Mount Rainier

One of the biggest dangers relating to Mount Rainier and it's explosive nature are lahars racing down the volcano at highway speeds carrying with it a large amount of debris. Past lahars on Mount Rainier are evidence that future lahars will occur and if they are not monitored correctly, they will cause a high number of fatalities along with a massive amount of damage. Mitigation is important in ensuring the safety of the populations living on and near the volcano.
Figure 1: Mount Rainer and Tacoma
Figure 1: Mount Rainer and Tacoma

Mount Rainier: General Information - Mount Rainier is a stratovolcano located in Washington State that stands at 14,410 feet above sea level. The volcano is close to the heavily populated cities of Seattle and Tacoma (as shown in the figure 1), which could potentially pose a serious threat to the people living in those and other cities near the volcano because of the hazards associated with volcanism, especially lahars. There are five major rivers associated with Mount Rainier; they are the Carbon, White, Cowlitz, Nisqually, and Puyallup rivers. These rivers could assist future lahars, which would also be a serious problem because it gives the lahars more water and debris to carry down and along the volcano, flooding the cities below.
- Mount Rainier is one of the volcanoes in the Cascade Volcanic Arc and is near the border of the subduction zone of the Juan de Fuca plate system which is subducting under the North American plate. (Geller, Cascadia Subduction Zone)
- The volcano is made up of andestic rock and is the largest of the Cascade Mountains. It also “contains the largest system of alpine glaciers in the Cascade Range”. (Camp, How Volcanoes Work) Mount Rainier is voluminous and the most dangerous volcano in the range, which could be because its “load of glacier ice exceeds that of any other mountain in the conterminous United States.” (USGS, Mount Rainer Volcano)
- Because Mount Rainier is a stratovolcano, it does not erupt frequently, but when it does, the eruptions will be dreadfully explosive and destructive. These eruptions could lead to catastrophic lahars, killing residents and destroying nearby cities.
Lahars: General Information
- Lahars are essentially volcanic mudflows made up of a mixture of hot ash and some sort of water, such as melted ice or snow that form on the slopes of a volcano. Lahars are usually associated with stratovolcanoes because of their violent eruptions and their tall snow-covered peaks, which are “
constructed of weakly consolidated rock debris that is easily eroded”, especially when the volcanoe erupts. (USGS, Lahars and their Effects) They can be of any size, from a few centimeters wide and deep to hundreds of meters wide by tens of meters deep, as well as being a variety of temperatures, depending on how the flow began. (Camp, How Volcanoes Work) Lahars travel at rapid speeds and are usually made up of very dense material, making it seems as if it was wet concrete. When traveling down the volcano, lahars often pick up and carry debris down the slope, which could pose a serious hazard to the people and objects below and near the bottom of the volcano. They also can occur with little or no warning. (Camp, How Volcanoes Work)
- Lahars start mainly by intense rainfall during or after a volcanic eruption or by massive landslides. Volcanic rock, vegetation, and other debris are eroded away due to the rainwater (USGS, Lahars and their Effects). These lahars are the ones that cause the most damage and take the most lives. Lahars can also start by pyroclastic flows, which melt large amounts of snow and ice in a short amount of time. (Camp, How Volcanoes Work) Lava flows can also start a lahar in a similar way as the pyroclastic flow. Lahars: - The consequences of lahars can vary. Smaller lahars do not do a lot of damage, mainly because it cannot pick up enough debris, but the larger lahars are extremely destructive and cause more deaths than any other event associated with volcanoes.

Mount Rainer: Eruptive History
- Periodic melting of glacier ice has generated at least fifty major lahars over the past 10,000 years. (Camp, How Volcanoes Work) A few of the major lahars that occurred on Mount Rainier were the Osceola Lahar, the Paradise Lahar, the Round Pass Lahar, and more recently the Electron Lahar.
- The largest lahar was called the Osceola Lahar that occurred 5,700 years ago. It is thought to be triggered by a volcanic explosion, which resulted in a debris avalanche. The Osceola Lahar removed the top 2,000 feet, leaving behind a summit crater on top of Mount Rainier (USGS, Osceola Mudflow) This large mudflow traveled over 112 kilometers down the White River and spread out the mouth, which covered 300 square kilometers along the shoreline of Puget Sound. (Camp, How Volcanoes Work)
- Another major lahar that occurred on Mount Rainier was the Paradise Lahar, which occurred around the same time as the Osceola Lahar. This lahar temporarily filled the Nisqually River and covered Paradise Park and Paradise Valley on the south side of the volcano. It is thought that an avalanche a little bit smaller than the Osceola Lahar caused the Paradise Lahar. (USGS, Paradise Lahar)
- The Round Pass Lahar occurred around 2,600 years ago and was the result of an eruption on the far east flank of the volcano near the summit. This caused a huge avalanche to ensue, temporarily filling the Puyallup River (USGS, Round Pass Mudflow)
- A more recent lahar, the Electron Lahar, happened about 600 years ago and traveled down the Puyallup River, flowing close to the heavily populated city, Tacoma. (Camp, How Volcanoes Work) This lahar was the result of a slope failure on the west flank of the volcano and was not associated with an eruption. (USGS, Electron Mudflow)

Mitigation for Future Lahars on Mount Rainer
- The past lahars on Mount Rainier lead to a prediction of a catastrophic lahar occurring on the volcano, which would inevitably cause a disaster to ensue. The steepness and amount of ice and snow that Mount Rainier holds would allow a lahar to destroy the cities surrounding the volcano, bringing with it trees, rocks, and other debris, which could cause a large amount of damage and fatalities if there is little or no warning.
- Fortunately, Mount Rainier is monitored very closely with the hope to warn the local populations before the next lahar strikes because it has been predicted that the volcano could be the site of the next Cascade eruption. (Camp, How Volcanoes Work) The USGS scientists came up with a lahar detection system that would enable them to know when a lahar is heading down the volcano without the need of video cameras or observers. The automated system relies on acoustic-flow monitor stations, which detect ground vibrations of a lahar through a seismometer. The monitor then sends the signals to a microprocessor, which in turn, sends the signal to a radio station that communicates with a base station. The base station gets alert information every minute as long as the vibrations are above a specific level. The acoustic-flow monitor stations are located downstream of a volcano. (USGS, Lahars and their Effects)
- Along with the automated detection system, there are other mitigation techniques that would help the people living on or near Mount Rainier. One of the most important techniques is educating the public about the hazards of volcanoes and volcanic eruptions with a Volcanic Hazards Program. This program encourages people to participate in volcano-emergency planning, prepares educational material, meets with residents to inform them of volcanic hazards in their area, meets with media to get the information out to a large amount of people, and leads educational field trips to active volcanic areas. (USGS, Hazard Preparedness)

With the evidence of past lahars, we know that future lahars will surely happen. With the automated detection system, the Volcanic Hazards Program and other techniques, we can ensure that when the next major lahar strikes on Mount Rainier, the informed population with be ready with an evacuation plan to get as many people out of the hazard areas as possible.

Work Bibliography
Camp, Dr. Vic. "How Volcanoes Work: Lahars" How Volcanoes Work. 2006. San Diego State University. 31 Oct 2008

Geller, Doug. "Cascadia Subduction Zone Volcanism in British Columbia". Emporia. 2008. Emporia State University. 1 Dec 2008

USGS. "Description: Mount Rainer" Cascades Volcano Observatory, Vancouver, Washington. 2007. U.S. Geological Survey. 30 Oct 2008

USGS. "Description: Electron Mudflow, Mount Rainier, Washington" Cascades Volcano Observatory, Vancouver, Washington. 2001. U.S. Geological Survey. 28 Nov 2008

USGS. "Description: Osceola Mudflow, Mount Rainier, Washington" Cascades Volcano Observatory, Vancouver, Washington. 2001. U.S. Geological Survey. 29 Nov 2008

USGS. "Description: Paradise Lahar, Mount Rainier, Washington" Cascades Volcano Observatory, Vancouver, Washington. 2001. U.S. Geological Survey. 29 Nov 2008

USGS. "Description: Round Pass Mudflow, Mount Rainier, Washington" Cascades Volcano Observatory, Vancouver, Washington. 2001. U.S. Geological Survey 30 Nov 2008

USGS. "Hazard Preparedness: Preparing for Volcanic Emergencies" Volcano Hazards Program. 2008. U.S. Geological Survey 1 Dec 2008

USGS. "Lahars and Their Effects" Volcano Hazard Program. 2008. U.S. Geological Survey. 1 Nov 2008

Picture Bibliography
USGS. "Description: Mount Rainer" Cascades Volcano Observatory, Vancouver, Washington. 2007. U.S. Geological Survey. 5 Nov 2008