Lesson Plan

Lahar in a Jar!

Rocks, mud, and water mixed together to form a mini lahar slide down a plastic track into a pan.

Create your own lahar by mixing rocks, mud, and water together.

Overall Rating

Add your review
Grade Level:
Sixth Grade-Tenth Grade
Earth Science, Environment, Geography, Geology, Hydrology, Landscapes, Volcanoes
50 minutes
Group Size:
Up to 36
beaker, debris flow, flank collapse, glacier outburst flood, graduated cylinder, hydrothermal alteration, lahar, landslide, lava flow, pyroclastic flow, Mount Rainier National Park, mount rainier, Cascade Volcano Observatory


Explore how small amounts of water can mobilize loose rock to form lahars by making a small lahar within the safety of a beaker or jar and analyzing it using scientific methods. This lesson plan is part of the "Living with a Volcano in Your Backyard" curriculum, created through a partnership between Mount Rainier National Park and the US Geological Survey Cascades Volcano Observatory.


Students will:

  • Recognize lahars as the principal volcano hazard at Mount Rainier.
  • Become familiar with some of the more significant lahars that originated on Mount Rainier.
  • Recognize the role of lava flows, pyroclastic flows, landslides, and glaciers that initiate debris flows and lahars.
  • Recognize that an abundance of surface water and loose, weakened rock makes Mount Rainier highly susceptible to lahars and debris flows.
  • Observe how only a small amount of water is required to initiate a debris flow or lahar.
  • Become familiar with the nature of lahars and debris flows, and the proper usage of the terms.


Lahars are fast flowing torrents of rock, mud, and water
Lahars, also known as volcanic mudflows or debris flows, are worthy of attention because they are the principal volcanic hazard in the valleys that head on Mount Rainier. The word lahar is an Indonesian term that refers to any rapidly flowing and gravity-driven mixture of rock, mud, and water that rushes down the slopes of a volcano. Lahars have been known to travel distances of more than one hundred kilometers (60 miles) at speeds of 60 kilometers per hour (40 miles per hour).

While many scientists treat the terms lahar and debris flows synonymously, scientists and officials working at Mount Rainier seek to reduce confusion locally by modifying work usage. They reserve the word lahar for large flows of eruption or landslide origin with potential to travel to densely populated valleys, and use debris flow for much smaller events caused by glacier floods and precipitation, which stay generally within park boundaries.

Once witnessed, lahars and debris flows are seldom forgotten
The ground shakes and rumbles in a way similar to that of an approaching train. Dust plumes rise into the air above the flow front and small pebbles splash skyward. The flow, tan or gray in color, looks and behaves like a river of flowing concrete. Boulders crush and grind vegetation, which releases a strong stench of organic oils that hangs in the air long after the event is over. Where valley walls widen, lahars spread, drain, and cease motion. Boulders and trees that had been buoyed and pushed to flow margins come to rest as blocky ridges along the flow's margin.

The speed of a lahar and debris flow depends upon its volume and the slope gradient. Some of the faster flows have been clocked at speeds of 30 to 60 kilometers per hour (20 to 40 miles per hour). Lahars may last for hours or days; debris flows generally last for half an hour to several hours. Both leave behind an inhospitable surface of tightly-packed mud, boulders, and vegetative debris.

Abundant water and rock debris make Cascade volcanoes highly susceptible to lahars and debris flows
Eruptions have built vast volcanic slopes at high elevation that are scattered with lava fragments and that retain snow and glacier ice. Mount Rainier's slopes are covered by approximately 4.4 cubic kilometers (one cubic mile) of snow and ice, an amount equivalent to that on all the other Cascade volcanoes combined!

Most lahars form during volcanic eruptions, but landslides can also produce lahars
Almost all lahars happen during volcanic eruptions when hot pyroclastic flows and lava flows interact with snow and ice. This scenario repeated at Mount Rainier many times has resulted in thick sequences of lahar layers beneath the floors of some valleys.

Not to be discounted are large landslides, known as flank collapses, that can also produce lahars. The largest landslide-induced lahars have occurred during eruptive periods and involved rock that had been weakened by long-term exposure to hot acidic groundwater, a process called hydrothermal alteration. 

What triggers a flank collapse? Accepted mechanisms include instability at the onset of or during volcanic eruptions, large earthquakes, and intense ground deformation by rising magma and perhaps long-term exposure to gravity. While the chance of a flank collapse is greatest during eruptive periods, the possibility exists of failure during non-eruptive times.
Rocks at the head of the Puyallup River valley are more prone to landslides than rocks elsewhere on Mount Rainier, because they contain hundreds of millions of cubic meters (cubic yards) of hydrothermally altered and weakened rocks. At least seven landslide initiated lahars have covered valley floors in the southern Puget Sound area over the past six thousand years.

Small events caused by rainfall and glacier floods
Conditions that favor debris flow formation are glacier outburst floods in mid-summer and intense rainfall in late fall. These events are small when compared to lahars produced during eruptions, having a thickness of only tens of meters (feet) and traveling only a few kilometers (miles) from their source. Lahars can reach a thickness of 100 meters (300 feet) or more and travel far from the source. Debris flows happen once or twice a year at some Cascade volcanoes, whereas lahars happen much less frequently.

What to do if in danger from a debris flow or lahar
Most large-volume lahars are associated with volcanic unrest and eruptions. Usually earthquakes or other precursory activity at a volcano serve as a warning than an eruption is imminent. While debris flows happen frequently and large volume lahars happen infrequently, the necessary response is the same. Get to high ground off the valley floor.


Student pages are used to make observations and answer the questions provided by the teacher pages. Students will use the graphics pages to observe the area affected by lahars and debris flows, like those which have occurred on Mount Rainier.



Use the questions in the "Introducing Lahars and Debris Flows" and "Making a Lahar in a Jar" steps to assess students' understanding of the conditions required to form lahars and debris flows. Look for evidence that students understand the following concepts: debris flows and lahars form where there is an abundance of rock debris, ground water, and free-flowing surface water; that only a small amount of water is required to initiate a debris flow or lahar; lava flows, pyroclastic flows, landslides, and glaciers, and weakened rock make Mount Rainier susceptible to lahars and debris flows; that lahars are the principal eruptive hazard at Mount Rainier, even many kilometers (miles) distant. As the activity progresses, look for evidence that students think globally, and recognize that lahars and debris flows occur on volcanoes worldwide. Understanding the character and chronology of these events at a volcano help scientists identify communities at risk from lahars today. Students should recognize that knowledge of lahars and debris flows enables scientists to identify when and where people are at risk; this knowledge ultimately can help citizens live responsible and improves the health and well-being of communities.


Park Connections

Significant lahars and debris flows have occurred at Mount Rainier creating a lasting impact.



  • Instruct students to draw a diagram and/or flow chart that illustrates the initiation and activity of lahars and debris flows.
  • Use library and internet searches to learn more about the lahar history of Mount Rainier and the other snow-clad volcanoes of the Cascades.
  • This experiment does not account for porosity (air space between the particles) of the solids. Instruct students to design an experiment that accounts for porosity.

Additional Resources

Driedger, C.L., and Fountain, A.G., 1989, Analysis of recent glacial outburst floods on Mount Rainier: Annals of Glaciology, 1988, pp. 51–59.


Scott, K.M, Vallance, J.W., and Pringle, P.T., 1995, Sedimentology, behavior, and hazards of debris flows at Mount Rainier, Washington: U.S. Geological Survey Professional Paper 1547, 56 p., 1pl.


Vallance, J.W., and Scott, K.M., 1997, The Osceola Mudflow from Mount Rainier: sedimentology and hazard implications of a huge clay-rich debris flow: GSA Bulletin, February, 1997, v. 109: no.2: p. 143–163, 6 tables.


Vallance, J.W., Driedger C.L., Scott W.E., Diversion of melt water from Kautz Glacier initiates small debris flows near Van Trump Park, Mount Rainier, Washington: Washington Geology, vol 30, no 1 / 2 July 2002, pp. 17–19.


Walder, J.S., and Driedger, C.L., 1994, Geomorphic change caused by outburst floods and debris flows at Mount Rainier, WA: U.S. Geological Survey Water Resources Investigation Open-File Report 93–4093, 93 p.


Walder, J.S., and Driedger, C.L., 1994, Rapid Geomorphic change caused by glacial outburst floods and debris flows along Tahoma Creek, Mount Rainier, WA, USA: Arctic and Alpine Research, vol. 26, no. 4, pp. 319–327.