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.


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.


You will need:

  • Copies of "Lahar in a Jar" student pages
  • Graphic: "Three Prominent Lahars at Mount Rainier"
  • Graphic: "Mount Rainier and Emmons Glacier"
  • Graphic: "Mount Rainier Lahar Hazards Zone"
  • Graphic: "Extension Maps of Lahar Hazards Zone"
  • Graphic: "Debris Flow on Tahoma Creek, 1986"
  • Graphic: Tahoma Creek After Debris Flow, 1986"
  • 100 ml or larger graduated cylinder
  • Wide-mouth 1 liter beaker
  • Large wooden spoon or paint stirrer
  • 200 to 400 ml of lahar deposit or rock debris, as prepared in recipe (see next step)
  • Calculator
  • One-meter (three foot) long flat boarder gutter

What to do Before Class Begins:

  • Decide if you are going to a large group demonstration or have the students work independently or in small groups.
  • Collect materials.
  • Make copies of student pages for Lahar in a Jar.
  • Prepare to show graphics.

Lahar Debris Recipe

You will need:

  • 2.0 L gravel from a driveway (pea to marble-size)
  • 3.0 L sand from a sand box or river bank
  • 2.0 L garden soil (soil with high clay content and minimal organic matter works best- look for clay loam or silt loam commercially)
  • 0.6 L dry potters clay powder (available from pottery supply stores)
Obtain and mix ingredients in a dry 5-gallon bucket or other container. Mix a batch that is sufficient to provide 400 milliliters to each student group (amounts listed enough for 10 student groups). All ingredients should be dry.

Note that while scientists generally measure contents by weight, this activity provides a general conversion from percent weight to volume so that the activity can be conducted with ease in the field or classroom. This method does not account for porosity (air spaces between the particles) of the solids.

Introducing Lahars and Debris Flows
Introduce students to lahars and debris flows through class discussion and graphics, and then conduct the experiment.

  1. Instruct students to list some of the distinctive components and characteristics of Mount Rainier that are visible in the photograph entitled "Mount Rainier and Emmons Glacier". Lead the discussion towards these three features: glaciers, loose rocks, steep slopes. Ask them to explain how volcanic heat, glaciers, and snow might interact. Introduce the concept of a lahar as a mixture of rock, mud, and water that rushes down the slopes of a volcano and its river valleys. Rapid melting of snow and ice during eruptions generally cause lahars, though landslides can also initiate them. 
  2. Instruct students to hypothesize about what happens when an excess of stream water, originating with intense snowmelt in mid-summer or extreme rainfall, flows across valley bottoms strewn with loose rock. Explain that these conditions can lead to debris flows.
  3. Instruct students to hypothesize whether or not Mount Rainier is a potential site for lahars. They should explain their reasoning. Yes, it is the site of frequent debris flows (almost annually). There is an abundance of loose volcanic rock from eruptions and glacier action, and water from snow and ice melt and rain. In addition, rock fall and landslides can initiate lahars. 
  4. Tell students to use their knowledge of debris flows and lahars to identify to types of terrain over which lahars and debris flows generally travel. (Answer: in river valleys.)
  5. Use the graphic "Tahoma Creek Debris Flow, 1986" to illustrate the appearance of a lahar on Tahoma Creek on the west side of Mount Rainier National Park. Use the graphic "Three Prominent Lahars" to follow the pathway of three major Mount Rainier lahars- Osceola, Electron, and National. These three prominent lahars provide the basis for lahar hazard zones on the graphics "Mount Rainier Lahar Hazards Map" and "Extension Maps of Lahar Hazard Zones". Display the lahar hazards map and ask students which valleys would be affected by lahars and debris flows on each side of the volcano. Ask students to name the communities that are at risk.
  6. Explain to students that debris flows tend to happen during intense rainfall and periods of rapid snowmelt at Mount Rainier. By definition, debris flows travel over terrain within the park and do not generally flow beyond park boundaries. Follow the pathway of some recent debris flows.
  7. Determine whether you live, work, or go to school on the debris from one of the lahars shown on the lahar hazard map. Discuss how lahars are significant to your community. Ask students to find the safest locations near their communities.

Make a lahar in a jar
Learn how only a small amount of water in motion can mobilize loose rock to form a lahar. Conduct this activity either as a teacher demonstration or in small groups.

  1. Divide class into groups of 3-4 students.
  2. Distribute "Lahar in a Jar" student pages.
  3. Instruct students to place approximately 400 milliliters of loose lahar material from lahar recipe (see step 2) onto a large piece of paper and break up any large clumps of dirt and debris. Dump the loose rock into a beaker. Press it firmly with your hands to remove spaces from between the particles. Record the exact volume on the student page.
  4. Ask students to predict how much water they think is required to make the deposit flow like a lahar. 10 ml? 100 ml? More? Students record their prediction on the student activity sheet.
  5. Fill the graduated cylinder with water and record the starting amount of water on the student page.
  6. Instruct students to begin pouring water in the beaker in increments of 10 ml.
  7. Students should stir the loose rock after each addition of water.
  8. After each addition of water, students should tilt the beaker to the side and gently rotate it sideways to determine if the mixture "flows" around the jar sides as a lahar would. The consistency initially is like that of dry dirt, but with the addition of water, changes to the consistency of cookie dough and later to that of thick cake batter. Decrease the amount of water added each time as your lahar begins to flow. Remember, it does not take much water to get debris flowing.
  9. Students sum the amount of water used after the rock debris forms a lahar in the jar and record the amount on the student page.
  10. Instruct students to compare the total volume of lahar and water and determine the percent water required to produce a lahar in a jar. Ask whether the amount of water was as predicted. Answer: will probably be between 20-40%, depending on the material used. At Cascade Volcanoes, the water content in debris flows and lahars is generally between 30 and 45 percent.
  11. Each student group should pour their lahar onto the inclined gutter or board for all of the class to see. Ask for hypotheses about what happens when slop of the gutter or board is changed, then test the hypotheses. Inquire about any interesting observations made of the lahar mixture. The lahar may flow in single or multiple surges. Velocity of the flow increases with slope.
  12. Ask students to follow the path of energy transformation, and to write about this, or draw a diagram on the reverse side of their student pages. Students should report that the lahar while still in the jar has potential energy. Kinetic energy is releases as the lahar slides down the gutter or board.
  13. Ask students what conditions exist on Cascade volcanoes that promote development of lahars. Answers: loose rock, abundant water, steep slopes, heat.


  • Provide students with sand, clay, garden soil, and gravel and instruct them to hypothesize about what happens when the amount of clay is increased and decreased. Instruct students to design and conduct experiments with different portions of materials.
  • Obtain rock debris from other sources in your community, such as stream beds, lahar deposits, gardens, etc., and repeat Lahar in a Jar again. Compare results with your lahar recipe mixture.



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.