Piles of Fire Individual and Team Project Modified from http://www.spacegrant.hawaii.edu/class_acts/pilesfirete.html The purpose of this activity is to investigate how particle size affects the angle of a volcano's slope. Introduction The most exciting volcanic eruptions are dramatic explosions, involving gas and/or water mixed with magma. This potent mixture breaks the surrounding rock into pieces as the eruption occurs. This type of eruption is called a pyroclastic eruption. While large-scale eruptions of this type are associated with continental volcanoes such as Mount St. Helens and Mount Pinatubo, Hawaiian volcanoes also have smaller pyroclastic eruptions, usually more common during the last stages of the volcano's life. Pyroclastic flow Mt. Augustine (1996) Pyroclastic flow Mt. St. Helens (1980) Pyroclastic flow Mt. Pelée (1902) http://www.geology.sdsu.edu/how_volcanoes_work/pyroflows.html Pyroclastic materials range in size from ash (less than 2 millimeters) to lapilli (between 2 and 64 millimeters) to cinders (larger than 64 millimeters) to boulders (or bombs). ASH -- Very fine-grained fragments (< 2 mm), generally dominated by broken glass shards, but with variable amounts of broken crystal and lithic (rock) fragments. Courtesy of USGS. 1
LAPILLI -- Pea- to walnut-size pyroclasts (2 to 64 mm). They often look like cinders. In water-rich eruptions, the accretion of wet ash may form rounded spheres known as accretionary lapilli (left). Courtesy of USGS. BLOCKS AND BOMBS -- Fragments >64 mm. Bombs are ejected as incandescent lava fragments which were semi-molten when airborne, thus inheriting streamlined, aerodynamic shapes. Blocks (not shown) are ejected as solid fragments with angular shapes. Courtesy of J.P. Lockwood, USGS. http://www.geology.sdsu.edu/how_volcanoes_work/tephra.html The exact angle at which loose, cohesionless material remains stable is called the angle of repose. For most geologic materials, this angle is 25-40. 2
Work individually and as a team to collect the required data. Follow the procedure outlined below. Materials needed by each person in your group Team members need to discuss what materials they will use. Each member s data will contribute to the team s results; therefore, everyone must have the same materials. poster board round particles in 3-4 different sizes (e.g., sand, lentils, small white beans, kidney beans each person on the team must have the same materials) small containers for holding the round particles (e.g., plastic cup - each person on the team must have the same type and size containers) 4x7 index card protractor metric ruler Procedure Each person in the group will follow these steps, collect the data, and report to the team. 1) Lay the poster board on your work table (a flat surface). 2) Fill a container (plastic cup) with the material you want to examine (e.g. sand). 3) Place the index card over the top of the container and turn the container upside down onto the poster board. Lift the container. The particles should form a cone. 4) Lay the index card against the side of the cone and measure the angle of the cone from the horizontal base (poster board). This angle is called the angle of repose. 3
5) Repeat the above procedure five (5) times in order to insure a decent sample size. Record the angles in the table below. Include photos of you doing this in the report. 6) The mean or average of these sample angles of repose can now be used as an estimate of the true angle of repose of the material. Mean = (sum of all angles of repose) / (number of measurements made) 7) Using the metric ruler, measure the diameters of some of the particles you just used and record this measurement of particle size in the table below. Diameter = width of particle 8) Repeat these steps with the other three kinds of particles and record the information in the tables. 9) Make a graph of average particle size versus the average angle of repose. Use your measurements to label the axes such that use of the graph space is optimized. Mean = Mean = Mean = Mean = 4
Graph of Individual Results 5
Attach photographs of you conducting these experiments. Make sure you give a title to each photograph (i.e., Figure 1. Measuring the angle of repose for sand). 6
Questions: Answered by each individual 1) Is there a relationship between particle size and angle of repose? Write a statement that compares particle size to angle of repose. 2) What other properties of the material might affect the angle of repose? 7
3) Cinder cones tend to be steepest near the vent, and less steep at the outer edge. In other words, the angle of repose is greater at the vent, and less at the edges. How can you explain this by using pyroclast size? 4) How might this information be useful from a volcanic hazards point of view? 8
Team Work Assemble the data collected by all team members for each type of particle. Report the data in a table. Calculate the mean angle of repose and mean particle diameter for the team results. Graph the team mean angle of repose vs. mean particle size. Team Member 1 Team Member 1 Team Member 2 Team Member 2 Team Member 3 Team Member 3 Team Member 4 Team Member 4 Team Member 5 Team Member 5 Mean = Mean = 9
Team Member 1 Team Member 1 Team Member 2 Team Member 2 Team Member 3 Team Member 3 Team Member 4 Team Member 4 Team Member 5 Team Member 5 Mean = Mean = 10
Graph of Team Results 11
Question: Is there a difference between your individual results and the team results? Why could there be a difference? 12