|Piles of Fire
Students will be able to measure particle size and angle of repose, and graphically compare the relationship of both.
Students will be able to explain how the particle size affects the angle of a volcano’s slope.
8th grade Earth Science
One fifty minute class
Michigan Earth Science
E3.p3C—Describe the three major types of volcanoes (shield volcano, stratovolcano, and cinder cones) and their relationship to the Ring of Fire.
E3.4D--Explain how the chemical composition of magmas relates to plate tectonics and affects the geometry, structure, and explosivity of volcanoes.
Copies of student questions, pens or pencils, graph paper, poster board, round particles in three different sizes (e.g., lentils, barley, sand, gravel), small cups/bowls for holding round particles, protractor, stiff cardboard or index card, metric ruler.
-Draw the three types of volcanoes on poster (shield, stratovolcano, cinder cone) (see image)
-Draw a cone, w/ labels of A, B, C; showing location that pyroclasts are likely to fall. (See image)
(Write terms on the board)
Angle of repose—the greatest slope angle that loose, unconsolidated particles can lie on without sliding down.
Pyroclastics—Solid particles ejected during a volcanic eruption.
Lapilli—Rounded pyroclasts ranging from 2-64mm in diameter.
Cinders—Pyroclasts larger than 64mm in size.
Intro/Description: (10 mins)
Review the types of eruptions with the students (effusive/explosive) and what types of volcanoes have each eruption, as well as materials generated, and write on the board. (A good option would be to have a drawing of the 3 types of volcanoes, so the students could identify them.) Ask students to raise their hands to contribute to the discussion.
--What are the 3 types of volcanoes? (Stratovolcano, Cinder cone, Shield)
--What are the two types of eruptions? (Effusive / explosive)
--Who can explain what those two types mean? (Effusive=lava flow; explosive=ejecta)
--What types of volcanoes have effusive eruptions? (Shield)
--How about explosive? (Strato, cinder)
--Can anyone tell me any of the materials that are coming out of these different volcanoes? (Shield=lava, some pyroclasts; strato=ash, pyroclasts, lava; cinder=pyroclasts)
The most exciting volcanic eruptions are dramatic explosions from gas and/or water mixing with magma. The magma can be broken into pieces as it is ejected. These pieces are called pyroclastic ejecta or just pyroclasts. Large eruptions of pyroclasts are generally associated with stratovolcanoes (such as MSH or Pinatubo), cinder cones and even shield volcanoes can have smaller eruptions of these materials.
Pyroclastic materials can range in size:
Ash (< 2mm) or about the size of beach sand.
Lapilli (between 2 & 64mm), well rounded particles ranging from pea to walnut size.
Cinders (> 64mm) also known as volcanic blocks or bombs. Blocks are full pieces of rock that are launched from a crater, whereas bombs are ejected as a liquid, and so take on an aerodynamic shape as they fly through the air, like a football.
The larger pieces will fall nearer the vent, while smaller pieces can be deposited many km away from the vent.
Today, we will be looking at how the size of material can affect the shape of a volcano and its angle of repose (define).
-Break students into 2-4 person groups.
-Lay poster board on table for each group.
-Provide graph and table for each group.
-Using 2 volunteers, show class how to build the cinder cone using the smallest particle size, measure the angle of repose, calculate average grain size, and record into table.
-Begin by pouring about a cup-full of one kind of particle (e.g., barley) onto the poster board. Make a cone with the barley, as steep as possible.
-Lay the cardboard (index card) against the side of the cone and measure the angle of the cone from the horizontal base with the protractor. Called angle of repose. Record on table.
-Using the metric ruler, measure the diameters of some of the particles you just used. Compute an average diameter of the particles and record in the table.
-Have groups repeat with two other kinds of particles and record the information in the table.
-Show on graph paper, how to graph the first measurements of particle size vs. angle of repose.
-Have groups graph the other two points on their own.
-Clean up particle cinder cones by dumping back into cups and keeping for later classes, or using poster board to dump into trash.
Conclusion: (5 mins)
Write each group's results on the board and compare the similarities and differences. Talk about the differences that can arise from experiments. “Since each group has different data, what is the best way to come up with an accurate number?” (ie. Averaging, multiple trials, etc)
Have each group write a statement that compares particle size to angle of repose. Present this statement to class. “So as you can all see, larger pieces will make steeper-sided cones, and smallest pieces will make shallow-sided cones.”
To assess the students’ learning, have students answer the following questions (either as a group or all together, write on the board):
“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 pyroclastic size?
Draw a rough sketch of a volcano on the board, labeling 'A', 'B', and 'C', for the top, middle and base of volcano.
Where would you expect to find more volcanic bombs and cinders?
Where would you expect to find more ash and dust?
Optional questions based on time:
Is this useful information for a volcano? Explain why or why not.
What hazards could be related (or influenced) by this info?
It is the cinders and ash and other "ejecta" that are thrown out of the volcano. Most of it lands right near the vent and builds up a steep pile around the vent. That pile is at the "angle of repose" meaning that it builds the steepest angle that the particles can maintain. Once it gets to that angle, additional cinders roll down off the side, but it keeps that steep slope as it builds up.
During rainstorms, especially, the added weight of the water and the slurry created by water added to ash can cause big sections of the material to suddenly move downhill.
Another thing that can happen is that the size of the ejecta changes periodically for a composite volcano so that sometime it is mostly ash and other times it is cinders of a particular size. Each of these materials has a different angle of repose. When a layer of one material with a high angle of repose builds up over a layer that has a low angle of repose, the added weight on top of the lower layer can cause massive slippage along the surface between the two.
Hawai'i Space Grant Consortium, Hawai'i Institute of Geophysics and Planetology, University of Hawai'i, 1996 http://www.spacegrant.hawaii.edu/class_acts/PilesFireTe.html