Hawai'i Space Grant College, Hawai'i Institute of Geophysics and Planetology, University of Hawai'i, 1996

Piles of Fire 

To investigate how particle size affects the angle of a volcano's slope. 

Key Words




angle of repose 



poster board 

round particles in
three different sizes
( e.g., barely, small white beans,
pinto beans; or sand,
pea gravel, gravel, etc.) 

small paper cups,
plastic bowls, or
small containers for holding
the round particles 

stiff cardboard or
4x7 index card 


metric ruler

Lay the poster board on your work table.


Begin by pouring about a cup-full of one kind of particle (for example, barley) onto the poster board. Make a cone with the barley, as steep as possible.


Lay the stiff cardboard against the side of the cone and measure the angle of the cone from the horizontal base. This angle is called the angle of repose. Record the angle in the table below.


Using the metric ruler, measure the diameters of some of the particles you just used. Compute an average diameter of the particles and record this size in the table below.


Repeat steps 2 - 4 with the other two kinds of particles and record the information in the table.


Make a graph of particle size versus the angle of repose.


Write a statement that compares particle size to angle of repose.


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?

Where would you expect to find more volcanic bombs and cinders?

Where would you expect to find more ash and dust?

Is this useful information for a volcano? Explain why or why not.


Kind of Particle Angle of Repose Average Particle Size

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