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

Piles of Fire
Teacher Page

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


The most exciting volcanic eruptions are dramatic explosions, involving gas and/or water mixed with magma. This potent mixture breaks the magma into solid 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. A special type of pyroclastic eruption involves just water mixing with the magma, and it is called a phreatomagmatic eruption.

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.

The larger pieces fall nearer to the vent, while the smaller pieces can be deposited many kilometers away from the vent.


Review and prepare materials listed on the student sheet. Select three differently-sized, but similarly-shaped materials for this activity. Using barley and beans is very easy and light but can be expensive. Sand and gravels work well, but make sure that the sand is dry and that the gravels are well sorted into two distinct sizes.

In Class

The materials will create cones whose sides have various angles. This angle is called the angle of repose. The larger pieces will make steeper-sided cones, and the smallest pieces will make shallow-sided cones.


The results should mimic a pyroclastic cone's shape. The larger pieces fall nearer to the vent, and the smallest pieces fall farther from the vent. Thus the cone should have steep sides near the vent, and the angle of repose should become less at the edges of the cone.

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