Dear Stephen, I would like to know how I could experiment with
how the eruption affects the shape of the volcano.
Matt
Dear Matt,
Here are few ideas. Explosive eruptions fragment the lava to make
pyroclastic material (blocks, bombs, cinder, ash). This material tends
to pile up close to the vent (some of it gets carried down wind). The
slope of the flank of the volcano is related to the size of the volcanic
material. Larger pieces have steeper slopes. Smaller pieces have
gentler slopes. You might visit a local stone quarry and buy gravel and
sand of different sizes. Ask you students to make piles of the different
size materials and measure the angles of the slopes. Do they observe any
correlation between particle size and slope angle? Do they think this
would also be true for volcanoes? Repeat the experiment with any
volcanic materials you can obtain (cinders or ash).
The shape of volcanoes that are made of lava flows is controlled (mostly)
by the viscosity (stickiness) of the lava. Low viscosity (very fluid)
lava travels far from the vent, making shield volcanoes like those in
Hawaii.
Higher viscosity (very sticky) lava cannot travel very far from the vent,
making steep-sided volcanic domes like the one in inside Mount St.
Helens.
Your students might experiment with fluids of different viscosity
(perhaps cold molasses and cheap pancake syrup). Plan on them making a
mess. Pour an equal volume on a gently sloping board and measure the
distance they travel. Relate their observations back to types of volcanoes.
Steve Mattox, University of North Dakota
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