You may wish to do this demonstration outside. This apparatus will use air pressure to propel tephra (sand) into the air. A fan disperses the tephra downwind where it accumulates to form a cinder cone. Visibility of the tephra is enhanced if a white sheet or cardboard is used as a backdrop and drop cloth.
You will need:
- sand or cinder (best to sieve and use dry, fine grained sand)
- pressurized air (air jets in lab or compressed gas in can)
- a plastic funnel
- plastic tubing
- plastic T-joint
- a white sheet or white cardboard to serve as a backdrop
Assemble the apparatus as shown in the above diagram.
This diagram compares the apparatus to a real eruption:
Air pressure in this set up is supplied by an air jet in a lab.
Feed sand down to the "eruption" using the funnel. Gravity will help pull the sand down the tube. If the tube gets clogged tap it with your fingers or a pencil. Keep the tube as dry as possible.
Here is an alternative set up. Air pressure is supplied by a can of compressed air.
The fan blows air across the sand as it leaves the "vent." The sand is deposited downwind just like tephra blown downwind from lava fountains. If this set up ran long enough a cone would form. Coarse sand particles would be close to the vent and finer sand particles would be farther away.
This set up with the air jet worked differently. By placing a thumb over the open nozzle by the T-joint pressure would build up in the tube and then eject much of the sand in the tube all at once.
This reminded us of eruptions at Stromboli.
Eruption at Stromboli. Copyrighted photo courtesy of Steve O'Meara of Volcano Watch International.
Because we were making a mess we decided not to use the fan.
We were surprised by the results. After the sand was ejected it made beautiful cones, much like cinder cones or tuff rings. The cone even had a central crater.
Here's another look at our sand volcano.
This type of model is fun. If you have trouble try different set ups until you can get the sand to shoot out of the tube. Keep trying. It will work!
This model is a great way to introduce the factors that influence the distribution of tephra. These factors include:
- size of materials: large particles accumulate near the vent
- small particles accumulate farther away
- thicker deposit near vent, deposit thins rapidly farther away
- fountain height: high fountains cause greater dispersal
- wind speed: strong winds carry tephra farther from eruption
- wind direction: tephra is carried downwind
Vary as many conditions as possible and note the results. Let the demonstration run as long as possible and compare your cone to cinder cones and tuff cones shown in VolcanoWorld.