(Lava Flows and Pyroclasts)
Modified and adapted from John Farndon's book
"How the Earth Works"
- 4 plastic jars
- 4 spoons
- fine grained sand
- stop watch
- 4 plastic plates
- 1 tablespoon
- liquid dish soap
The students will need their science notebooks, pencils, and stop watches ready. One student will measure one tablespoon full of one of the liquids. They will slowly pour that liquid onto a plastic plate. Another student will time how long it takes for the liquid to stop spreading. Repeat this procedure with the other three liquids.
The liquids that have the longest spreading times have the highest viscosities. Tell the students there are lavas with very low viscosities (very thin and runny) and lavas with very high viscosities (thick and pasty). There are also many different lavas in between.
Low viscosity lavas are found in Hawaii and Iceland and are usually not violent. High viscosity lavas are very violent and erupt with little or no lava. High viscosity lavas shoot pyroclasts such as pumice, cinders and ash high into the air.
Have the students add one teaspoon of sand to one cup of the four liquids used in part 1.
Stir the mixture thoroughly.
Have the students repeat the pouring and timing portion as in part 1. Have all students record the times and compare Part 1 to Part 2 times.
Explain to the students that lavas with a high silica content(sand and quartz) have high viscosities(aa) and lavas with low silica contents have low viscosities (pahohoe)
Add sand to a cup of molasses until its viscosity is so high that will not flow. Spoon the mixture onto a dish and explain that they have just created a lava dome.
High or Low Viscosity
- 2 plastics jars
- 2 straws
Fill a small plastic jar to within one inch of the rim with water and the other jar with molasses. Put one straw into each jar. Have one student blow bubbles with the same pressure. Record what happens. The students will see rapid bubbling in the water because the water has a low viscosity. This is what pahoehoe lava is like. The gases escape quickly from the low viscosity lava and usually are not very violent.
The students will see a slow bubbling from the molasses because of its high viscosity. Lava with a high viscosity will hold a lot of gas and will loose the gas as it nears the surface of the Earth and the pressures become lower (like opening a bottle of pop and releasing the pressure). These magmas erupt violently frequently.
Clear plastic container
Float pumice in a container full of water. Have the students draw what they see. Show the students the holes in the pumice explaining they were formed as the rock hurtles through the air.