Hot Magma, Loose Lava

In this experiment about volcanic eruptions we were seeing how the viscosity and temperature of the molten rock effect the type of eruption we see on the earth's surface.  What is viscosity?  It is how thick or thin a fluid is.  Viscous means thick and sticky.  We are using corn syrup to represent thick magma (or molten rock) and water represents the thinner magma.  Since purple lava is so much cooler than regular lava we added blue and red food coloring to the corn syrup. 

 Some thought this mixing was the coolest part of the experiment.

Air, blown through a straw, models the force of gasses that push through the magma in a volcanic eruption.  You have to blow much harder to make bubbles come up through the corn syrup.

In the thick magma, the bubbles slowly expand and when they break, plumes of hot, purple lava are flung against the side of the glass.  Since volcanoes don't usually have glasses around them it might actually be flung against your village.

The bubbles can get pretty big as they leave the thicker magma.

This glass of water, the thinner magma, take much less air pressure to make bubbles.  The bubbles are small and break gently on the surface.

So what makes magma thick or thin?  First, hot magma and lava is thin and runny while the cooler (but still really hot--about 1000 degrees C) stuff is thick and sticky.  Secondly, even really hot magma can be thick and sticky depending on what minerals are in the mix.   Silica, a very common mineral, makes the magma really thick. 

Hot, thick magma with a high silica content means explosive eruptions with gas and rocks blasting from the volcano.  Thin, runny magma with low silica content gives us quiet eruptions with slow lava flows.