Molasses Lava

    Kids, imagine rocks getting hot enough to actually melt and flow like molasses! Let’s find out how viscosity affects the way lava flows and volcanoes grow. When lava is very hot it’s thin and runny, but as it cools down it gets thicker and stickier. Temperature, along with the specific minerals contained in the melted rock, affects the way that volcanoes grow.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    All you’ll need to see viscosity-in-action are two cans of molasses, 2 plates, a heatproof bowl and boiling water. First, put one unopened can of molasses in a refrigerator overnight. Open the can the next day and pour the contents onto a plate. Notice how thick the molasses is, and how it piles up to form a heap. When a liquid is thick like this it’s called “viscous”.

    Next, have an adult partner boil water and pour it into the bowl. Put the other unopened can of molasses into the bowl and allow it to warm up for 30 minutes. Now open this can and pour the contents onto the other plate. Notice that the molasses is much runnier and spread quickly into a wide, flat puddle.

    What does this mean for volcano shapes? Conical volcanoes grow from thick, viscous lava, together with ash and rubble, because the lava cools before running far away. Shield volcanoes have long gently sloping sides because they grow from runny lava that can travel a long way before cooling. Dome volcanoes are created from lava so viscous and cooling so quickly that it barely flows at all.

    Here is some chemistry to explain the viscosity. Thick, highly viscous lava has a large amount of silicate (silicon dioxide type, SiO2) minerals. Runny lava has less silicate minerals. Instead, there are more iron and magnesium minerals; the more "ferromagnesian" the content, the less viscous the lava.

    Which type of volcano do you think is responsible for the Hawaiian Islands? (Answer: shield volcanoes).


    Kathleen Carrado Gregar, PhD, Argonne National Labs
    March 2011


    “Rock and Fossil Hunter”, a Smithsonian series book by Ben Morgan; DK Publishing, Inc. NY; 2005; page 18, “On the Lava Trail”.
    Also Wikipedia at