Ice cream is an igneous rock

Geologist Maria Brumm makes a compelling case for considering ice-cream to be a sort of igneous rock:

Ice cream is an igneous rock. You begin with a liquid slurry containing a hodgepodge of chemicals, and by bringing it below its freezing point, you create something solid - or at least solid-ish. Good ice cream or sorbet needs a little give, a bit of liquid remaining between ice crystals so that you can comfortably dig into it with a spoon. This is what it looks like: [A scanning electron microscope image of ice cream. The ice crystals and air bubbles are separated by sugar solution From Clarke, 2003, "The Physics of Ice Cream" Physics Education 38 (3)]

Compare that to a thin section of glassy lava from the Pacific Northwest: [Small, separated mineral crystals in a glassy groundmass]

Much like igneous rocks, the same liquid mix can turn out very differently depending on what happens while it is freezing. The goal of most ice cream and sorbet is to have a smooth and creamy texture, which would be ruined by the presence of large ice crystals. To achieve this, you want to cool your ice cream so quickly that the crystals don't have time to grow, and keep the mixture stirred up while it freezes. There's a lot of energy involved in the transition from liquid to solid water, and a home ice cream maker can't do the heat transfer quickly enough to keep the ice crystals small, so you have to sit there and turn the crank until your arm is sore while the mixture slowly freezes (or invest in a fancier machine that will do the stirring for you).

The Igneous Petrology of Ice Cream (Thanks, Marilyn!)



  1. Weird as it may sound at first, it makes sense. My students are often blown away by the revelation that water is in fact a mineral that happens to be liquid at room temperature. (Just as mercury is a metal with a seriously low melting-point.)

  2. Water is a mineral? It’s a molecule, a substance and the unversal solvent, but a mineral? I’ve never heard that. Although, under extremely high presure hydrogen can act as a metal, but that doesn’t make it one: liquid metalic hydrogen. And I do love Rocky Road ice cream.

  3. @1: By definition, minerals are solids.

    “. . .you want to cool your ice cream so quickly that the crystals don’t have time to grow.”

    This is what happens with extrusive igneous bodies. Exposed to the surface, they cool very rapidly, so crystallization is minimal and the rocks are aphanitic (fine-grained).

  4. (or invest in a fancier machine that will do the stirring for you)

    Or do liquid-nitrogen ice-cream! Be sure to wear those isotherm gloves, kids!

  5. #4, or use a molecular filter that ensures the ice crystals will be very small. Just like Ben and Jerry’s.

  6. I used to make ice cream for a couple years, and one of the keys to keeping it creamy was to get it as quickly as possible from the mixing machine to the shock freezers (which were around -20°F). You didn’t have to know what you were doing to be able to tell if somebody took their time with the ice cream, because those ice crystals grow quickly.

    Glad to know I can now add Rock Manufacturing to my résumé.

  7. Except ice cream isn’t a rock at all…otherwise, we’d have to consider ice cubes to be gemstones.

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