The Igneous Petrology of Ice Cream

It was hot out last weekend. Some of you might scoff at what I consider "hot", but the glorious thing about Seattle is that the entire city seems willing to join me in whining and wilting whenever the temperature breaks 80 (that's 25 of your Earth units). Naturally, I spent a lot of time thinking about ice cream.

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:


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:


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. However, 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. The alternative approach is to keep the mixture stirred while it freezes, so that you can avoid growing giant crystals where it's coldest. So you have to sit there and turn the crank until your arm is sore (or invest in a fancier machine that will do the stirring for you).

Or, you could acquire some liquid nitrogen - pouring a -321° liquid (that's a mere 77 Earth units above absolute zero) into your ice cream mix will freeze it so quickly that stirring is neither difficult, nor tremendously important. The crystals will be tiny no matter what. This method has the added advantage of being able to freeze mixtures whose melting point is below what you can get with home freezers and rock salt, enabling such monstrosities as a silky-smooth 80 proof rum raisin.

The difference between ice cream and sorbet is that ice cream contains cream and sometimes eggs or other emulsifying agents, while sorbet is just fruit, water, and sugar. The presence of fat and emulsifiers in ice cream provides another way to control the crystal structure (and another way to ruin the texture, if you let the fat globules get too big) but there's no direct analog for this in igneous petrology.

Occasionally, you do want a dessert with a sharper texture to it, or maybe you're just too lazy to stir very often. That's how you make a granita: Leave the sugar slurry in the freezer, so that the crystals have time to grow, but interrupt the process every half-hour or so, so that they don't get too big. You end up with a slushie or crystal mush.


Granite... or granita? Magma that is left to cool slowly underground will also produce large crystals.

Coming tomorrow: The metamorphic petrology of ice cream, or, why I need to stop cooling off by staring absent-mindedly into the freezer.

More like this

*picks up glassy lava*
*attempts to nom*

Iz not so much like eis krem, akshully... ouchies.

Although I liiiiike the sound of 80 proof rum raisin... indeedy I do....


Unfortunately, I suspect that metamorphic ice cream isn't as yummy as igneous. (I've recrystallized Ben & Jerry's seconds before...)

I bought a 35 liter dewar on e-bay and now I keep a supply of entertainment LN2 in my basement. I make a lot of ice cream.
I made some LN2 Ice Cream from Baileys Irish Cream a couple of weeks ago. It was smooth and frosty, but it froze my tongue. Just a warning to those who would try it with 80 proof. Be afraid, be very afraid.

I have been craving ice cream all day, and have been telling myself that I should not get some, because I am low on money.

You are SO not helping!

I will think about decidedly less texturally-yummy metamorphic ice cream. Maybe it will help a little.

Where's my igneous hot fudge lava?


@Eric: Heh, I just ebay'd a 10L dewar so we can serve LN2 ice cream at the wedding. If you don't count the food or the camp rental it was the single most expensive item in the shebang... but we'll be able to use it more than once.

I lied about stirring not being important with LN2 - the more air you incorporate into the ice cream, the less likely you are to freeze your mouth while eating it. For boozy ice cream that needs to be eaten while it's still super-cold it might make sense to think about folding in some whipped cream or egg whites or something before freezing it.

IF you are looking for an igneous analogy to the fat globules in ice cream--would vesicles work? They *seriously* change the texture of an igneous rock, particularly when they are large. (e.g. rocks which are so vesicle rich they float)

Not strictly ice cream but a related issue:

I recently forgot some 250 ml beer bottles in the freezer for about 45 minutes (I originally wanted my beers to cool off real quick after a shopping trip so I could have one nice and cold, then got distracted by sthing on ScienceBlogs). When at last I remembered and took them out, it felt like the beer was still liquid, but when I uncapped one, pscchhhwiiit, I got a rapidly-oozing fountain of beer granita.

I'm presuming it was the pressure that kept them liquid, but when the pressure was released, ice crystals formed nearly instantaneously and combined with the volume expansion, well, the result was quite unlike beer.…

I've been wanting to use this since we first met and spoke of geological issues:

Maria, this is a real gneiss post!

Abel, your comment was pretty wacke. I guess it's tuff to come up with good puns.......

That is amazing!

Never before has looking at pictures of rocks made me hungry. I'm going to have to make some ice cream and granita into a rock strata model.


Thanks for the advice. I think I will buy another bottle of Baileys and try making some more ice cream with it it this weekend, following your suggestion of course. I don't know what the hold time of your 10L dewar is, but in my experience with the 35L is that it keeps the LN2 for a surprising amount of time. I really thought it would boil away in a few days but mine seems to only lose about 250ml / week. Quite impressive.

As a note, one other fun experiment is filling a metal cone with LN2 and condensing oxygen out of the air. Lots of fire experiments possible there.

LN2. 1L soft plastic bottles. Put LN2 in bottle. Cap it. Shake it so the plastic freezes and gets brittle, while the evaporating LN2 builds up pressure inside. LN2 bomb! Remember to throw it after a few shakes so it doesn't blow up in your hand.

Used to scare the hell out of people at the Oregon Museum of Science and Industry when I was a kid (we had a very large dewar of LN2 at our disposal sans adult supervision, and used to steal one liter plastic bottles from the little science supply store inside the museum at night, using our filched master key to get in).

And, yeah, you probably can poke your eye out doing this, it was long ago, we didn't worry about such things.

I love ice cream, but have doubts that it's an igneous rock. A rock has to be composed of minerals or mineralloids, and be solid. Ice cream is neither one. Think of it as a magma though, and the analogy works. The point that it doesn't really have a solidus might be instructive for igneous petrologists!

BTW I will use this post when I teach Ig Pet this term.