Fun Geo-Thread: A Lithological Classification Scheme for Cakes and Pastries 
Geologists love rocks. Geologists love food. No field trip in Australia is ever really complete without a stop at a country bakery to fuel up on sweet and savoury baked treats.
So, I’ve decided that its time for geology to properly embrace our love of food by treating food as rocks – starting with cakes, pastries and other goods.
So, here is a tasty thread on the first ever lithological classification scheme for cakes and pastries.
So, here is a tasty thread on the first ever lithological classification scheme for cakes and pastries.
The love geos have for baked goods runs deep. Things can be rough out in the field. It can be rainy, scorching hot, remote. You can be in the field for days, weeks, months. A bakery stop or pub meal represents rare & blissful luxury to be relished while you tramp the outcrops.
Cakes and Geology also have a long close relationship. There are mud cakes, marble cakes, lava cakes, rock cakes. People now put rocks into cakes, like this geode wedding cake.
Geos have even started competing to make the best geologically cakes as part of the Geo Bake Off.
Cakes & rocks come in a huge variety of forms, & vary by composition, texture, formation, structure, colour. So, can we classify cakes as rocks?
Cakes & rocks come in a huge variety of forms, & vary by composition, texture, formation, structure, colour. So, can we classify cakes as rocks?
Rocks are first classified as igneous, sedimentary or metamorphic, depending on how they formed. Igneous rocks crystallised from a molten state. Sedimentary rocks are deposited in air or water. Metamorphic rocks have been changed (recrystallised) by heat and/or pressure.
Cakes don’t really have a formal classification scheme, though are sometimes split into shortened or unshortened types. Shortened cakes contain fat & use leavening agents like baking powder. Unleavened cakes have no fat & get leavened by air/steam. Not so useful for rock types.
Shortened & unshortened are end members of cake types, with lots of hybrid varieties in between. Butter cakes are classic shortened cakes, while sponge & angel food cakes are unshortened. Things like chiffon & coffee cakes are hybrids.
So, how to classify cakes as rocks? You could argue that cakes are igneous, because they start as some sort of fluid mixture that becomes solid. But, igneous rocks typically cool from a molten state – whereas cakes are usually heated (baked) into a solid form.
If cakes are baked, that means they must be metamorphic, right? Well, it certainly does fit pretty well. The baking results in many chemical reactions that change the mixture into cakey goodness. So, a lot like a metamorphic rock. But, it is more complex & there's other options.
Cakes look granular & porous, and lots are made of layers, like sedimentary rocks. Furthermore, cakes are organic, & organic rocks like coal are sedimentary. Plus, the heat at which cakes are baked is typically <200degC, so more in the range of sedimentary diagenesis.
So. Cakes can really be igneous, metamorphic or sedimentary. So, let's propose a way of dividing them up so that all three rock types get to share in the tasty joy. Let’s look at each rock type in turn, fit cakes into their classification schemes and show some sweet examples.
Igneous. These are classified as being either extrusive (volcanic - cooled quickly at the surface) or intrusive (cooled underground). Extrusive rocks are very fine grained, with crystals often not visible (aphanitic). Intrusive rocks cool slowly and tend to have big crystals.
Igneous rocks are next classified by color – felsic (light) or mafic (dark), which relates to the minerals in them. Igneous rocks may have additional textures to further describe them, like crystal shape or having some big crystals (phenocrysts). Cakes can be like this!
For cakes, I am going to classify anything that is poured and cooled (in order to ‘set’) as extrusive igneous. I will classify any cake that is still partially fluid, or especially gooey, as intrusive igneous.
‘Extrusive’ igneous cakes include things like cheesecake, sweet pies and tarts. These are often made by a rapid ‘eruption’ of a fluid cake mixture flooding over the existing surface (e.g. a ‘sedimentary’ biscuit base). This chocolate tart is the cake equivalent to a flood basalt.
Extrusive igneous also includes ice cream cakes (extremely aphanitic) and sweet treats of the cream based pudding variety like crème caramel (obsidian of the cake world), chocolate mousse (a cakey scoria). We should also classify quiche as an extrusive igneous rock.
I’ve put together this guide to help geologists identify different igneous extrusive cakes and desserts in ‘the field’. Note that you normally wouldn’t lick igneous rocks to ID them, but taste is frequently necessary (& encouraged) for cake ID.
You'll need to zoom in.
You'll need to zoom in.
What about intrusive igneous cakes? I’m regarding these as anything still super moist, gooey or semi molten. I know that’s not quite right, but just seems to fit better as intrusive igneous. So, examples are things like steamed puddings, soufflé, molten cakes & fondant.
Controversially, chocolate lava cake, a type of molten cake, is classified as a mafic intrusive – the cake equivalent of a gabbro. Many geos note that lava cake should be called magma cake. Lava cake is characteristically similar to fondant, but has less to no flour.
Here’s my classification scheme for intrusive igneous cake varieties. One must first determine how felsic or mafic the cake is, and then look at its composition, and particularly the relative amount of flour, butter and egg in the cake.
Christmas pudding is a great example of an igneous intrusive cake. It’s intermediate to mafic in color and has spectacular rounded phenocrysts of raisins & currants, and often visible crystals of cherries & walnuts. These are sometimes found with xenolithic silver coins.
Molten or liquid masses injected into cake under pressure are also intrusive igneous features. For example, a cakey ‘country rock’, like a doughnut, that has been internally filled with jam, custard or chocolate. Éclairs and profiteroles also display intrusive igneous features.
Next, sedimentary cakes. These are cakes comprised of multiple cake/pastry layers or compressed cakey grains. I know that metamorphic rocks can be layered too, but most layered cakes are laid down layer upon layer, like sedimentary rocks (rather than through recrystallisation).
If we look at layered cakes/pastries, then we can count as sedimentary things like Rainbow cake, pancake stacks, lapis legit, many tortes like dobos, and puff pastry based yummies like danish and croissants.
Sedimentary rocks can be clastics, carbonates or evaporites. However, when it comes to cake, we are really looking at clastics, which are first classified according to grain size and then according to things like composition, sorting and grain roundness.
Layer thickness tends to be a good proxy for grain size. Fine grained rocks, like shales & siltstones have many thin beds, while coarser grained sediments, like sandstones & conglomerates, often form thicker beds. So, that leads to this sedimentary cake/pastry grading scheme.
These colourful sedimentary rocks really make it clear why rainbow cake must be considered sedimentary!
Croquembouche is a spectacular sedimentary rock that is a well sorted and well-rounded profiterole conglomerate with syn-depositional glucose cementation.
My family cake is the Hungarian Dobos, which is comprised of repeating fine sandstone sequences of sponge and chocolate buttercream, and always with an angular unconformity at the top overlain with an unusual igneous (or possibly evaporitic) caramel layer.
Baklava is characterised by its yellow shaley filo pastry beds inter-layered with angular nutty sandstones and honey cementation.
It should be noted though that sedimentary cakes show an inverse relationship to depositional energy than sedimentary rocks. Fine grained sediments usually mean low energy environments. However, finely layered cakes take much more energy to make than thick-layered cakes!
Many savoury items are sedimentary. Pizza is sedimentary (with a cheesy igneous extrusive top). Pasties are thick conglomerates of angular well-sorted vegetables overlain & underlain by fine-grained shale-like pastry.
I’m not very good at sed logging. I keep eating the outcrop.
I’m not very good at sed logging. I keep eating the outcrop.
Finally, metamorphics. Personally, I find metamorphic rocks horribly complex, & more like wizardry, than science. But, cakes are also akin to magic – turning simple ingredients, like flour, butter & eggs, into amazing treats. So, cakes are, I think, most like metamorphic rocks.
I think we have to consider most of the ‘classic’ or ‘simple’ cakes as metamorphic. Butter cakes, sponge cakes, chiffon cakes, bread and plain doughnuts. These all undergo an incredible transformation from their batter/mixture to their baked form. This embodies metamorphosis!
Metamorphic rocks are classified primarily according to their grade of metamorphism – essentially how much temperature and pressure they were under. Now, pressure isn’t much of an issue for cakes/pastries – but temperature and time are very important.
Whilst I feel that most cakes can be classified as metamorphic rocks, fitting them into the metamorphic classification scheme is problematic. The issue is that the primary descriptor is usually the degree of foliation (mineral alignment), and cakes really don’t show this.
There are non-foliated metamorphic rocks, like quartzite & hornfels, but these don’t fit well with most cakes. So, my classification uses how hard baked the cake is as a metamorphic grade, rather than foliation. Like a dehydration reaction.
So, we can imagine fudgey brownies & soft cookies as low grade (e.g. slaty). Typical cakes/breads as medium grade (e.g. Phyllitic) and hard baked things, like shortbread/biscuits as high-grade metamorphic (e.g. Gneissic). They are further described by their primary composition.
It is no secret that I love mud and mud is sedimentary. However, much to my chagrin, according to my classification scheme I must consider all types of chocolate cake as metamorphic, including mud cake.
Here we see some lovely soft cookies, which could be considered Butter-Flour-Sugar Phyllites with characteristic choc chip porphyroblasts.
Fruit cake is a fruit-walnut-flour-sugar metaconglomerate – you can clearly see that it has been baked from a extremely coarse-grained sedimentary protolith.
So, there you have it. The first ever lithological classification for cakes! We can now all consider cakes and other yummies as geological features!
It’s not a perfect scheme, and really needs your help getting it right - but now every visit to a cafe or bakery is a field trip!
Next time you’re asked if you want to see a dessert menu, you can proudly say ‘yes! I will take a look - for science!’
So that wraps up this geo-cake thread - go out there and enjoy your geological cake adventures!
