The chock cycle

My high school geology/geography/RE teacher (she taught a lot of subjects over the years, it was a small school with a lot of teacher overlap) used to do fun things with us in geology class including what she liked to call edible geology. This involved taking a geological concept and explaining it with food.

This one wasn't one of her ideas per se but does slightly borrow from her idea of how Mars bars are a good analogy for folding.

Also, before continuing know that I've made this for a bit of fun and any student of geology who happens across this should know that that is the case and bare that in mind!

OK, so first of the ground rules so to speak, how rocks and chocolate are similar:

Both can melt.
Both can be fractured or mechanically weathered.
Both have variable composition.
Both can be found in different forms.
Both have variations in colour and texture.
Both have a variable viscosity as they cool and solidify.
Both can be combined with other chocolate/rocks to form new rocks of variable composition. i.e. Magma mixing or those nice sea shell chocolates with the praline in the middle.

How chocolate and rocks aren't similar:
There's only a handful of rocks on the planet that make their way into foodstuffs. The main one is salt.

OK, so this is the rock cycle:

http://upload.wikimedia.org/wikipedia/commons/6/6b/Rock_cycle.gif

So now with the ground work done let's think about the rock cycle:

Imagine your block of nice solid chocolate. What are the things you can do to it? (Apart from eat it!)

Well, you can grate it. This would be like mechanical weathering of a rock leading to particles that can be transported.

This grated material can be transported then compressed with minimum heat and temperature to form a new, if crumbly, slab or be set inside some kind of matrix, like on top of the icing on a cake. This represents a sedimentary rock.

Next you could deform the chocolate. Apply enough heat to cause folding but not for it to melt. This represents low grade metamorphism. If this happened to a rock it would still be identified as the original rock. If you really bent and squished the chocolate beyond all recognition this could be seen as higher grade metamorphism. Throw in some raisins and hazelnuts and you've got some fruit 'n' nut metamorphosed chocolate with porphyroblasts.

Finally, you can melt the chocolate. This would be your magma stage of a rock. In reality rocks don't so much melt due to a temperature rise than a fall in pressure causing melting. Think about it this way, you can boil water on top of Everest at 30 degrees because the pressure is much lower. The atmosphere is thinner. As mantle rock rises the temperature will remain within the rock because it's a poor conductor of heat but the pressure drop leads to melting.

But since this is the rock cycle there are many different ways all this can happen so I've made a handy chocolate diagram:

Now here's another thing about chocolate, it's called tempering.

Nice shiny chocolate with a crisp 'snap' is caused by the alignment of the fat molecules in the slab. If you melt the chocolate beyond a certain temperature the chocolate is no longer in 'temper' that is when it resets it will not set to a crisp shine and instead it will melt at room temperature! (Depending on your type of chocolate temper is about 40 degrees C, approximately). If you melt an igneous rock and cool it again the the same way you will get the same rock. But, if you cool it down differently you'll end up with a different grain size and rock texture. This is the reverse of the chocolate since it's the heating not the cooling that effects chocolate.

OK, so in the end not the most perfect analogy for the rock cycle but still fun to think about.

TTFN. 

Fun geology fact: A trilobite fossil is named after Harrison Ford's character from Star Wars, the fossil is called H. Solo.