On the subject of minerals/rocks, if naturally occurring ice is a mineral, does that make glacial ice a weird type of regional metamorphic rock?
Thanks so much for your fun question, anon! Apologies that it’s taken the team a little while to respond!
The short answer is, yes! Ice is the crystalline, or solid, form of water - it’s naturally occurring, inorganic, has a definite chemical composition (H₂O), and has a defined and organised crystal structure (as seen below) - so by all accounts, a block of ice is what we would call a mono-mineralic rock!
Fig. 1: Crystalline structure of ice, or solid water. Doesn’t that structure look familiar? There’s a reason that snowflakes look the way they do, and that’s because of that beautifully organised crystalline structure!
You are absolutely correct in that glacial ice meets all the requirements to be classed as a metamorphic rock, too!
Well, we won’t divulge too much as we’ll be talking about the different rock types in our next big post - but for the purpose of this ask we’ll do a quick crash course in sedimentary and metamorphic rocks.
Let’s start with sedimentary basics. When talking seds one of the best ways to imagine your rock is like a layer cake. Let’s say you’re making a chocolate and vanilla layer cake. Your chocolate batter represents mud, and your vanilla batter is sand - they each have a different colour and texture. You lay your muds down first, then your sands. You can keep doing this again and again if you wish, until you have lots of layers. Then, you bake! The baking represents the many millions of years it takes for these sediments to solidify into a solid rock (although you technically don’t need heat for real rocks). Do note that in reality it’s not quite as simple as this, there are many things that affect ways that rocks solidify!
Now imagine just a plain old chocolate cake. No frosting or filling. Just chocolate sponge. We layer up the chocolate sponge and bake. Boom, we have a whole unit of just mudstone.
The origins of glaciers are exactly the same, except your mudstone is snow! Lots and lots of snowflakes that have built up over time.
Okay, but what about the metamorphism? Metamorphic rocks start out pre-baked, if we’re using our cake analogy, so we already have our snow layers. Metamorphism of rock occurs either due to heat or pressure - or both. We don’t have heat when it comes to ice, but we do have pressure! Lots of pressure, actually - caused by the weight of all that snow piling up and up. As the weight increases, the ice at the bottom gets squashed which causes the crystals to change shape or recrystallise into ice.
Now think back to our cake. If we kept layering up layers and layers of baked cake, eventually the bottom layer would get squashed, altering the texture and making it really dense. The same thing is happening here! Turning soft snow into hard ice. And then this keeps happening, compressing the ice even more to form glacial ice! Also cool to note that the “texture” of normal ice under the microscope is very different to that of glacial ice!
Fig. 2: Diagram showing how glacial ice is formed. Note the different texture and size of the "crystals" between the fine-grained and coarse-grained ice! Original image from Let's Talk Science.
This recrystallisation is a really key part of metamorphism in rocks, too, with metamorphic textures being one of the main diagnostic features that geologists use to categorise different types of metamorphic rocks - limestone to marble, for example!
Fig. 3: Petrographic slide images showing limestone (left) and marble (right) showing the recrystallisation effects of metamorphism. Note how different the shapes are! Marble and limestone are made of the same building blocks (CaCO₃), but have different properties because of this! Original images from Alex Strekeisen's incredible website for all things optical petrography.
Now you can be a cool geologist at parties and let folks know the ice in their drink is technically a rock!
(DekkiiDan does this. Merrydock does this. So does Mohs. We are cool.)
Anyway, hope that answers your question! It was a really fun one, hehe!
See you in the next loop!
The OWGS Team
