I have no idea in what community to post it because none seems suitable, so I'm going with this one. Hope it's ok.
Recently, I picked up an interest of rockhounding and started thinking about how the local stones form. We have a lot of different rocks on our beaches, and one of them is Granite.
How often do you go to a beach or rocky area and think about why this rock looks the way it does and why it's here? Yeah, probably never😄. Well, I'm going to answer this about granite, the answer you never thought you wanted to know.
Btw I'm no geologist or scientist, just a curious person who is interested to learn this stuff and now I'm spreading this, without going too deep into science.
Our rocky beach
What is Granite?
It's a very old( hundreds of millions to billions of years) volcanic rock that forms deep underground from magma that cools slowly. Granite is literally everywhere and makes up a massive part of the continental crust. You are most likely sitting on it right now.
It forms deep underground but gets on the ground by the formation of mountains(the ground gets pushed up by movement of tectonic plates) and granite comes along or erosion( wind, rain, rivers, ice) that removes softer upper sediments and leaving granite exposed.
Granite in Estonia, where I live, is not native. It came here during the ice age about 10k years ago with glaciers and originates from Scandinavia ( Finland, Sweden, Norway) and got left here after glaciers melted.
Why is it shiny and colorful?
Well, the quick answer is different minerals. When magma cools slowly then the different minerals that are in the magma have time to form bigger crystals. The main color, shine, and type of crystals depend on what the magma consisted of the most when cooling down.
This simple table tells you the main minerals and what color each mineral is, and therefore, you know what mineral each granite rock consists mostly of. Of course, most granite is a mix of several minerals, that's why it's colorful and speckly
| Mineral | Color |
|---|---|
| Quartz | Clear, Gray, Glassy&Shines |
| Feldspar | Red/Pink, White |
| Mica | Black, Shiny flakes |
| Hornblende | DarkGreen/Black, usually Matte |
Some examples now of Granite I took photos of.
This is a mix of feldspar and likely hornblende minerals. Also a bit of quartz because skiny clear flakes can be seen.
This is mostly made of K-feldspar since it's pink/reddish. Some lighter specks are quartz and a little bit of mica or hornblende.
This one has mostly black tones and is most likely hornblende granite with a bit of feldspar.
This might not be a real granite but it's definitely granite-like. It's very pale and a lot of glassy crystals, which are Quartz and a mix of some feldspar.
Next to this one on either side you can see red/black granite. Its K-feldspar with mica minerals.
So as you can see, it's easy to identify granite because its big different crystals are very much visible.
A couple of examples that are not granite.
This is fine-grained/dense and has no visible big crystals; therefore does not classify as granite. It's most likely gabbro, a magnesium and iron-rich volcanic origin rock.
This one? I honestly have no idea haha. The brown suggests it's iron-rich something and the layer between is a sandstone or shale. Interesting find.
Granite is Radioactive?
Yes. Is it dangerous? No. It is very slightly radioactive but does not exceed normal background radiation levels. If the room is made of granite and has poor ventilation, then the raised radiation levels can be measured, but it's still not anywhere near dangerous.
If you have a granite countertop, then it just very slightly raises the background radiation level.
It's radioactive because granite often contains small amounts of uranium, thorium and potassium, especially feldspar.
Here is a table of what color granite is the "most" radioactive. Again, the most means just small trace amounts of radioactivity.
| Most radioactive color | Reason |
|---|---|
| 1.Red/black | Lots of K-feldspar(potassium-40)+accessories(mica,uranium,zircon) |
| 2.Gray/light | Balanced( little K-felspar and maybe mica) |
| 3.Just Black | Not true granite, low K content |
| 4.White/Glassy | Low K content, low mica |
So, this granite in the photo below is technically the most radioactive due to Potassium-40 + accessory elements.
This red/black granite is also quite a rarity. Its plenty of it in our area of the world( Scandinavia and the Baltic), but its a little of it in other parts of the world. Yes, it can be found, but you'd have to be lucky.
Overall, the granite here in Estonia is very old. This is because it originates from Scandinavia and this area around here is in fact one of the oldest crusts in the world. It's named the Fennoscandian shield. So it's safe to assume that the granite on our beaches is 1.4 to 2 billion years old. What a history when you think about it.
Well, hope this post gave you some knowledge you definitely didn't think you needed. I will probably do more of these kinda posts in the future, about random stuff I want to teach myself and then share it.
