Granite is a light-colored, igneous rock containing large mineral grains. It forms from the slow crystallization of magma, which is underground lava were some of the first rocks on Earth. Considering the Blue Ridge Mountains are between 250,000,001 billion years old, and the granite countertops you see in our region are older than the first dinosaur that walked the Earth. Think about that the next time you’re making lunch.
Granite gets its unique coloring because while it hardens deep underground, it cools slowly enough for mineral crystals to grow, becoming visible to the naked eye. The most common colors are pink and white and variations of gray and black. Some of the world’s finest white Granite can be found in North Carolina. The white comes from an abundance of quartz in our mountains. Mt. Airy, North Carolina, nicknamed the Granite City, is home to the largest open-faced granite quarry in the world.
15 Fun Facts About Granite
Granite is some of my favorite rocks. They also make up a large part of the Earth’s continental crust. We can also find them in the human world, used as decorative stones on the front of buildings, on kitchen counters, in cafes, and our own homes. Granite is one of the oldest, hardest, and most abundant rocks in the Earth’s crust. Here are 15 interesting facts about Granite.
1. Granite is a grand igneous rock rich in silica, aluminum, and potassium-bearing minerals.
2. Even though Granite is some of the most common crystalline rocks, they’ve been a bit of a mystery for a long time. Geologists had no idea how they formed. We know where basalt and other volcanic rocks come from because we can see them erupt from volcanoes.
So geologists reasoned that if we can see basalt erupt from a volcano. Gabbro has the same chemistry and mineralogy as basalt, except it’s coarse-grained. It must have had a similar origin derived from partially melting the Mantle. It’s been in place at depth, so the crystals have cooled slowly and had a long time to grow.
We can’t quite do that for granite soil because even though there are things like Rhyolite, which is silica, rich volcanic rocks have a composition very similar to Granite.
3. The occurrence of Rhyolite doesn’t explain the types of chemistries and mineralogy and occurrences that we see in all the different types of granites.
4. Granites formed underground where humans and our instruments can’t get to. So we can’t watch them as they form, as we. The same way that we can watch basalt being erupted and forming.
5. Granites are different from basaltic rocks in how they interact with the host rocks you find them with. Basaltic rocks tend to erupt and form sheets, and walls called sills and dikes, whereas granites form huge underground blobs called basalts. These structures seem to have violently intruded, or the rocks or the Granite blasted into the host rock below. Often there’s very little of that original host rock remaining. So where do that millions of tons of rock go? It can’t disappear.
6. While some granites are associated with basaltic rocks, and in volcanic settings, we’d expect to find igneous rocks or the granites found in places that are not associated with any volcanism, like in the core of mountain ranges or the middle of thick continental crust.
7. The mysterious origin of granites for a long time was known as the granite problem, which is the most interesting fact.
8. Some geologists pointed out that the chemistry of granite and rye lights is very similar. The Granite must be the final stage in a process called fractional crystallization, and it’s the same process that forms basaltic rocks from partially melting the Earth’s Mantle.
Therefore, they reasoned that Granite must be the intrusive equivalent of Rhyolite in the same way that gabbro is an intrusive version of basalt. But that doesn’t explain the chemistry of all granites or the fact that some granites don’t occur associated with Rhyolite or volcanic rocks at all.
9. Other geologists found granites with chemistry almost identical to sedimentary rocks like mudstones. These grants were often found intruding into sedimentary rocks and contained deformed and cooked chunks of the host rock.
10. Geologists argued that Granites were formed by altering pre-existing sedimentary rocks by melting a process called Ana Texas or by altering existing sedimentary rocks with mineral-rich fluids. That’s a process called metal Semitism. It doesn’t explain the granites with chemistry and the third type of Granite.
11. There’s a set of granites with unusual chemistry. For example, many rare earth elements occurred in the middle of continents, a long way from volcanic activity or mountain-building events.
12. Some geologists proposed that the ants were formed deep under the continent when they had already metamorphosed. Rocks have been melted by unusually hot areas of the Mantle that we call hot spots. Again, though, this doesn’t explain all granites.
13. Different kinds of granites form in different tectonic settings. That’s places within the Earth where active geology is happening.
14. The Buncombe County judicial complex used over 23,000 square feet of white Mt. Airy Granite. So the next time you see Granite in a kitchen building or monument, think about all the science that went into this beautiful rock.
15. Granite is used for everything from countertops to buildings, facades, and sculptures.
Different types of Granites and their facts
The research I’ve been talking about became the basis for the modern granite classification system, also known as the alphabet system. The alphabet system has three main types of Granite, covering most of the granites that you’ll see out in the wild and as building stones and countertops.
The M stands for Mantle. These granites were originally derived from the Mantle by fractional distillation. They represent the final silica-rich contents of a magma reservoir where all the poor silica material has already crystallized and been erupted as basalt or intruded as cobras and in other volcanic products.
You can see my gabbro episode for more information about how that works. An example of an M-type granite would be the iron granite on the Isle of Arran in Scotland. It was 50 million odd years ago! After all, basalt, gabbro, and other types of volcanic products erupted right at the end. We had all of this rich silica material that bobbed up in various places like Arran and Skye.
We ended up with these large bath lists of M-type Granite and S-type granites produced by melting pre-existing igneous rocks for I-type and sedimentary rocks for S-type. It usually occurs during the final stages of ocean closure and mountain building. Subduction zones carry much water into the crust in all that mud and oceanic crust rock.
This lowers the melting point of rocks, allowing granitic magmas to form because they’re warm and less dense, and the surrounding material blows up into the edges in the middle of mountain chains. So a good example of this is The Caledonian Granites in Scotland, classic S-type Granites. Some of the Jurassic granites in the Sierra or some of the Jurassic granites in the Sierra Nevada in the US.
They’re nice type granites as well. The final type of Granite we’ll talk about is an air-type granite, which is androgenic they’re not involved with. They’re not formed from mountain building. These granite types form when a mantle plume, like a big jet of heat in the Mantle, heats the underside of a thick continental crust called a craton. That causes the melting of rocks that have already been heavily metamorphosed. So what we call granite rocks and the melting of these rocks produces these air-type granites.
Yellowstone Park is one of the places on modern Earth we can find this happening. So you get a generation of air-type granites and Rhyolite there. This process can also lead to continental rifting.
So A-type granites can often be found in early rift settings as well. Granite intrusions, especially S-type Granites, can produce all kinds of exotic minerals, but for a rock to be classed as Granite, it needs to contain at least 20 to 60% quartz, and the feldspar needs to be composed of 35 to 90% alkali feldspar.
Have a look at some granites on your own. If you find any cool ones, let me know in the comments below.
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