I was recently looking at my rock collection that is sitting in boxes in my basement. Unlabeled and not in any cases…I just got my hands on a new case, and am trying to decide which to put int it. I’m looking at some lava I have from a lava tunnel I was in in Arizona. It’s full of holes, it quite light and is black and brown.
I then look at a piece of hard as hell, granite with a really cool vein of marble running through it. I can see how this must have taken some serious pressure and heat to make…That got me wondering.
How hot does granite have to get to liquify? Or turn into Molten granite? I’m assuming it’s in the 10,000+ degree mark, or even uncalculable…which I doubt.
So how do I get my hands on some molten granite? Figuratively speaking of course.
The Wikipedia article on granite answers some of your questions and gives some supplemental information that confirms what I thought I knew – granite normally forms from the cooling of magma under substantial pressure deep underground. The same substance emitted from a volcano is rhyolite, and characterizes the explosive Pelee-Krakatoa-St. Helens type of eruption. (Hopefully a geologist will wander by and tell how to distinguish the two rocks.)
For semi-obvious reasons (a melting point varies greatly with differences in pressure), the Wiki article doesn’t give a specific melting point, but suggests that granites normally form around the “eutectic temperature” – the temperature at which different constituents of a magma mixture begin separately settling out as solids.
If that’s the case we have the interesting dichotomy that one can melt granite moderately easily, if with a high temperature – but one cannot then fuse (“freeze”) the molten product back into granite, as at surface pressure it will form rhyolite instead.
No – magma is simply the term for molten rock at depth (lava being magma that has emerged onto the surface but not yet cooled past its melting point). You can have different varieties of magma, just as you have different kinds of oil.
Actually, dacite is quite predominant at Mount St. Helens. The explosive nature of St. Helens is because the magma is like that of your mother’s home-made fudge, as compared to the molasses-type magma emitted by the Hawaiian volcanoes. St. Helens eruptions are quite explosive because “the fudge” caps building pressures up to a point when all hell breaks loose.
Well, all right, but the obvious next question is: if I want to melt some granite at home, how do I go about doing it? Like, say I want a new poured-lava countertop in my kitchen.
The rate of temperature loss has more to do with it than the pressure, I seem to recall from school. The longer the magma takes to cool down, then the larger the individual mineral crystals are- that’s why you’ll have a blackish gabbro or granite that cooled slowly with visible crystals, and a solid black basalt that cooled quickly from mafic magma or a lighter granite/rhyolite from felsic magma. (mafic = Mg/Fe rich, felsic = SiO2 rich).
Basically since granite is formed underground, it tends to solidify much slower than magma that gushes out onto the surface, and therefore has a much more developed crystalline structure.
I’ve grown salt crystals (up to about 800 C, or about 1500 F) from the melt, but that’s wayyyy below the mp of granite. My furnace was a resistance heated furnace, and 800 C is hot enough, thank you. I’m not sure if you can get to 3500 F with an RF furnace, but you’d need something like that. I don’t think I’d want to be around when the melt hit your mold, even if you did have a heated mold. I suspect making granite counter tops is something better done at the factory (underground) than in the kitchen.
Exactly. Even if you had the right ingredients, it would be something other than granite if you poured it into a mould here at the surface. It takes time and pressure for it to become granite. You’d probably get something like obsidian if you cooled it quickly.
That would still be an awesome counter-top!
What *would * you get if you take the granite ingredients, melt them up here at the surface and then cool it quickly?
