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A general guide for using the Forum with some rules and tips
Rocks melting
  
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Tom Mazanec




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PostPosted: Feb 25, 2019 13:11    Post subject: Rocks melting  

How do most rocks melt if they are heated enough? Do they just all turn liquid at once, like ice at 0C? Does some of them melt and the rest not, like a sludge or slurry?
Does it just gradually soften like glass?
Something else?
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Kevin Schofield




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PostPosted: Feb 25, 2019 14:45    Post subject: Re: Rocks melting  

Tom Mazanec wrote:
How do most rocks melt if they are heated enough? Do they just all turn liquid at once, like ice at 0C? Does some of them melt and the rest not, like a sludge or slurry?
Does it just gradually soften like glass?
Something else?


The answer to this question would be sufficient material to fill a semester-long petrology class in the second or third year of a Geoscience BSc!

The very short answer is that minerals have different melting points under differing conditions of temperature and pressure. In very very simplistic terms then:

The conditions under which rocks melt are generally encountered in subduction zones, where one continental plate "dives" beneath another. Eventually, the combined forces of friction, increasing temperature and increasing pressure will melt the plate going down, and sometimes part of the plate that it is going under. The melts produced are relatively buoyant, and then work their back towards the surface by a variety of mechanisms. Depending the chemical composition of the melt, they may recrystallize at depth as basic intrusive rocks (gabbros for example) or acid intrusive rocks (granites), or eventually work their way to the surface as volcanic lavas (which again may be basic basalts or acidic andesites).

The different melting points of minerals are maybe more easily visualized from the way they drop out of a melt (ie crystallize). If you look at any slab of granite in a stone yard, it will likely be obvious which crystals crystallized first (big, well-formed crystals) and which dropped out later (filling the spaces between the big perfect ones). Most melts as they crystallize will, at some point, resemble the "slurry" you mention above. In some cases the crystal "mush" circulates in the magma chamber as a result of convection, and the solid crystals act as "sand grains" in the melt and form cross-bedded structures like those seen in sand-dunes. If you ever get the opportunity to go to the Isle of Mull, Western Scotland, you can see this phenomenon in the gabbroic intrusions exposed there in the hills.

More interestingly (maybe?!), there are places deep in the earth where the rocks are neither strictly a solid nor a liquid, but have a physical consistency known as "rheid". This is a form of plastic solid that is able to flow. It is the layer at the base of the crust upon which the continents "float" and which enables plate tectonics to "work".

Possibly more than you ever needed to know!

If you Google all the geological BS Bingo words in the paragraphs above, you will likely learn even more :-)

Have fun,

Kevin

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Pete Modreski
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PostPosted: Feb 25, 2019 15:07    Post subject: Re: Rocks melting  

Hi, Tom,

Kevin's answer to you was pretty good, and as I was getting to ready to write myself anyway, I'll just try to add a few more comments.

For different kinds of rock, you pretty much answered your own questions; the answer is, some rocks, behave in about all the different ways you described yourself.

Some, like obsidian, might soften gradually. Others, depending on how high a temperature you brought them to, would form a "mush", partly crystalline, partly molten. Others, like pure quartz or most sandstone, and limestone, have such a high melting temperature that they would, in most furnaces, not melt at all, or would require a super-hot heating source, like an oxy-acetylene or oxy-hydrogen torch. Most rocks, even when molten, would be very thick (viscous), like thick tar, and would scarcely flow at all, unless heated well above their minimum melting temperature.

I'll just leave my answer at that!
Sincerely, Pete
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