MadSci Network: Earth Sciences

Re: How is Granite formed

Date: Mon Sep 18 11:56:34 2000
Posted By: David Kopaska-Merkel, Staff Hydrogeology Division, Geological Survey of Alabama
Area of science: Earth Sciences
ID: 968893153.Es


Granite is an igneous rock, which means it forms when hot liquid rock 
cools. You are probably familiar with volcanic rocks (rocks that exit 
volcanoes in a liquid state and cool to solidity at the Earth's surface). 
These rocks are extrusive; they come out of the earth, or are extruded. 
There is another kind of igneous rock called an intrusive rock (because it 
forms where liquid rock pushes its way in somewhere in the subsurface, or 
intrudes). Granite is an intrusive rock: it forms far underground. If you 
have ever looked at a lava rock, you probably did not see any crystals. 
That is because the crystals are tiny; the rock cooled too quickly for the 
crystals to grow large. But the rock is made of nothing but crystals (or 
glass, which forms when even the tiniest crystals do not have time to 
grow). Granite is made out of big crystals: you can easily see them with 
the naked eye. This is because the granite cooled slowly underground, 
shielded by the rock surrounding it so that its great heat could escape 
only slowly. Granite is made out of feldspar and quartz, plus minor 
amounts of other minerals. Feldspar and quartz are light minerals. They 
are light in color and they stay liquid until relatively low temperatures 
(less than 1,000 degrees!). Therefore, when hot molten rock is coming up 
from the mantle, some minerals grow crystals and then later others form. 
If the magma (liquid rock) keeps moving, those early crystals are left 
behind, until finally what's left is mostly the ingredients for feldspar 
and quartz. The result: granite. If a melt like one that could make a 
granite cools EXTREMELY slowly, then the crystals get very large. Single 
crystals can be meters long. This kind of rock is called a pegmatite, and 
it is a good place to collect beautiful mineral specimens. 

Hope this helps.

David Kopaska-Merkel
Geological Survey of Alabama
PO Box 869999
Tuscaloosa AL 35486-6999
(205) 349-2852
FAX (205) 349-2861

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