Crystals Formed Above and Below Ground
Country: United States
Date: September 2006
What is the biggest observable difference between a
crystallized above ground rock versus one that is crystallized deep
Typically, longer crystallization times underground yield larger crystals
than shorter crystallization times above ground. This is why granite, which
cools underground, has large crystals. Basalt, which cools above ground, has
smaller crystals. This difference in crystal size is the most obvious
visible characteristic distinguishing intrusive from extrusive igneous rocks.
In general, rocks that cool underground have large, visible crystals.
The slow cooling gives the crystals longer to grow.
Rocks cooled at, or near the surface cool quickly and tend to be glassy
or have small, almost invisible crystals. Their crystals have almost no
time to grow big.
Some rocks that cool at or near the surface may have large crystals in
them. But, these phenocrysts are the result of the liquid rock
dissolving older rocks with and capturing old crystals, or the rock may
have partially cooled underground and then come to the surface to
complete its cooling. In these cases, look at the rock between the
crystals. It is fine grained or glassy.
Rock cooled completely underground, is made up of crystals alone.
For some interesting information on how liquid rock (magma) cools, look
up the work of N. L. Bowen or Bowen's Reaction Series.
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Update: June 2012