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Name: Liz
Status: Student
Grade: 9-12
Location: NC 
Country: United States
Date: September 2006

What is the biggest observable difference between a crystallized above ground rock versus one that is crystallized deep underground?

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.

Pat Rowe

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.

Robert Avakian

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