Basaltic Glass & Scoria

Both of these extrusive igneous rock are made of some or all of the same minerals: Plagioclase feldspar, biotite, pyroxene, amphibole, and olivine (see green zone in chart below). Both formed extremely rapidly on the earth's surface, so fast that crystals had no time to grow. These non-crystalline rocks are said to have a glassy texture because they are glass.

The difference between them is in the gas content of the magma from which they formed. Basaltic glass formed from magma without gas. It has a smooth, glassy appearance much like obsidian. Scoria, on the other hand, is formed from gas-rich magma. As it cools, gases escape forming a very rough, vesicular texture (bubbles and gas pockets).

Basalt Glass

Coal. Image from http://www.mii.org/Minerals/photocoal.html

 

 

 

 

 

Basalt Glass

Scoria

Scoria

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