Photograph by J. Lowenstern in September 1990.
Scoria
Scoria is a vesicular (bubbly) glassy lava rock of basaltic to andesitic
composition ejected from a vent during explosive eruption. The bubbly nature
of scoria is due to the escape of volcanic gases during eruption. Scoria is
typically dark gray to black in color, mostly due to its high iron
content. The surface of some scoria may have a blue iridescent color;
oxidation may lead to a deep reddish-brown color.
Photo: Scoria exposed in
the cross section of a cinder or scoria
cone.
Did you know?
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Originally, scoria was the name for the silicate slag left over from
the smelting of ore. The word scoria, in fact, stems from the Greek
word for refuse or trash. Scoria resembles silicate slag; thus its
name.
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Because scoria is very hard and porous, it makes a wonderful base for
roads (road metal). In volcanic areas, scoria is often quarried from
cinder cones and laid out as the foundation for small country roads, or
used during winter to provide friction for automobiles.
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A highly expanded form of scoria is called reticulite, where bubble walls have
burst, leaving a three dimensional network of delicate glass threads.
Because reticulite is so expanded, some people call it basaltic pumice.