Tuff Rock
"Tuff rock
is a common pyroclastic
rock."
It forms when explosive stratovolcanoes erupt and throw ash, debris, other rock parts, lava, gases and steam into the air.
The fragments solidify and later, are cemented together.
If it is hot enough they are "welded" together, forming welded tuff.
If the mixture contains rhyolite, rhyolite tuff is formed.
It contains biotite, feldspar, quartz, and glassy, pumiceous fragments.
If the mixture contains trachyte, trachyte tuff forms.
It contains mainly sanidine or anorthoclase, and lesser amounts of feldspar, quartz, hornblende, augite and biotite.
If the mixture contains andesite, andesitic tuff forms.
It contains quartz, chlorite, calcite, chalcedony and epidote.
Basaltic tuffs form from basaltic rock material.
They are dark in colour and vary in grain size.
While andesitic and basaltic tuffs are common, ultramafic ones are very rare.
They are associated with kimberlites - the rock that contains diamonds.
By Jon Wiley
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