VOLCANIC TUFF
Volcanic tuffs have the same origin as volcanic breccias - explosive
volcanic eruptions. Volcanic tuffs lack the abundance of large, angular
grains present in volcanic breccias. Tuffs are fine-grained to very
fine-grained ash deposits that have become lithified. The degree of lithification
seen in tuffs varies. The sample shown below is very loosely consolidated
- individual grains can be easily separated from the rest of the rock.
Some ash beds remained so hot after initial deposition
that the heat was sufficient to self-solidify the beds. Such deposits are
called welded tuffs. Some welded
tuffs have glassy cores, where the ash was heated past the melting point after
deposition (for example, see the famous Resting Spring Pass Tuff east of
Shoshone, eastern California, USA)
Volcanic tuff