Answer: Tuff is a volcanic rock made up of a
mixture of volcanic rock and mineral
fragments in a volcanic ash matrix.
Wherever there are explosive volcanic
eruptions you can expect to find tuff.
Tuff forms when some combination of
ash, rock and mineral fragments
(pyroclastics or tephra) are blasted into the
air, then fall to the ground as a mixed deposit.
Most of the rock fragments tend to
be volcanic rocks that were once solidified parts
of the volcano that erupted to
produce the tuff, but sometimes other types of
rock are blasted out and
incorporated into the tuff as well.
Welded Tuff:
Sometimes erupted material is so hot when it
reaches the ground that it fuses together to produce a welded tuff.
Tuffaceous Sandstone:
A
sandstone
which contains volcanic ash.
-- Excerpts from:
USGS/NPS Geology in the Parks Website, 2003, and
North Dakota Geological Survey Website, 2003
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U.S. Geological Survey