Fragments of rocks and lava that are ejected
from volcanoes, and the rocks that are made up from these
fragments. The term excludes lava flows but includes most
other material that is thrown out of volcanoes during eruptions.
The term is closely related to volcaniclastic sediments
and rocks, another general term for fragmental rocks or
material that includes some volcanic fragments. Volcaniclastic
material may be reworked and redeposited away from the
volcano, whereas pyroclastic rocks are those deposited directly
by the eruption. Volcanic blocks are solid materials thrown
out of the volcano, whereas volcanic bombs are hardened
pieces of lava that were ejected while still in the molten state.
Pyroclastic deposits typically contain a mixture of material
that ranges in size from fine ash to blocks and bombs
that may be as large as houses. Most of the very large blocks
and bombs are deposited very close to the volcanic vent and
may form a deposit type called an agglomerate, in which
large volcanic clasts are mixed with fine-grained ash. Smaller
particles are deposited farther away, and ash may travel hundreds
or thousands of kilometers and spread around the
globe if it is ejected into the upper atmosphere. In some cases,
hot volcanic ash clouds known as nuée ardentes rush down
the sides of volcanoes, destroying all in their path. Rocks
formed from hardened ash are known as tuffs, or welded
tuffs if they are hardened by the heat of the ash, and ignimbrites
if they contain flattened pieces of ash known as
fiamme. Some volcanic eruptions generate rainstorms, and
when the rain drops fall through the ash cloud they grow
concentric rings of ash particles known as lapilli that fall to
the ground as mud balls. When preserved, these mud ball rich
ash deposits are called lapilli tuffs.
See also NUÉE ARDENTE; VOLCANIC BOMB; VOLCANO.
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