The term pyroclastic
is commonly used to refer only to volcanic materials ejected(噴出) from a volcanic
vent. But there are several other ways to make volcanic particles. Volcaniclastic
includes all volcanic particles regardless of their origin.
A sediment may be
defined as a mass of particles that has been deposited or is being transported
on the earth's surface from one place to another and deposited by flow or
fallout processes, a combination of these, or by chemical precipitation. By this
definition, volcanic particles are deposited as sediments, the principal
differences with nonvolcanic sediments being in some of the physical processes
by which the particles are formed. Some volcanic particles are generated by
weathering and erosion (epiclastic, discussed below) and therefore differ only
in composition from nonvolcanic clasts. Other volcanic particles are formed
instantly by explosive processes and are propelled at high velocities (>100
m/s) along the surface of the earth or high into the atmosphere (>40 km above
the earth).
Generic types of
Volcaniclastic Particles
- Pyroclastic particles (pyroclasts)
form by disintegration of magma, as gases are released by decompression and
then ejected from a volcanic vent either in air or beneath water.
- Hydroclasts form by steam explosions
from magma-water interactions.
- Autoclastic fragments form by
mechanical friction during movement of lava and breakage of cool brittle
outer margins, or gravity crumbling of spines and domes.
- Alloclastic fragments form by
disruption of pre-existing volcanic rocks by igneous processes beneath the
Earth's surface.
- Epiclasts are lithic clasts and
minerals released by ordinary weathering processes from pre-existing
consolidated rocks. Volcanic epiclasts are clasts of volcanic composition
derived from erosion of volcanoes or ancient volcanic terrains.
It is advisable to
distinguish between epiclasts and other volcaniclastic fragments to determine
contemporaneity(同時代) of volcanism and
sedimentation(沈澱作用). Terms such as pyroclastic,
hydroclastic and epiclastic refer to the processes by which the fragments
originate. A pyroclast therefore cannot transform into an epiclast merely from
reworking by water, wind glacial action, etc. This difference is important for
reasonable interpretations because the sediment supply rates commonly differ by
orders of magnitude between degrading ejecta piles and eroding epiclastic
terrain(地勢).
Varieties of
Pyroclastic Ejecta According to Origin
- Essential (or juvenile).
These are pyroclasts derived directly from erupting magma and consist of
dense or inflated particles of chilled melt, or crystals (phenocrysts) in
the magma prior to eruption (phenocrysts).
- Cognate (or accessory).
Cognate particles are fragmented comagmatic volcanic rocks from previous
eruptions of the same volcano.
- Accidental. Accidental
fragments (or "lithoclasts") are derived from the sub-volcanic
basement rocks and therefore may be of any composition.
Three common names of
pyroclasts that depend in part on their degree of vesicularity are pumice,
scoria and cinders. They are named without reference to size, but usually are in
the lapilli(火山爍)
or larger size range.
Pumice is a
highly vesicular glass foam, generally of evolved and more rarely of basaltic
composition with a density of <1 gm/cubic cm; bubble walls are composed of
translucent glass.
Scoria (also
called cinders), usually mafic, are particles less inflated than pumice.
They readily sink in water. They are generally composed of tachylite,
that is, glass rendered nearly opaque by microcrystalline iron/titanium oxides.
Spatter
applies to bombs, usually basaltic, formed from lava blebs that readily weld
(agglutinates) upon impact and contrasts with scoria that do not stick together.
Scoria (or cinder) cones, for example, are composed largely of loose particles;
spatter cones are composed mainly of agglutinated blebs or larger isolated lava
tongues.