Geology Reference
In-Depth Information
Figure 5.4
Cross bedding tractional structures in the basal parts of the Minoan
ignimbrite deposit, Santorini, Greece.
about 1000 - 2000 kg m
−
3
) to poorly vesiculated or non-vesiculated rock
fragments (density over 2000 kg m
−
3
). Pumice is generally taken to apply
to fragments of intermediate or evolved (felsic) chemical composition of
density about 1000 kg m
−
3
or less. Scoria is the term used for dense vesicular
fragments of basic or intermediate composition. The term
glass shard
is applied
to ash-sized fragments. (Table 5.1) resulting from the fragmentation of bubble
or scoria walls, so tending to have Y-shaped or cuspate shapes (Figure 5.8a).
Crystals
may be derived by fragmentation of partially-crystallised porphyritic
magmas and often may behave as a distinct population during transport and
deposition within pyroclastic deposits (Figure 5.8c - e).
Lithic fragments
(Figure 5.8f) are usually the densest components of a
pyroclastic deposit and include non-vesiculated juvenile fragments which are
genetically related to the volcanic host, termed
cognate lithics
, and accidental
country rock fragments incorporated during eruption or transport and which
are, respectively, termed
accessory
and
accidental lithics
.
Tephra
(Greek, for ash) is a generic term for any airborne pyroclastic
accumulation usually given to unconsolidated deposits. The use of
tephra
layers, which bear their own unique chemistry and character, as temporal
marker horizons is known as
tephrochronology
.
