Geology Reference
In-Depth Information
Single-pulse collapse from
Vulcanian/Plinian column
Sustained low fountaining
highly unsteady current
quasi-steady current
(a)
(c)
Single-pulse lateral blast(s)
Sustained fountaining
in Plinian column
highly unsteady
current
quasi-steady current
(d)
Single pulse dome collapse
and avalanche
Styles of pyroclastic flows
from explosive eruptions
(from Branney & Kokelar 2002)
highly unsteady
current
(b)
(e)
Figure 5.14
Schematic of the styles of pyroclastic eruptions (from Branney and
Kokelar, 2002).
slope as density currents known as pyroclastic flows (previous terms include
Nuee ardent - firey cloud, a term now more commonly associated with bock
and ash flows). It is important to recognise that such mixtures of fragmented
material can form in a number of ways during an eruption which will result in
different currents and concentrations of material, for example Figure 5.14.
Another important thing to remember is that an ignimbrite deposit may well
have formed over a protracted time, during which different currents and concen-
trations of material are involved in the deposition. You must think of the deposit
building up through time (which could be minutes-hours even days in duration)
from the base up, with the deposit being created at a boundary zone (the progres-
sive aggradation model in Figure 5.15). As the flow velocity increases (waxes)
and decreases (wanes), the style of the resultant deposit will reflect this.
5.2.2 Common ignimbrite deposit characteristics
Non welded ignimbrites
can be quite un-consolidated and are susceptible to
weathering very quickly after their deposition. Sometimes units can be lithified
early as the constituents form a natural cement type mixture so may be preserved
