Batur volcanic field (BVF) in Bali, Indonesia, underwent two successive
caldera-forming eruptions, CI and CII (29,300 and 20,150 years B.P.,
respectively) that resulted in the deposition of dacitic ignimbrites.
The respective ignimbrites show contrasted stratigraphies, exemplify the
variability of dynamics associated with caldera-forming eruptions and
provide insights into the possible controls exerted by caldera collapse
mechanisms. The Ubud Ignimbrite is widespread and covers most of
southern Bali. The deposits consist dominantly of pyroclastic flow with
minor pumice fall deposits. The intra-caldera succession comprises three
distinct, partially to densely welded cooling units separated by
non-welded pyroclastic flow and fall deposits. The three cooling units
consist of pyroclastic flow deposits only and together represent up to
16 distinct flow units, each including a thin, basal, lithic-rich
breccia. This eruption was related to a 13.5x10 km caldera (CI) with a
minimum collapsed volume of 62 km(3). The floor of caldera CI is
inferred to have a piecemeal geometry. The Ubud Ignimbrite is
interpreted as the product of a relatively long-lasting, pulsating,
collapsing fountain that underwent at least two time breaks. A stable
column developed during the second time break. Discharge rate was high
overall, but oscillatory, and increased toward the end of the eruption.
These dynamics are thought to reflect sequential collapse of the CI
structure. The Gunungkawi Ignimbrite is of more limited extent outside
the source caldera and occurs only in central southern Bali. The
Gunungkawi Ignimbrite proximal deposits consist of interbedded
accretionary lapilli-bearing ash surge, ash fall, pumice lapilli fall
and thin pyroclastic flow deposits, overlain by a thick and massive
pyroclastic flow deposit with a thick basal lag breccia. The caldera
(CII) is 7.5x6 km in size, with a minimum collapsed volume of 9 km(3).
The CII eruption included two distinct phases. During the first,
eruption intensity was low to moderate and an unstable, essentially
phreatomagmatic column developed. During the second phase, the onset of
caldera collapse drastically increased the eruption intensity, resulting
in column collapse. The caldera floor is believed to have subsided
rapidly, producing a single, short-lived burst of high eruption
intensity that resulted in the deposition of the uppermost massive
pyroclastic flow.