Strato-volcano formation.The lowermost exposed volcanic deposits in the walls of the Tambora caldera are termed the strato-volcano formation. This formation is particularly prominant in east and south walls of the caldera, but reaches also the caldera rim, at peak 8870 ft in the northwest, in dike ridge, in peak 9039 ft in the west and in forest ridge in the southwest.
The old Tambora strato-volcano is composed of interbedded sequence of lavas and pyroclastic deposits, with approximately 40% of the succession represented by lava flows. The lavas are typically 1 to 4 m in thickness, often discontinuous laterally, and always steeply dipping (20° to 30°) away from the caldera center. The formation was studied closely at peak 8870 ft in the northwest, where the lavas are interbedded with scoria, tuffs and pyroclastic flows and falls. Near dike ridge, a dome-like unit occurs in lower part of the formation, associated with extensive dome breccias.
There are numerous dikes in the strato-volcano formation, mostly 0.5 to 3 m in thickness, and their orientation may be radial to the volcano. In the eastern wall of the caldera some of the dikes extend all the way from the caldera floor to within a few meters of the rim. An unconformity occurs within the formation, with generally older deposits in north and east sectors, and younger part of the formation in south and east sectors of the caldera walls.
The characteristics of the strato-volcano formation suggests the existence of a high volcanic cone, with a central vent emitting frequent lavas that cascade down a steep slope. Much of the pyroclastic material may be produced by fragmentation during flow, producing thick scoria beds. Intermittent explosive eruptions have produced pumiceous pyroclastic flows, e.g. below abandoned gulley.
The strato-volcano formation forms the sockle and flanks of Tambora and is thus the pricncipal foundation of the volcano. In lower parts of the caldera walls, the formation is extensively hydrothermally altered. Some of this alteration is due to the currently active hydrothermal field associated with the 1815 caldera ring-faults, while some alternation is much older.
Early caldera. The evolution of the strato-volcano was terminated by an event that produced a caldera-like in western part of the stratocone. This early caldera extends from forest ridge in the south to peak 8870 ft in the north, or approximately 4 to 5 km in diameter. It is asymmetric with the 1815 caldera, and western limit of the early caldera is about 1 km west of the 1815 caldera, as defined by the peak and ring-faults at the east end of the camp ridge. Eastern extent of the early caldera is unknown but less than the 1815 caldera. Depth of the early caldera is approximately 500 m less than the young caldera, or about 700 m. Early caldera is suspected formed as a result of an explosive eruption, or an ignimbrite forming event.
Caldera lava formation. The early caldera is in part filled in by a series of horizontal and thick lava flows which have banked up against the steep caldera walls. There was no evidence that these lavas have extended to the flanks of the volcano and they seem to be entirely confined within the early caldera. These lavas filled in about two-thirds of the volume of the early caldera. Up to 16 lavas are present in the succession; many are 15 to 20 m in thickness, separated by scoria and tuff interbeds of several meters in thickness. Overall, the formation is about 300 m to 400 m in thickness.
The caldera lavas have not been intruded by dikes, nor have they been hydrothermally altered, and appear to be quite young. They are typically clinopyroxene-phyric, often with phenocrysts of plagioclase and olivine also. Their setting and character resembles that of the intra-caldera lavas extruded after the 1815 eruption.
Black sands formation. In western part of the 1815 caldera wall, a 100 m thick deposits of black sands is exposed. This unconsolidated formation is easily eroded and forms a prominent ledge in the caldera wall, separating the caldera lavas below from the 1815 pyroclastic deposits above.
Brown tuff formation. Overlying the black sands formation is a 5 to 10 m thick deposit of brown to grey-brown, semi-consolidated tuffs interbedded with yellowish grey pumice and scoria falls. The brown tuff formation is generally conformable with the black sands formation, but sometime interval must have elapsed between the eruption of the two, as indicated by extensive plant remains in the brown tuffs.The brown tuff formation is the last formation erupted prior to 1815. It represents both low-level plinian activity and pyroclastic surge generation.
1815 proximal deposits. This proximal deposits consist of pyroclastic fall and pyroclastic flows deposits. Proximal deposits of the 1815 eruption are very well exposed in the caldera walls. Their deposition was profoundly influenced by the morphology of the early caldera, which was not yet filled when the 1815 eruption began.
Post 1815 activity.
During or shortly after the caldera formation, large slumps occurred from
the steep caldera rim. These slumps form several very prominent debris
flows at the foot of the caldera walls, and extend one or two km onto the
caldera floor. Some of these slumps contain brecciated strato-volcano formation
that still retain their stratigraphic characteristics.Following the slumping
phases, the caldera floor has been covered by fluvial sands and gravels
washed in from gullies in the rapidly eroding caldera walls. These fluvial
deposits form mudflows in and around the ephermal caldera.