**5.2 Emplacement of rhyolite domes and flows**

Many rhyolitic obsidian flows and domes are commonly preceded or accompanied by explosive episodes [2, 3, 36, 37]. Two contrasting models have been proposed to explain the common sequence of initial, explosive plinian eruptions followed by quite effusions of lava: (1) the volatile stratification model; and (2) permeable foam model. In the former case, a stratification of volatiles in the source magma body is invoked to explain the initially explosive phase [1]. In the later case, one envisions a fairly uniform batch of magma that can release gas as it ascends through the fracture and porous conduit rock [7].

The Badi volcanic edifice is entirely constituted by several clusters of coalescing silicic domes and lava flows; there are no any explosive products associated with the effusive activity. This is in contrary to many rhyolitic obsidian flows and domes (e.g., Fentale and Gedemsa, Ethiopia; Inyo Dome, USA; Pantelleria, Italy) which are commonly associated with pyroclastic deposits [1–3]. These features are also common in most silicic volcanic centers of Afar in which pyroclastic rocks are usually scarce [15].

The fundamental question is whether extrusive rhyolite lavas of Badi volcano represent quenched dry rhyolite magma or have somehow degassed during ascent and eruption to prevent build-up of a magmatic gas pressure. The lack of hydrothermal manifestation, represented by direct escape of exsolving volatiles through the vent immediately before eruption, strongly suggests that the coherent lavas from Badi did not erupt from degassed magma source. Furthermore, amphibole phases are not observed in Badi rhyolites. The absence of amphibole phase in Badi rhyolites demonstrates that the water content of the pre-eruption magma was not enough to stabilize amphibole which requires about 3 wt.% H2O in a silicic magma to crystallize [3]. The absence of amphibole phase suggests that the Badi rhyolite domes and flows resulted from initially volatile-poor silicic magmas. Hence, the lack of progression from tephra ejection to lava extrusion, contrary to many rhyolite eruptive sequences, at Badi volcano reflects the lava must be nearly as dry as obsidian to escape fragmentation up on extrusion.

Effusions of silicic lavas often pile up over the vent area rather than traveling long distances (e.g., [1–3]), due to their high viscosity that prevents them from flowing far from the vent from which they extrude. It seems that the Badi rhyolites advanced outward. This might be related to their composition (**Table 1** and **Figure 6**) in that the Badi rhyolites are predominantly pantellerite with relatively high Na+ and K+ ion concentrations which act as network modifier (i.e., lowering the degree of melt polymerization) thereby relatively lowering the viscosity of the silicic magma [51]. Once extruded, the Badi lava flows outward (the average Badi flow is about 1.5 km) due to their relatively low viscosity.

In addition, the Badi rhyolite lavas are aphyric (with total phenocryst contents of <5%, **Table 1**), suggesting an extremely high magma temperature at the time of eruption. The high emplacement temperature implies that the rhyolite lavas reached the surface through a circular conduit, which presents a much smaller cooling surface to the country rocks [52]. The aphyric condition of rhyolite lavas has been

ascribed to unusually low viscosity [53]. The Badi lavas have flowed outward up to 1.5 km. Hence, these lavas may have had reduced viscosity due to their high magma temperature and peralkaline affinity, as the cause of the increased fluidity.
