**Abstract**

We report field observation, textural description (thin section and scanning electron microscope (SEM)) and mineral chemistry (backscattered electron imaging and dispersive X-ray analysis) for rhyolitic obsidian lavas from previously under described effusive Badi volcano, central Afar within the Ethiopian rift. These rhyolitic obsidian lavas are compositionally homogeneous and contain well developed flow bands. Textural analysis is undertaken to understand the formation of flow band, and to draw inferences on the mechanism of emplacement of this silicic volcano. Flow band arises from variable vesicularity (i.e., alternating domains of vesicular, light glass and non-vesicular, brown glass). Such textural heterogeneities have been developed during distinct cooling and degassing of the melt in the conduit.

**Keywords:** Afar, Badi, flow band, lava, pyroclastic

## **1. Introduction**

Many rhyolite lavas are usually associated with pyroclastic deposits [1–3]. In fact each lava eruption is almost invariably associated with preceding phases of explosive pyroclastic activity [4–6]. This suggests that lavas could be a terminal event of many explosive eruptions during which most of the volatiles of the magma have been removed. Even while the rhyolite lavas being growing explosive activity may continue, as evidenced by the presence of unusually large amounts of obsidian ejecta among the pyroclastic deposits [4].

The principal requirement for the effusive (not explosive) eruption of magma as coherent lava is that the exsolved volatile content of the magma immediately before eruption should be sufficiently low to prevent the build-up of a gas pressure which could cause explosive fragmentation of magma and country rock [7]. Nevertheless, sufficient water is initially available in the magma source regions [3]. Therefore, for coherent magmas to be erupted from magma sources with high volatile contents the magma has to degas [7].

Volcanologic and petrologic studies on the silicic centres which lie within the Afar axial range or off the axis are very scarce. This is partly because of the remoteness and inaccessibility of the area that practically inhibits field investigation. The very few previous studies mainly focused on the extensive basaltic flow fields

#### **Figure 1.**

*Topographic relief map of central Afar, showing the location of the off-rift axis Badi volcano. Dashed line represents the 2005 dyke injection (after [21]).*

[8–13], which have been interpreted as incipient oceanic ridges. However, there are also comparable volumes of silicic magmas to that of the basaltic counterparts in the region [14–16]. Very little is known about these silicic centers which form a conspicuous central edifice in the axial range of the Afar magmatic segments [15, 17–20].

This study presents field observation; textural description (thin section and scanning electron microscope); and mineral chemistry (backscattered electron imaging and dispersive X-ray analysis) for previously undescribed Badi volcanic edifice from central Afar, Ethiopia (**Figure 1**). Contrary to many localities, whereby rhyolitic lava domes and flows are usually associated with pyroclastic deposits, the Badi volcanic edifice contains several clusters of coalescing silicic domes and lava flows, and sparse evidence for pyroclastic rocks. Thus, this volcano offers a relatively rare opportunity to study effusive silicic volcanic phenomena. The motivation of this work is to understand the mechanism of emplacement of Badi silicic domes and flows with hope to draw inferences on the formation of the flow bands. In order to address these questions we employ textural analysis of the lavas using petrographic microscope and scanning electron microscope (SEM), accompanied by mineral chemistry. To the knowledge of the authors, there are no earlier studies of this kind documented in the literature from the region so far. Petrologic and geochemical studies of some silicic volcanic rocks from Afar including those from Badi have been considered elsewhere [17–20, 22] and are omitted from the forthcoming discussion.

## **2. Geologic background**

Afar depression (the Afar triple junction), roughly 300 km wide, marks the intersection of three rifts: the Red Sea, Gulf of Aden and east African rifts. This extensional province formed within a Palaeogene Ethiopian flood basalt province associated with the Afar mantle plume [23, 24]. Rifting within the Red Sea and Gulf of Aden arms of the triple junction has progressed to oceanic spreading [25], whereas the less-evolved Ethiopian rift is transitional from continental rifting to

*Effusive Badi Silicic Volcano (Central Afar, Ethiopian Rift); Sparse Evidence for Pyroclastic Rocks DOI: http://dx.doi.org/10.5772/intechopen.98558*

oceanic seafloor spreading [26]. The crust of the Afar depression is highly extended and intruded with mafic dykes [27]. Crustal thickness ranges from 16 km in the north beneath Erta'Ale range to 24 km in the south [28].

Within the southern Red Sea rift and Afar, the initial development of border faults was roughly coincident with the 31–29 Ma flood basalt sequences in the same area [29]. Strain migrated riftward from 19 to 12 Ma [29], and by ~5 Ma, an oceanic spreading ridge had developed within the south central Red Sea rift [30]. Southward propagation (south of 16°N) of the ridge runs inland through Ethiopia, whereby extension (faulting and dyking), seismicity and volcanism are localized in discrete narrow (<10 km wide) ~60 km long rows/zones within the Afar depression [31]. These rows are referred to as magmatic segments and are characterized by aligned chains of basaltic cones with associated flow fields, shield volcanoes, shallow seismicity and positive gravity anomalies [26]. The available K-Ar data for basaltic and silicic rocks along the terrestrial portion of the Red Sea rift system indicate an age range of 1.46–0.52 Ma [32]. Within these magmatic segments, volcanism tends to be bimodal, with extensive basaltic flow fields and axial silicic centers [15, 18–20, 22]. Profuse fissure basalt volcanism, referred to as "Stratoid Series" [33] covering most of the central and southern part of the Afar depression, occurred about 5 Ma ago where it was most active between 4.5 and 1.5 Ma [34].
