**4. Result**

#### **4.1 Field relation**

Badi volcano (located at 12.387°N and 40.366°E) lies off the axis of the main rift and is associated with a deviation in the strike of the faults (**Figure 1**). It is a welldefined rounded volcanic center with diameter of emerging from fissural basaltic lava fields. The summit of the center is about 1280 m high above mean sea level (msl) and the base is around 640 m high above msl. The total volume of the volcano is estimated to be about 31.5 km3 . The silicic lava consists of a cluster of several rhyolite domes and flows. There is no central vent; rather each dome/flow has its own vent. The only age constraint available for the silicic part of the Badi edifice is a K-Ar age of 290 Ka for one of the basal silicic domes [32].

The sub-aerial Badi volcanic edifice has essentially two parts (**Figure 2**): the base of silicic domes and flows and then, the upper basaltic flows which have been erupted on to the silicic material. There are no exposed explosive products associated with the effusive activity in Badi volcano, unlike many rhyolitic obsidian flows and domes in the Ethiopian rift valley which are commonly associated with pyroclastic deposits [4, 6, 35–37]. It has been noted that in most volcanic centers of Afar, pryroclastic products are scanty [15]. However, there are large silicic caldera complexes in Afar away from the rift axis [16]. The absence of fragmental magmatic materials at Badi volcano clearly reflects that the effusion of lava domes and flows resulted from the different rheology of the magma.

**Figure 2.**

*Outcrop photos illustrating the eruptive sequence of Badi volcano; Basal silicic domes and flows, and upper basaltic flow.*

During our preliminary field investigation, we found evidence for a single, coarse-grained pumice cone deposit, on the side of the volcano. The pumice fall deposits are quite high up elevation wise and lay directly on top of some basaltic scoria cones (**Figure 2**), so they post-date at least some of the later basaltic volcanism and have been erupted after almost all of the silicic domes/flows that make up the main body of the Badi Mountain. Accordingly, the volcanic stratigraphy of Badi volcano from old to young is silicic dome, then basaltic scoria and finally pumice fall deposit. There are certainly no silicic flows interbedded with the pyroclastic material (**Figure 2**). The very large pumices (0.5 m in size or more) suggest reasonably close to the vent. All of the dates for the late stage basaltic activity are around 50 Ka and younger (Ar-Ar and cosmogenic 3 He datings, [13]). This indicates that the pyroclastic deposits are much younger than 50 Ka. An interpretation might be that the pumice deposit is the product of a small explosive eruption, sourced from a body of silicic melt that was rejuvenated by the later injection of basaltic magma.

### **4.2 Texture of flow bands**

Field inspection reveals that the rhyolite lavas show a vertical zonation of lava textures related to the mechanism of emplacement (**Figure 3a**). The upper surface of obsidian usually fractures into blocks, probably related to the movement and cooling of the interior of the flow. Beneath these layers is the core (interior) of the dome which is unlaminated and shows columnar joints. The upper outer surface of the dome is made up of obsidian layer which displays a very pronounced layers, or flow bands (**Figure 3b**) defined by a color variation (i.e., alternating domains of light and brown glasses). The flow bands are frequently folded (**Figure 3c**) and exhibit intricate fluidal textures as indicated by highly contorted and intensively crenulated layers (**Figure 3d**). Folds arise as flow layering deforms during flow advance [38].

Petrographic observation of the flow bands (**Figure 4**) shows that the boundaries between the light and brown glass bands are abrupt, reflecting laminar flow state. The brown lamella is relatively thickener than the light one. The flow banding is locally deflected around phenocrysts (**Figure 4**), suggesting that crystallization took place before the cessation of flowage of the lava.

As seen both in the hand specimen and thin section, SEM observation (**Figure 5**) illustrates that the Badi lavas have flow banding/layering defined by alternating lamellae of light and black glasses. Black bands are represented by non-vesicular obsidian, while light layers are vesicular glass. The obsidian domain shows abundant, very small microlites of mainly alkali feldspar, quartz *Effusive Badi Silicic Volcano (Central Afar, Ethiopian Rift); Sparse Evidence for Pyroclastic Rocks DOI: http://dx.doi.org/10.5772/intechopen.98558*

#### **Figure 3.**

*Outcrop photos illustrating the lithological variability (a) Textutal differences through a rhyolite lava, with a chilled glassy carapace top and a columnar jointed bottom. (b) Black, vitreous obsidian occurring as interbanded layers. (c) Flow-folded obsidian. (d) Fluidal characteristic as evidenced by contorted and crenulated layers.*

#### **Figure 4.**

*Photomicrographs illustrating flow banding defined by alternating domains of brown and light glasses (×30, ordinary light). Note flow bands deflected around phenocrysts.*

#### **Figure 5.**

*SEM image illustrating the differences in abundance of vesicles between the flow bands. Note microlites are randomly oriented. Field of view is 9 μm.*

and pyroxene set in a glassy matrix. Microlites are generally randomly oriented. It is important to note that there is no notable difference in the abundance of microlites between the two glass domains. Furthermore, the Badi lavas contain neither xenocrystic nor xenolithic materials.

## **4.3 Petrography**

The rhyolite lavas, which form the main part of the Badi edifice, display a wide variety of textures ranging from sparsely porphyritic through aphyric to almost completely glassy obsidians (**Figure 6**). The phenocrysts are unbroken which provides textural evidence that distinguish the flows and domes as lava rather than rheomorphic ignimbrite. They appear to have been in equilibrium without

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

#### **Figure 6.**

*Photomicrographs illustrating the petrographic characteristics of pertalkaline rhyolites from Badi volcano with phenocrysts of alkali feldspar (euhedral), quartz (rounded), aegirine (green) aenigmatite (dark brown) set in a microcrystalline or glassy matrix (×30, ordinary light).*

embayment or resorption. The porphyritic lavas (e.g., samples 01–04, 02–06, 25–02) contain very few phenocrysts or microphenocrysts (< 5 vol.%) of alkali feldspar, quartz, green clinopyroxene and aenigmatite enclosed in a microcrystalline or glassy groundmass which is mainly alkali feldspar, quartz and pyroxene. The aphyric lavas (e.g., samples 01–07, 02–04, 29–03) exhibit very scarce microphenocrysts of alkali feldspar, quartz and green pyroxene embedded in a microcrystalline groundmass which mainly contains alkali feldspar and quartz. They are slightly altered as indicated by a dirty appearance of feldspar. The rhyolitic obsidians (e.g., samples 01–09, 02–04, 03–01, 30–04(1), 30–12, 31–01) contain microlites of alkali feldspar, quartz and pyroxene set in a glassy matrix. The groundmass/matrix is relatively fresh and unaltered devoid of post eruption divetrification and hydration products such as spherulites.

The mineral assemblage in Badi lavas, in order of decreasing abundance, includes alkali feldspar, quartz, green clinopyroxene and aenigmatite, although not all phases are found in every sample. **Table 1** reports the main petrographic characteristics of phenocrysts and matrix of the Badi rhyolite lavas. Accessory Fe-Ti oxides and apatite are present in trace amount and occur as inclusions. Fe-sulfide, possibly


#### **Table 1.**

*Main petrographic characteristics of phenocrysts and matrix of the Badi rhyolites.*

pyrrhotite occurs as tiny bleb inclusions within oxides. Alkali feldspar, quartz, green clinopyroxene, and aenigmatite are ubiquitous in the phenocrysts and microphenocrysts. Alkali feldspar is volumetrically the most abundant crystal in the Badi lavas. Phenocryst and matrix compositions of the Badi rhyolite lavas are presented in **Table 2**. Composition of alkali feldspar is anorthoclase or sanidine. Alkali-pyroxene is the most abundant mafic mineral and is mostly aegirine and subordinate aegirineaugite. Aenigmatite is commonly the second most abundant mafic mineral.

The modal presence of alkali pyroxene and aenigmatite, which are considered to be index minerals in the peralkaline salic rocks [39], in Badi rhyolites surely confers a peralkaline affinity. Nicholls and Carmichael [40] indicated that aegirine is the dominant phase in strongly peralkaline composition (pantellerite), whereas hedenbergite seems to be dominating in less peralkaline composition (comendite).


#### **Table 2.**

*Representative energy dispersive (EDS-SEM) x-ray analyses of minerals and glass of rhyolites from Badi volcano.*

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

The presence of modal aegirine in Badi lavas implies a pantelleritic composition. This affinity is also supported by chemical composition (Hutchinson et al., 2018) in which the silicic lavas from Badi volcano are predominantly pantellerite with minor comendite. The absence of Fe-Ti oxides in the mineral assemblage suggests that the magma was crystallized at low oxygen fugacity which lies at or below the FMQ buffer curve in the T-*f*O2 space [41]. Recent works (e.g., [42]) have shown that the nature of co-existing phases, especially pyroxene, in peralakline rhyolites is controlled by the redox conditions; aegirine crystallizes in more reduced conditions (i.e., in no-oxide field). The co-existence of aenigmatite and aegirine in Badi rhyolites strongly suggests that the original silicic magma was generated, evolved and crystallized in a more reduced condition; at low oxygen fugacity which lies at or below the FMQ buffer in no-oxide field.

It becomes increasingly apparent that some workers (e.g., [37]) have shown the presence of fayalite, hedenbergite and plagioclase in the mineral assemblage of peralkaline rhyolites from Ethiopian rift valley. These minerals are not found in Badi rhyolites. We only observed plagioclase and hedenbergite as xenocrysts in xenolithic material in a single specimen (30-01(4)). These less-evolved inclusions show angular contacts, suggesting that they were solid while the host rhyoltic lava was liquid. We emphasize the importance of indentifying the mineral assemblage found in rhyolites as phenocrysts and xenocrysts.

## **5. Discussion**

#### **5.1 Origin of flow banding**

Thin flow banding, defined by discrete lamellae/layers with contrasting color, is a common feature of many effusive volcanic rocks. It is a ubiquitous texture in very viscous, highly siliceous lavas, such as rhyolites (e.g., [3, 43, 44]). Flow band in rhyolite lavas has been described from varying crystallinity and vesicularity [45]. Differences in abundance of microlites and/or vesicles appear to develop either during flow of the melt in the conduit or during late stage cooling and degassing during flow emplacement [45, 46]. Flow banding is thought to be a reflection of laminar flow.

Flow banding in rhyolite lavas may have a variety of origins, including mixing of compositionally distinct magmas [47, 48], or incorporation of xenolithic material in a shear flow [49], or fracture-healing processes of texturally distinct magma [46, 50]. Another type of flow banding origin seems to arise from deformation of domains in the melt that had contrasting water concentration in the melt prior to flow [43]. There is yet little consensus on any of these alternatives.

One of the most important questions to answer is whether or not the banding displayed by the Badi rhyolite lavas is due to textural (i.e., differences in abundance of vesicles) or compositional (i.e., differences in abundance or preferred orientation of microlites) heterogeneities. Flow banding in Badi lavas is defined by alternating domains/layers of contrasting glass colors (light and brown glasses). Brown bands are represented by non-vesicular obsidian, while light layers are vesicular glass (**Figure 5**). Our data set shows that there are no extreme differences in mineral composition or proportion between the light and brown glasses. This appears to indicate that the banding observed in the studied lavas is not due to compositional heterogeneities at least on the basis of mineralogical grounds. Instead it is due to textural differences, caused by variations in vesicle concentration of the glass bands (**Figure 5**).

Such textural heterogeneities due to differences in the abundance of vesicles of the glass may develop either during magma flow in the conduit [46], or during flow emplacement [49]. All of the samples from Badi volcano surveyed both in thin section and SEM do not contain xenocystic and/or xenolithic material. This further provides evidence against incorporation of xenolithic material during the course of the flow of Badi lavas at the Earth's surface. Hence, this textural (vesicularity) heterogeneity could not have developed during late stage cooling and degassing during flow emplacement. Rather such textural variations (heterogeneities) imply distinct cooling and/or degassing histories, and must have formed during flow in the conduit prior to magma extrusion.
