Monogenetic Volcanics

The transitional relation between the lapilli-bombs agglomerates and clastogenetic lava (Fig.12B)proposes that theemitted lava blebs for each lava was virtually continuous throughout a singlefountains episode. Likewise, each eruptive unit has its own cooling joints.The occurrence of inversely coarse-tail-graded beds, poorly defined bedding, lithic clasts and lapilli/bombs-rich concentration zones are indicative of an emplacement of the lateral flow, akin to pyroclastic density currents forming spatters ( Freundt and Schmincke, 1995; Valentine et al., 2000). Occasionally, the record of flow laminations, flattened vesicles and clasts, and brittle shear zones (Figs.

9E-G & 14G) in these deposits reflects rheomorphism during their eruption (cf. Soriano et al., 2002;Sumner and Branney, 2002).

Finally, all the evidence imply that the whole Hawaiian sequence can be interpreted as lava-like bodies according to view of Valentine et al. (2000). In various lava fountains, clastogenic lavaflows are produced by the abrupt breakdown and coalescence of spatter masses(e.g., Sumner, 1998). Under such condition, some of the scoriaceous clasts in the spatter agglomerates may possibly be conserved in lava flows, as the result of rapid cooling.

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Generally, when acone is broken, seepage lava would be capable of agitating cone fragments to produce rafts (Valentine and Gregg2008). No rafted blocks have been detected at G. Marssous scoria cone, like those observed in Izu-Oshima (Sumner 1998). It may hence be presumed that the lava erupted gradually through the cone base and extended everywhere without rafting the scoria cone.

The feeder dykes marks the final eruption that feeds both the pyroclastics and the lava flows (Figs.

15F-G). Theycreate the shallow plumbing pattern of the G.Marssous volcano because of the load pressure derived either from the stagnant lava over spatter deposits or feeder dykes or plugs.These lava may penetratethrough predominant fractures producing a grid of dykes and sills through which the lava discharge to produce stagnant lava flows,as regularly happens in monogenetic lava(e.g. Valentine and Gregg 2008).

Explosive vs. effusive volcanism

Discrepancies in the magma-ascent rate, gas content, viscosity, and contact degreewith superficial waterare though to be liable forswitching betweenwet (phreatomagmatic sequence) and dry (strombolian/Hawaiian sequence) eruptions (Parfitt et al., 1995; Keating et al., 2008). The high magma flux and accumulation rate in Hawaiian eruptions permits partial cooling, forming coalescence of the juvenile clasts (Head andWilson, 1989), whereas these clasts rapid calm prior to landing during Strombolian eruptions ( Vergniolle and Mangan, 2000). Pioli et al. (2008) interpreted that the initial volatile content and competence of gas segregation may consider major causes in Strombolian-Hawaiian transition.Anoverall evolutionary successionfrom initial lava flow, hydromagmatic eruptions through strombolian/violent to dry Hawaiian eruptions accompanying clastogenic lava flows marks a decreasingeffect of magma-water interfacesduring cone evolution.The lack of major unconformitiy surfaces or time rangebetween the explosive and effusive facieshas proved to reflect continuous and simultaneous eruption of the exposed rock units (Fig.6), analogous to scoria cones worldwide (Pioli et al.2008).

The field geometry of effusive-explosive rocks proposes that degassing process plays an effective rolein creation the main coneforming phases (Vespermann and Schmincke, 2000). The actual gas segregation arises in vertical and lateral outlets of a plumbing system, whereas the lava is placed for a smallperiod,resulting into a gas-rich and a gas-poor phase (Menand and Phillips 2007;Pioli et al. 2009). These phases flow back and emitalong with the rising melt in conduit.Emphasising this view into interpretation of G. Marssous scoria cone, the beginning of eruption may be the consequence of lava intrusionat shallow depth accompanying actual gas dissection.After that, the gas-rich and gas poor phaseserupt concurrent and detach into two separate materials for feeding lava flows and strombolian/Hawaiian pyroclastics, as advocated by Pioli et al. (2009). The overpressure and local stresses because of the volcanics loading are the causes for the initiation of such intrusion in the focal conduit (Hintz and Valentine, 2012; Re et al., 2015). Another explaination for the gas segregation is an annular flow, whereas magma degassing and gas-rich flows deviate laterally producing explosive-effusive rocks (Doubik and Hill 1999).

Low magma ascent rates may facilitate the emission of gas-poor material as the result of low rate for both gas segregation and magma supply, allowing the ascending magma degassingfrom the intrusion (Menand and Phillips 2007)and transferring from explosive to effusive activity,as previously reported by Di Traglia et al. (2009) and of Pioli et al. 2008).Numerical modeling showed that the distribution between gas-rich and gas-poor magma is controlled by the system architecture, mass flow rate, and magma pressure/viscosity(Pioli et al. 2009). The latter authors suggested that the concomitant tephra-lava eruptions occur if mass flow rates equal to 103-105 kg/s , but formation of Plinian activity happen when this rate exceeds 105 kg/s. Gabal Marssous stratigraphy clarifies a decrease in Tephra/lava manufacture towards the end of its eruptive history,which isperhaps the principal keyof eruption-controlling processes, signifying gas segregation-gas loss path.Therefore, this latter path is favoured for elucidatingan instantaneousemission of pyroclastics and lava flows-rich columns in the construction of Marssous Volcano, and is undoubtedlyresponsible for an increase in the magma effusion rate compared with the early stage.

Comparison of Marssous scoria cone with other pyroclastic cones

The Bahariya volcanics are a part of low-volume igneous products of North Egypt that were erupted during the rifting of Red Sea in Late Tertiary period. The intraplate alkaline Miocene mafic rocks at the Bahariya Oasis show broad petrological similarities with the extensive Neogene-Quaternary volcanism in North Africa, northeast Arabian shield (Harret volcanic fields), and Mediterranean province (e.g. Massif Central Volcanic Field, France; Eifel volcanics, Germany, Calatrava volcanics, Spain)(Cebri? et al., 2000). The studied scoria cone has main three eruptive types involving phreatomagmatic, strombolian, and Hawaiian styles. These styles vary in morphology of lapilli/bomb-sized tephra from angular, wood-like, to spheroidal-like outline, respectively.The spheroidal lapilli/bomb-rich deposits are petrographically complex consisting of spinning/spherical droplets, achnoliths, crystal droplets, and lithics/crystals-cored lapilli/bombs. These cores-rich lapilli/bombs are interpreted either as mantle xenoliths or juvenile clasts and crystals that have been verified in alkali mafic volcanism, as recorded in scoria /cinder cones like Cabezo Segura volcano, Spain (Carracedo S?nchez et al., 2009; Sottili et al., 2010).

Compositionally, spherical composite lapilli/ bombs are related to low viscosity, high temperature mafic and ultramafic magmas (Carracedo S?nchez et al., 2009).They are comparativelyscarce in monogenetic cones compared with the uniqueclasses of cauliflower, tubular and craggy or breadcrusted bombs (Alvarado et al., 2011).However, spherical composite lapilli/ bombs are recorded in Bahariya scoria cone, as have been documented in several intraplate scoria and cinder cones having hydromagmatic and Strombolian eruptions (Fig. 19,Poblete, 1995; Ancochea 2004) worldwide like an alkaline basalts and basanites at the Calatrava volcanic field, Spain (Carracedo S?nchez et al., 2009), La Palma, Canary Islands(Schmincke and Sumita, 2010), Monta?a Rajada (Carracedo and Rodr?guez, 1991), Rothenberg cone in the East Eifel, Germany (Houghton and Schmincke, 1989), and Colli Albani cone, Italy (Sottili et al., 2009).

At G.Marssous scoria cone, Bahariya, complex comagmatic spheroidal lapilli/bombs deposits of strombolian and Hawaiian eruptive styles are pervasive, identical similar to the above mentioned Mediterranean-Europian volcanics and southAfrica (e.g.Uganda, Lesotho) as well as Rotomahana (New Zealand) scoria cones. These late Miocene to Quaternary alkaline volcanics have been related to either anintricatemega-rift system or to asthenosphericmantle/hot spot (Ancochea 1982). At Pelagatos, Pacaya, and Cerro Chopo scoria cones, Mexico, there are abundant cannonball bombs, in contrast with the common spheroidal bombs and lapilli at G. Marssous, Bahariya Oasis. This is further supported by compositional dissimilarity involving subducted-related high-Mg tholeiitic basalts and basaltic andesite at Pelagatos (Guilbaud et al., 2009), and Chopo, compared with rift-related within plate alkaline mafic volcanics at the study area.

Summary & conclusion

This paper presents for the first time the chief characteristics of one ofthe Red Sea rift-related Miocene monogenetic volcanics in North Western Desert, Egypt, Marssous alkaline basaltic volcano, located at the Bahariya Oasis. Marssous volcano is an exclusive example of composite scoria cone that evolves from effusive toexplosive explosions, revealing a multiple eruptive history. Five lithofacies have been identified stratigraphically towards the top section : (1) coherent lava flows, (2) crude cross bedded Ash tuff-lapilli tuffs, (3) medium-grained lapilli-bombs, (4) strongly oxidized, semi-consolidated, reddish spheroidal lapilli-bomb complex, and (5) clastogenic lava flows. These complex sequences reflect distinct variation in eruptive fragmentation style ranging from effusive ( facies 1) through phreatomagmatic ( facies 2 ) and strombolian/violent strombolian ( facies 3) to Hawaiian phase(facies 4 & 5). No major breaks and soil horizons occur in Marssous cone sequence which reflect cone derivation from a single continuous eruption.The first phreatomagmatic phase was produced by the mixing of the rising magma with theBahariya Fm. aquifer, as supported by the occurrence of quartz/feldspar clasts.

Two common populations of scoriaceous juvenile clasts are recorded: wood-shaped scoria with stretched bubble and complex breadcrusted- spheroidal scoria with rounded-shaped vesicles characterizing strombolian and Hawaiian style, respectively. The high degree of aggulination from strombolian to Hawaiian eruptive style having fluid-like "spattered" appearance is suggestive of hot state emplacement of pyroclastic deposits.The occurrence of scoriaceous juvenile clasts, together with stretched vesicles are strong proofs of fragmentation by magmatic volatile exsolution and glass quenching (Schmincke et al., 2018). Advanced increase in the magma flux,magma pressure and velocity, with subsequentwaning in magma-water interface produced the diminution in fragmentation efficiency which lead to an increase of clast size and plenty of the vesicles-rich juvenile components together with degassing toward the top of the strombolian-Hawaiian deposits.The petrographic (olivine and clinopyroxene)and geochemical resemblances (alkaline affinity) of both lava flows and pyroclastic deposits point to homogeneous magma source in spite of its fragmentation diversity.

The change in the eruptive styles maybe ascribed to deviations in magma flow circumstances as the result of fluctuations incrystallinity and gas content of theemitted lava and separation of gas-rich and gas-poor products (Cimarelli et al., 2010). In fact, discrepancies in stratigraphy and hydrogeological features of the substrate as well as rock strength display a noteworthy role in the studied cone architecture , as has been recorded in equivalent volcanic regions (Auer et al.,2007; Mart?n-Serrano et al., 2009).