科研之友
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摘要
It is generally assumed that spheroidal volcanic bombs are formed during cooling of molten clots pulled up into spheres by the surface tension of the magma. Also, volcanic bombs are regarded as non-fragmental structures, i.e. made of coherent material. We describe non-coherent, spheroidal bombs of mafic composition from the tuff/cinder cone of the Cabezo Segura volcano (Calatrava volcanic province, Spain) that exhibit features consistent with welding of essentially non-vesicular juvenile spherical droplets. It is assumed that the eruption of a low-viscosity magma containing suspended phenocrysts, mantle clasts and cognate lithic clasts resulted in the efficient separation of volatiles and melt, and the explosive fragmentation of the magma into different types of pyroclasts. The finest magma fragments would yield melt blobs (achneliths or lava spray, ca. 1-5 mu m), small droplets largely made up also of melt, and isolated solid clasts (phenocryst, microphenocryst and lithic). The largest lava fragments incorporated phenocrysts, microphenocrysts and lithic clasts. All these types of clasts would be erupted in a rapid gas thrust that prevented rapid collision among particles, massive aggregation and premature collapse by defluidization. The rotation of the largest magma droplets during ascent in a fluidized system produced spinning droplets similar to pelletal lapilli. The collision and aggregation of the fluidal clasts might start as soon as the pyroclasts-gas mixture was thrust out and progressively decelerated. A model is proposed for the formation of spheroidal composite bombs assuming fluidal clasts sintering, either by coalescence or agglutination, and welding of constituent pyroclasts inside the eruption column prior to bomb accumulation on the field.
- 出版日期2009-10-10
