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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Fractures in a trachyandesitic lava at OEraefajoekull, Iceland, used to infer subglacial emplacement in 1727-8 eruption
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Fractures in a trachyandesitic lava at OEraefajoekull, Iceland, used to infer subglacial emplacement in 1727-8 eruption

机译：冰岛OEraefajoekull的tranchandesitic熔岩中的断裂曾被用来推断1727-8年喷发的冰下侵位

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We present detailed field observations of cooling fractures in a small-volume trachyandesitic lava, informally named the Slaga lava, on the south west flank of Oeraefajoekull volcano, south east Iceland. Columnar joints, pseudopillow fracture systems, and curved platy jointing occur in the lava, whose exposed section is approximately 600 m in length and generally 2-3 m in thickness. Columnar jointing may occur at the base of flow lobes, whereas pseudopillow fracture systems occur throughout the lava in an outer, glassy, fractured carapace, and curved platy fractures occur in the centres of larger flow lobes. Pseudopillow fracture systems, composed of a single master fracture and multiple subsidiary fractures formed normal to the master fracture, are of two types: G-type pseudopillow fracture systems have very narrow striae (chisel marks) on their master fractures, indicative of rapid cooling and brittle fracture; SR-type pseudopillow fracture systems display alternating smooth and rough master fracture surface textures, evidence of alternating brittle and ductile fracture propagation mechanisms. Subsidiary fractures in both types show curved striae on their fracture surfaces, which enable the determination of fracture propagation directions. Pseudopillow fracture systems are thought only to form in the presence of water, including water caused by the melting of ice and snow. The curved platy fractures display prominent river lines and may have resulted from cooling contraction, post-emplacement degassing, flow deflation or shearing in the flow against the outer solid crust of the flow during inflation. Due to recent advances in the understanding of the formation mechanisms of pseudopillow fracture systems they, and the other fractures present in the flow, can be used to reconstruct the cooling environment. The lava is inferred to have been emplaced within subglacial drainage channels incised into or beneath a thin alpine-type glacier, with coolant infiltrating the lava radially from all sides and ponding or draining of water at or along the lava base. We link this flow to the 1727-8 eruption of Oeraefajoekull volcano on the basis of historical records, which describe lava effusion in this approximate location and striking chemical compositional similarity between 1727 tephra and the Slaga lava.
机译：我们在冰岛东南部Oeraefajoekull火山西南翼的小体积trachyandesitic熔岩中非正式地命名为Slaga熔岩中冷却裂缝的详细现场观察。熔岩中存在柱状节理，假枕形断裂系统和弯曲的板状节理，熔岩的裸露部分长约600 m，厚度通常为2-3 m。柱状节理可能发生在水流的底部，而伪枕形裂缝系统发生在整个熔岩的外部，玻璃状，破裂的甲壳中，弯曲的板状裂缝发生在大水流的中心。由单个主裂缝和垂直于该主裂缝形成的多个副裂缝组成的伪枕形裂缝系统有两种类型：G型伪枕形裂缝系统在其主裂缝上具有非常窄的条纹（凿痕），表明快速冷却和脆性断裂SR型假枕断裂系统显示出交替的光滑和粗糙的主断裂表面纹理，证明了脆性和延性断裂传播机制的交替。两种类型的辅助裂缝在其裂缝表面均显示弯曲的条纹，从而能够确定裂缝的传播方向。伪枕头破裂系统被认为仅在水存在下形成，包括由冰和雪融化引起的水。弯曲的板状裂缝显示出明显的河流线，并且可能是由于冷却收缩，就位后脱气，流量放气或流量膨胀过程中对流量外部固体结壳的剪切作用所致。由于最近对伪枕形裂缝系统形成机理的理解上的进步，它们以及流中存在的其他裂缝可用于重建冷却环境。推断熔岩已被置于切割成薄高山型冰川或在其下方的冰川下排水通道内，冷却剂从各个侧面径向渗入熔岩，并在熔岩底部或沿熔岩底部积水或排水。根据历史记录，我们将这种流动与1727-8年的Oeraefajoekull火山喷发联系起来，该火山喷发描述了该近似位置的熔岩流，并在1727 tephra和Slaga熔岩之间达到了化学成分相似。

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