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Abstract

Lava-rise structures form by endogenous growth where lava is injected under, and lifts up, its surface crust. They are especially common in pahoehoe where it flows on shallow gradients (.48). Examples are described from the Mauna Loa (Hawaii) 1859 and Xitle (Mexico) 1.5 ka basalt flow-fields, and range from ten to hundreds of metres wide by up to 13 m high. The lobes that host the lava-rise structures were initially thin (≥1 m) and it is estimated that at least 80% of their total volume was emplaced by subcrustal injection. They are bounded by lava-rise edges that are highly distinctive margins with steep banded and striated surfaces (BSS) upon which lava inflation was accommodated. The BSS are emergent surfaces formed where newly injected hot lava cooled and solidified against the air. The surface textures indicate the direction of motion of the lava crust and edges relative to the point of emergence. Some BSS are steep and inwardly dipping reverse faults. Others are divergent-facing pairs of curved surfaces, the walls of lava inflation clefts. Many such clefts commonly transect the main BSS and subdivide them into stacks of lava wedges. A steep foliation defined by the plane of flattening of vesicles also occurs and bulges out into the wedges, attributed to a general expansion of the lava-rise. ‘Lava-rise sutures’ occur between contiguous lava-rises or lava lobes that inflated synchronously against each other. They are common where cross-sections through pahoehoe lavas are seen, such as in flood basalt provinces. In cross-section these features have the aspect of facing stacks of lava wedges, previously interpreted in the Xitle lava and elsewhere to be spiracles (steam-escape structures).

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