| Summary: | Pillow lava is the most common lava morphology on Earth. It is found in high abundance in many environments, from the ocean floor to the surface of Mars. Because pillow lava forms during subaqueous eruptions, it is often inaccessible for study. Thus, being able to analyze pillow lava using visible physical characteristics without requiring samples could improve pillow lava research. This paper analyzes the distribution and emplacement mechanisms of vesicles in uniquely accessible pillow lavas found in Undirhlíðar Quarry. These pillows are the only lasting record of a preexisting englacial lake formed during a subglacial eruption, so understanding the details of their vesicles may provide new insights into the timing and nature of their emplacement. I propose that the type of vesicle distribution in a pillow cross section may be used to infer proximity to the source, the rate at which the pillow cooled, and eruption events during pillow emplacement. In this study, I define six types of vesicle distribution in pillow lava based on the presence of a vesiculated core, vesicles in the body, and a hollow pocket. Four pillow types were identified in the field. The observed pillow types all had vesiculated bodies but different cores. I propose that vesiculated cores are the result of rapid pressure changes, that non- vesiculated cores indicate slow cooling and proximity to the source, and that hollow pockets indicate that the source was cut off or diverted. Vesicle rings and cores may be indicative of different events, like magma pulses or sudden changes of pressure of the surrounding environment.
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