| Summary: | Volcanic eruptions generate continuous or episodic tremor, which can provide unique information about activity changes during eruption. However, the wealth of information in episodic tremor patterns is often not harvested and transitions between patterns remain obscure. The 2021 Geldingadalir eruption of the Fagradalsfjall Fires, Iceland, is an exceptional case, where the lava effusion caused continuous tremor, and 8696 tremor episodes spanning two orders of magnitude in duration and repose. Based on seismometer and video camera data, we associate several-minute-long, symmetrical episodes with an open vent system, where lava remains in the crater bowl during repose, connected to a shallow magma compartment. Ramp-shaped episodes, lasting several hours, are associated with a temporary closure of the vent system, where no lava remains in the crater bowl during repose and more time is required to resume effusion. The transition from continuous to episodic effusion is related to the cumulative time spent in effusion and repose, and to external factors like crater wall collapses.
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