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.