File:Tephra in Grindavík from the February 2024 Sundhnúkar eruption.jpg

English: Tephra is frothy and vesicular material which forms when splashes of lava cool quickly in the air in the lava fountaining activity. This process took place when an eruptive fissure opened this morning (8th of February 2024). Due to the tephras‘ vesicularity and lightness it can be transported far within the volcanic plume with wind. It falls in Grindavík now, 3-5 km from the source of formation due to the height of the lava fountains, north-easterly wind-direction, thermal upwelling from the lava and low air temperature.

Tephra is a synonym for all airborne, solid, eruptive materials, regardless of size an type. Tephra formation is well known in Iceland, such as in the explosive eruptions in Grímsvötn 2011 and 2004, and Eyjafjallajökull 2010. In explosive eruptions, the lava is more fragmented in the air an produces finer-grained tephra but tephra can also be produced in effusive eruptions such as is now ongoing on the Reykjanes peninsula. In effusive eruptions, the majority of tephra is deposited close to the vent and thus is not well detectable outside of the lava flow field. In May 2021, when lava fountaining was most active in the eruption in Fagradalsfjall, large pieces of tephra (up to 10 cm in diameter) fell at about 1 km distance from the eruptive source.

The image is pre-cropped from the original.