[Thor]

A good article on this area and what it means becoming active again:

https://www.sciencealert.com/experts...fter-800-years

The island straddles the Mid-Atlantic Ridge, a crack in the ocean floor separating the Eurasian and North American tectonic plates.

Sunday's eruption was the fifth in fewer than three years on the Reykjanes peninsula, which had not previously seen one in centuries.

"After eight centuries of a relative break and a complete cessation of surface activity, we have entered a new episode of plate separation which could last several years – possibly decades," volcanologist Patrick Allard from France's Institut de Physique du Globe de Paris told AFP.

Even before the first of the five eruptions on March 2021, scientists "saw the ground distorting, with magma rising from the depths and seeping into" an area three to 10 kilometers (two to six miles) below the surface, he said.

As that magma bubbled its way upwards through fissures in the Earth, eruptions started to occur.