James Stewart

As magma rises from below, it intrudes into any nook and cranny it can find.

At first it stalls at depth, not erupting, but building pressure.

For centuries this process unfolds quietly, steadily, relentlessly.

The apparent calm above ground is deceptive.

Eventually, the combined force of plate motion and magma pressure just becomes too much, and the crust gives way.

Existing fractures are forced open, new ones tear through rock, and magma surges sideways for kilometres at a time through networks of thin underground dikes, triggering these seismic swarms that signal what comes next.

When those intrusions finally break the surface, long fissures open and eruptions ignite along the rift.

Each eruption releases some of the accumulated stress, and the rift switches on and off over decades, until centuries of tension are relieved.

Then the system quietens and the cycle simply resets.

And terrifyingly, the most recent seismic swarms and eruptions fit the expected cycles uncannily well.

This activity is right on schedule.

But when this cycle last played out, it was medieval communities who were pushed to the brink.

What can modern Icelanders really do in the face of a cycle that cannot be stopped?

Well, they're going head to head with the lava, obviously.

Modern GPS and Interferometric Synthetic Aperture Radar, or INSAR, from satellites show that before eruptions, the land inflates as magma accumulates.

In 2021, some areas rose by tens of centimetres before subsiding again.

Taken together with thousands of small earthquakes, magma pathways can be mapped in real time.

This allows geophysicists to estimate the volume of magma involved as well as the depth and shape of the reservoirs.

And all of this gave Iceland time to plan.

Well, as much planning as you can do when it comes to huge impending lava flows.