James Stewart

When paired with radiocarbon dating of the sediments above and below the ash layers, as well as observations from old lava flows, the timing and sequence of historical eruptions can be pieced back together.

In March 2020, Christian Sirmutsen from the Icelandic Geo Survey and his colleagues published evidence of at least three major rifting and eruptive episodes over the last 4,000 years, each lasting a few hundred years and spaced roughly 800 to 1,000 years apart.

Now, to be honest, this would have been quite good to know before Iceland set up the majority of its economy, infrastructure and housing in this area.

But even so, what on earth was causing them?

The timing of all these cycles comes back to that uniquely Icelandic geological boiling pot that we talked about at the start.

Yes, Iceland as a nation sits on the Mid-Atlantic Ridge, but the Reykjanes Peninsula in particular sits directly astride it.

There is no single centralised volcano here.

The plate boundary itself is the volcano.

As the North American aneurysm plates slowly pull apart, lateral strain builds within the crust.

It stretches and over time fractures.

This creates underground networks of fissures that are invisible to us at the surface.

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.