Nothing like a good Nature paper to get the media's attention, especially when it was about the biggest air traffic disruption in almost a decade. Of course, the headlines I saw were just plain confusing on the whole: "Iceland Eruption linked to weird magma plumbing", "How the Icelandic volcano issued warnings for months before its eruption", "Magma mixer and odd plumbing made Iceland go pop". So, why is everyone so excited about Iceland again?

I sat down with the brand new paper in Nature by Freysteinn Sigmundsson (and about 15 other authors) to see what was being said. I have to admit, the paper was a nice, succinct, clearly-written paper that tells us three important things.

• Eyjafjallajökull did not behave like a highly active volcanoes when one of those volcanoes (think Kilauea) is going to start erupting.
• There were numerous subtle clues that magma was moving under the volcano, some signs as far back as 1992.
• The magma intruding Eyjafjallajökull was a series of sills that pressurized the magmatic system over many months to years.

The key distinction that the authors want to draw is that a lot of what we know about the signs of an impending eruption are from highly active volcanoes (which they don't specifically define, but my guess is something that erupts at least one a decade). This means that moderately active volcanoes that eruption once a century or longer might have different signs that an eruption is coming. This might be because the magmatic system is "cold" relative to a highly active volcano, so that new magma has to define its space with each intrusion.

What is their evidence for these observations? Long-term geodetic and seismic information! For example (see below):

• Swarms of earthquakes were noted at Eyjafjallajökull since 1992 after 20 years of quiet. Larger swarms occurred in 1994 and 1998, where it is though that sills of basaltic magma were intruded at ~4.5-6.5 km depth. Between 2001-2009, the volcano returned to quiet (~1-4 earthquakes/month).
• Starting in mid-2009, deformation was noted on the volcano using GPS information. Then, starting in 2010, the deformation increased and it is thought that ~0.05 km3 of basaltic magmatic intrusion grew under the area. This GPS deformation is corroborated by inSAR (satellite interferometry) images taken of the volcano.

A compilation of seismic and geodetic data leading up to the Eyjafjallajökull eruption. Figure 2B from Sigmundsson et al., 2010.

These lines of evidence suggest that a series of magmatic sills and dikes (horizontal or vertical intrusions of magma) were intruding under the volcano. Now, the exact geometry of the sills and dikes is impossible to know because we can't see them, but inferences can be draw based on how the land surface was deformed (pdf). By modeling the deformation, the authors concluded that the deformation couldn't be from a single magma chamber filling up. Instead, we get a series of sills from 4-6 km and a single dike intruding under the flank fissure vent location (see below). 

One of the most surprising aspects of the behavior of Eyjafjallajökull was that when the first fissure vent eruption started, the volcano did not immediately start to deflate, which is what you might expect as lava is erupted from the system. The suggestion of the authors is that the inflation was being caused by a pressurization of the system as magma flowed into the sill complex. The eruption relieved that pressure, but the rate of magma entering the sill complex at depth (20-30 m³/s) was still higher than the rate of the eruption (~13 m³/s for the fissure). Deformation didn't start again until the summit eruption, when eruptive rates reached 30-60 m³/s, generating (with the help of water), the 6-9 km plume that closed Europe. It is all about magma flowing into the system at depth and exiting the system during the eruption, and how the flow balance (what is coming in versus going out) could dictate the style of deformation.