Figure 2. Key eruption parameters.

The graphs are based on tephra sampling (Figs. 3–4), plume, weather and crater observations and microprobe analyses (see Methods for details).In addition to a 3–4 hour long inital subglacial phase (not shown on graph) when the eruption melted its way to the glacier surface, the eruption is subdivided into four main phases (I–IV on graph): I: The first explosive phase, 14–18 April; II: Low-discharge mainly effusive phase, 18 April–4 May; III: Second explosive phase, 5–17 May; IV: Declining phase, 18–22 May. (a) Six hourly mean and maximum values of plume altitude based on weather radar in Keflavík (155 km west of the volcano); (b) Wind speed and direction at the 500 hPa level (approximately 5 km) over Eyjafjallajökull April 14–May 23 (see Methods); (c) Magma discharge during the eruption based on combining plume and tephra dispersal data (see Methods for details). (d) Cumulative volume of ice melted in the eruption based on changes in ice cauldrons and ice surface elevation. (e) SiO₂ content of groundmass glass ( = melt) in tephra grains from individual days with 3–37 grains behind each data point (one standard deviation equal to symbol size). The groundmass glass falls into distinctive compositional groups, designated as benmoreite, trachyte and rhyolite. The relative abundance of more evolved groundmass glass compositions (highlighted) in the initial and second explosive phases is given by the percentage above the black squares. Shifts in the melt composition are in phase with shifts in magma discharge and style of activity.