The eruption of Eyjafjoell followed a long, threatening warm-up in which the mountain growled and its icy flanks bulged, say scientists.
Vulcanologists from the United States, Iceland, Sweden and the Netherlands used satellite images and data from Global Positioning System (GPS) movement monitors to build a step-by-step tableau of events.
"Several months of unrest preceded the eruptions, with magma moving around downstairs in the plumbing and making noise in the form of earthquakes," says Professor Kurt Feigl of University of Wisconsin at Madison.
The eruption of Eyjafjoell - the first since 1821-1823 - disgorged a cloud of ash which affected more than 100,000 flights and eight million passengers.
Reporting in the journal Nature, the scientists say they stepped up their watch in mid-2009, when a GPS station on the south side of the volcano suddenly pointed to movement of between 10 and 12 millimetres.
By January, the volcano was being shaken by several seismic episodes a day, signalling the inflow of magma into its roots. Eventually, the sides of the volcano bulged by more than 15 centimetres before the first eruption began on 20 March.
It took almost a month for the volcano to start to deflate, which is a surprise given that volcanoes 'shrink', rather like a poorly-knotted balloon, as molten rock and scorching gases flow from the chamber.
On 14 April, after two days of quiet, a second eruption occurred in which lava broke out through a channel under the ice on the mountain's summit.
This caused an "explosive" reaction as water flashed to steam and gas escaped from bubbles in the magma.
The result was an ash plume that rose to heights of up to 9000 metres, causing misery for millions of stranded air travellers.
The scientists say they were unsure why Eyjafjoell erupted when it did.
One idea is that magma intruded into the volcano's heart through a network of conduits and chambers.
"The explosiveness of the eruption depends on the type of magma, and the type of magma depends on the depth of its sources," says Feigl.
"We're a long way from being able to predict eruptions, but if we can visualise the magma as it moves upward inside the volcano, then we'll improve understanding of the processes driving volcanic activity."