Eyjafjallajökull Volcano
The Eyjafjallajokull volcano began erupting in March. On 14 April, the eruption entered a new explosive phase which was to bring European airspace to a standstill. The eruption threw thousands of tonnes of ash into the air forced higher by steam plumes created as glacial ice melted. Most of it was very fine particles which formed an ash cloud, rising to about 20,000-35,000 feet into the atmosphere.
The ash plume from Eyjafjallajökull caused turmoil in the air for nearly a month. Still, the eruption was a relatively small event. For instance, the plume never reached more than about 6 miles in height, and the volcano only spewed out about 9.5 billion cubic feet (270 million cubic meters) of ash over the course of several months, while some eruptions can spew out many times more than that in the span of a single day.
The reason that Eyjafjallajökull had such widespread influence was due to how the volcano's ash spread unusually far and stayed for an oddly long time in the atmosphere. To learn more about why this was, a group of scientists collected ash samples from across Iceland.
The investigators found Eyjafjallajökull's vent, upwelling magma reacted with nearby glacial water. This rapid cooling made the magma contract and fragment into fine, jagged bits of ash. Near the end of the eruption, ash was generated when small gas bubbles trapped in the molten rock expanded as the magma neared the surface.  
The researchers found that the width of all the ash grains was less than 1m wide. Starting about 10 km from the vent and moving outward particles smaller than 16 microns. Computer models suggest the shapes of these jagged ash grains made them very aerodynamic, increasing how long they spent aloft. This helps explain why a small eruption still impacted a large area.