Eyjafjallajökull

Iceland sits on a crack where the Earth is pulling apart.

Hot melted rock comes up through the cracks. It builds volcanoes. In cold Iceland, some volcanoes wear caps of ice.

Iceland lies on the boundary between two of Earth's plates, which are slowly moving apart. Molten rock, called magma, rises in the gap and builds volcanoes. Eyjafjallajökull is a stratovolcano, a cone built up from many layers of lava and ash.

Because Iceland is cold, ice collects on its high mountains and forms ice caps. When a volcano erupts under ice, the heat melts the ice very fast. The meltwater can rush down the mountain as a flood, and when water meets hot magma, it can blast the lava into tiny pieces of ash.

The Global Volcanism Program's index map places Eyjafjallajökull in the context of Iceland's plate-boundary spreading, close to Katla and the Vestmannaeyjar. The volcano is a stratovolcano, built of successive lava and tephra layers, and in 2010 its summit caldera was capped by glacier ice.

That ice changed the eruption's character. The Global Volcanism Program reports that once magma reached the ice-covered caldera, magma-ice interaction amplified the explosivity and produced fine-grained ash able to stay suspended in the atmosphere for long periods. Meltwater also caused significant local flooding, Britannica notes, and the Icelandic Meteorological Office recorded that glacial bursts on 14-15 April deepened canyons below the Gígjökull outlet glacier.

In March 2010, the volcano woke up. First, glowing lava came out on the side of the mountain.

In April, it erupted at the top, under the ice. It sent up a big cloud of gray ash. By the fall, the volcano was quiet again.

The eruption came in two parts. On 20 March 2010, lava burst out of a crack on the Fimmvörðuháls pass, about 9 kilometers east of the summit. Fountains of glowing lava shot up to about 150 meters high.

On 14 April the eruption moved to the summit crater, under the ice. The ice melted, and the mix of water and hot magma made a cloud of very fine ash that rose high into the sky. Ash kept coming for weeks. The last day of steady activity was 22 May, and later in 2010 scientists said the eruption was over.

The Global Volcanism Program, summarizing Höskuldsson and others (2010), describes the first phase, 20 March-12 April, as a basaltic flank eruption at Fimmvörðuháls, about 9 km east of the summit: initially up to 15 lava fountains reaching 150 m, building ramparts 60-80 m high and an ʻaʻā-dominated lava field of about 1.3 km², with about 0.025 km³ of magma at a mean 13 m³/s.

The summit eruption began on 14 April in the ice-covered caldera and went through three phases, producing about 0.11 km³ of tephra and 0.025 km³ (dense-rock equivalent) of lava at about 40 m³/s, roughly four times the Fimmvörðuháls rate. Sustained activity ended on 22 May; by 23 June the Icelandic Meteorological Office and the University of Iceland's Institute of Earth Sciences stopped regular status reports, and in October a University of Iceland scientist said the eruption was over. On 27 May scientists measured tephra up to 40 m thick around the craters.

The wind blew the ash cloud over Europe.

Ash can hurt airplane engines. So many airplanes stayed on the ground for a few days. Pilots and passengers stayed safe.

Winds carried the fine ash south and east over Europe. Britannica explains that the ash clouds were a danger to aircraft engines, and air traffic was disrupted across northern and central Europe.

For about a week, most passenger planes in those areas could not fly. EUROCONTROL, the organization that coordinates air traffic in Europe, said more than 100,000 flights were canceled between 15 and 21 April 2010, and about 10 million passengers could not travel. In May the ash drifted again, and some flights had to change their routes.

Fine ash from the summit phase blew over the Atlantic and, for long periods, directly over Europe. The Global Volcanism Program describes the result as the halting of most commercial flights for nearly a week, in what it calls a controversial shutdown with economic impacts in the billions; Britannica notes the threat that ash poses to aircraft engines.

Al Jazeera, reporting EUROCONTROL's figures in May 2010, said more than 100,000 flights to, from and within Europe were canceled between 15 and 21 April, preventing about 10 million passengers from traveling. When the plume drifted again in early May, EUROCONTROL reported that most flights between Europe and North America were being rerouted north and south of an ash cloud about 2,000 km long, with potential contamination expanding between the ground and 20,000 feet.

Below the ice cap are green valleys and rivers.

After the eruption, ash covered the land. Wind blew some of it far away. Rain and snow slowly covered the rest.

Around the ice cap are steep slopes, glacier tongues and valleys, such as Þórsmörk. During the eruption, ash fell over nearby land, and wind sometimes picked it up again. On 4 June 2010, the Icelandic Meteorological Office reported loose ash blowing across south and southwest Iceland for a few hours.

The eruption also changed the land. Floods from melting ice cut deeper canyons below the Gígjökull glacier, and a lake formed in the summit crater in June. By October, the first winter snow lay on the cooling new lava at the top.

The Icelandic Meteorological Office's eruption diary documents the landscape changes: glacial bursts on 14-15 April enlarged the canyon below the Gígjökull outlet glacier in the palagonite and formed another to the west, with a sedimentary fan built about 50 m above the former lake level; a crater lake had formed by 11 June; and ice at the summit advanced rapidly toward Gígjökull.

Ash remained mobile after it fell. The office recorded high airborne dust concentrations in Reykjavík on some days and a few hours of wind-blown ash across south and southwest Iceland on 4 June 2010. A photo taken on 9 October 2010 shows the first winter snow on the summit, with snow melted from warm ash- and lava-covered surfaces, and the Global Volcanism Program notes that the summit area was still steaming and that full cooling was expected to take a few years.

Icelanders keep watch on their volcanoes all the time.

Visit with a grown-up and stay on the marked paths. Never walk on a glacier without a guide. It can have deep cracks.

Iceland's volcanoes are watched closely. During the 2010 eruption, the Icelandic Meteorological Office and the University of Iceland's Institute of Earth Sciences issued regular status reports about earthquakes, ash, steam and floods, and stopped only when activity had died down.

Glaciers and volcano slopes can be dangerous even when all is quiet: there can be hidden cracks in the ice, sudden floods and changing weather. Families should stay on marked trails, check warnings before setting out, and walk on glacier ice only with a trained guide.

Monitoring was central to the response in 2010. The Icelandic Meteorological Office and the Institute of Earth Sciences issued joint status reports through the eruption, using seismic tremor, earthquake locations, GPS deformation, web cameras and overflights, and continued to watch for floods from the crater lake after the eruption ended; the Global Volcanism Program notes earthquakes along the volcano's north-south axis during 14-29 April.

For visitors, the hazards are those of any ice-capped volcano: glacial floods, crevasses and fast-changing weather, plus the possibility of renewed unrest. The Icelandic Meteorological Office publishes current information on volcanic and seismic activity, so hikers should check it before crossing high ground such as the Fimmvörðuháls pass, stay on marked routes and go onto glacier ice only with qualified guides.