The Eldgjá eruption produced a noticeable cooling of the climate, with resulting cold winters and food crises across Eurasia.

[5] Glaciation has influenced volcanic activity on Iceland, and the occurrence of large eruptions—such as the 25 km3 (6.0 cu mi) Þjórsá Lava 8,600 years ago—in the early Holocene has been attributed to the unloading of the crust caused by the melting of Pleistocene ice.

[9] The rocks erupted by Eldgjá are mainly alkali basalts, which have a uniform composition and contain phenocrysts of clinopyroxene, olivine, magnetite and plagioclase.

[32] The Eldgjá is part of the wider Katla volcano, which features a series of fissures, as well as a caldera covered by the Myrdalsjökull Ice Cap.

[24] To the northeast, the lineament runs 5 kilometres (3.1 mi) away from and parallel to that of the 1783-1784 CE Laki eruption fissure,[33][34] which is part of the Grimsvötn volcano.

[38] It involved a 75 km (47 mi) long area of the volcano, including both the central caldera and the Eldgjá lineament.

[34] These episodes did not occur simultaneously across the entire length of the Eldgjá; rather the eruption commenced in the caldera and propagated northeastward.

[51] The eruption produced two fields of (mostly pahoehoe[52]) lava flows[27] emanating from the southern and central sectors of the Eldgjá fracture.

[54] The lavas buried traces of earlier eruptions[12] and obstructed river valleys, forcing the rivers to change their course, and altered the terrain so that large parts of the plains east of Katla can no longer be reached by jökulhlaups (glacier meltwater flood) from the volcano.

[60] External water (such as from ice melt) did not play a major role in driving the explosivity of the eruption.

[70] Large volcanic eruptions can produce veils of aerosols in the atmosphere from sulfur dioxide, which reduce the amount of sunlight reaching Earth's surface and alter its climate.

[80] Conversely, increased flooding in Europe after the Eldgjá and other volcanic eruptions during the 10th century have been correlated to declines in Poland's Alnus trees.

[88] The events and impact of the eruption may have stopped the settlement of the island[89][90] and could have played a role in stimulating the Christianization of Iceland.

[91] However, it is possible that Iceland's population at the time was more resilient than during the 18th century and thus the Eldgjá event had less impact than the Laki eruption.

The area became populated before it was overrun by jarðeldur (an earth fire), then they fled west to Höfðabrekka and set up a camp at TjaldavellirUnlike the local impacts on Iceland, the effects of the Eldgjá eruption on Europe appear in the historical record.

[95][96] Reportedly, winters in Europe and China between 939-942 were severe, with the sea and canals freezing, while droughts occurred during the summer months.

Food crises reported in China, the Maghreb, the Levant and Western Europe at that time have been linked to the Eldgjá eruption.

[80] More tentatively, the downfall of the Later Jin Dynasty[97] and locust plagues in China,[98] epidemics of animal diseases in Europe[99] and a decrease of human activity on Ireland[100] and rebellions in Japan have been connected to the Eldgjá eruption.

The much smaller (0.27±0.07 km3) 2010 eruption of Eyjafjallajökull caused worldwide disruptions of air travel, with economic losses of over $1 billion for airlines alone,[36] because volcanic ash can interfere with the operation of airplane engines.