It poured out an estimated 42 billion tonnes or 14 km3 (18×10^9 cu yd) of basalt lava as well as clouds of poisonous hydrofluoric acid and sulfur dioxide compounds that contaminated the soil, leading to the death of over 50% of Iceland's livestock population, and the destruction of the vast majority of all crops.

[5] The Laki eruption and its aftermath caused a drop in global temperatures, as 120 million tonnes of sulfur dioxide was spewed into the Northern Hemisphere.

On 8 June 1783, a 25 km-long (15.5 mi) fissure of at least 130 vents opened with phreatomagmatic explosions because of the groundwater interacting with the rising basalt magma.

One study states that the event "occurred as ten pulses of activity, each starting with a short-lived explosive phase followed by a long-lived period of fire-fountaining".

[13] The parish minister and provost of Vestur-Skaftafellssýsla, Jón Steingrímsson (1728–1791), grew famous for the eldmessa [ˈɛltˌmɛsːa] ("fire mass") that he delivered on 20 July 1783.

This past week, and the two prior to it, more poison fell from the sky than words can describe: ash, volcanic hairs, rain full of sulfur and saltpeter, all of it mixed with sand.

[16]There is evidence that the Laki eruption weakened African and Indian monsoon circulations, leading to between 1 and 3 millimetres (0.04 and 0.12 in) less daily precipitation than normal over the Sahel of Africa, resulting in, among other effects, low flow in the River Nile.

[14][4] This outpouring of sulfur dioxide during unusual weather conditions caused a thick haze to spread across western Europe, resulting in many thousands of deaths throughout the remainder of 1783 and the winter of 1784.

In France, the sequence of extreme weather events included a failed harvest in 1785 that caused poverty for rural workers, as well as droughts, bad winters and summers.

At the time, the capital of the United States was situated on the Chesapeake at Annapolis, Maryland; the weather delayed Congressmen who were traveling there to vote for the Treaty of Paris, which formally ended the American Revolutionary War.

[26][29] Gilbert White recorded his perceptions of the event at Selborne, Hampshire, England: The summer of the year 1783 was an amazing and portentous one, and full of horrible phaenomena; for besides the alarming meteors and tremendous thunder-storms that affrighted and distressed the different counties of this kingdom, the peculiar haze, or smokey fog, that prevailed for many weeks in this island, and in every part of Europe, and even beyond its limits, was a most extraordinary appearance, unlike anything known within the memory of man.

By my journal I find that I had noticed this strange occurrence from June 23 to July 20 inclusive, during which period the wind varied to every quarter without making any alteration in the air.

The sun, at noon, looked as blank as a clouded moon, and shed a rust-coloured ferruginous light on the ground, and floors of rooms; but was particularly lurid and blood-coloured at rising and setting.

All the time the heat was so intense that butchers' meat could hardly be eaten on the day after it was killed; and the flies swarmed so in the lanes and hedges that they rendered the horses half frantic, and riding irksome.

The country people began to look, with a superstitious awe, at the red, louring aspect of the sun; ...[30]Benjamin Franklin recorded his observations in America in a 1784 lecture: During several of the summer months of the year 1783, when the effect of the sun's rays to heat the earth in these northern regions should have been greater, there existed a constant fog over all Europe, and a great part of North America.

Sir John Cullum of Bury St Edmunds, Suffolk, England, recorded his observations on 23 June 1783 (the same date on which Gilbert White noted the onset of the unusual atmospheric phenomena), in a letter to Sir Joseph Banks, then President of the Royal Society: ... about six o'clock, that morning, I observed the air very much condensed in my chamber-window; and, upon getting up, was informed by a tenant that finding himself cold in bed, about three o'clock in the morning, he looked out at his window, and to his great surprise saw the ground covered with a white frost: and I was assured that two men at Barton, about three miles (five kilometres) off, saw in some shallow tubs, ice of the thickness of a crown-piece.