The Volcanic Crack in Iceland That Killed a Sixth of Iceland and Helped Bring Down the French King

The eruption began on the morning of 8 June 1783 at a fissure in the unpopulated highlands of southern Iceland. It was not the polite kind of volcanic eruption that produces a single spectacular column over a recognizable mountain. The Laki fissure — Lakagígar in Icelandic, “the craters at Laki” — opened along a 27-kilometre crack in the basalt bedrock and erupted simultaneously from 140 distinct vents along its length. The lava flows that poured out covered approximately 600 square kilometres of southern Iceland over the following eight months. By volume of lava expelled (about 14 cubic kilometres), Laki was the largest basaltic fissure eruption in recorded human history.

The lava itself killed nobody. The eruption killed by gas, ash, and crop failure.

In Iceland

Approximately 122 tonnes of sulphur dioxide per second poured out of the fissure during the eruption’s peak phase. The total sulphur dioxide release over eight months has been estimated at approximately 120 million tonnes — about ten times the annual modern global anthropogenic sulphur release. The gas mixed with atmospheric moisture and fell back onto southern Iceland as a sulphate-rich acidic mist that killed approximately 80% of the island’s sheep, 50% of the cattle, and 50% of the horses over the following twelve months. The Icelandic agricultural economy collapsed.

The resulting famine — known to subsequent Icelandic history as the Móðuharðindin (“the mist hardships”) — killed approximately 10,000 Icelanders out of a pre-eruption population of about 49,000. The mortality rate was approximately 21% in a single year — the worst proportional natural disaster in any European country in the post-medieval period. The Danish Crown, which administered Iceland, briefly considered evacuating the entire surviving population of about 39,000 to the Jutland mainland.

The most-cited contemporary Icelandic account was the field journal of Jón Steingrímsson, the Lutheran pastor of the small parish of Síða, directly downwind from the eruption. His diary covers seven months in unbroken daily detail; he describes the gas, the death of his livestock, the death of his parishioners, the breakdown of the religious-social order under famine conditions, and his repeated direct sermons to the lava (the most famous Icelandic religious moment of the 18th century — the Eldmessa or Fire Sermon of 20 July 1783, in which he preached at the advancing lava front and the flow stopped 200 metres short of his church). The text was published posthumously in 1816 and is one of the foundational documents of modern Icelandic literature.

In Europe

The sulphate cloud was carried east on the prevailing summer winds and reached the British Isles within ten days. The eruption coincided with — and was partly responsible for — what English contemporaries called the summer of the great fogge: an unusually thick, persistent, sulphur-smelling haze that hung over much of northwestern Europe through July, August, and September 1783.

The naturalist Gilbert White at Selborne in Hampshire wrote the most thorough English account, recording the haze, the substantially elevated daytime temperatures (the sun could be looked at directly through the haze without injury), the substantial summer mortality (Britain experienced perhaps 23,000 excess deaths in the summer of 1783, mostly from sulphate-related respiratory damage), and the failure of multiple agricultural crops across the affected region. The American polymath Benjamin Franklin, then ambassador to France, wrote one of the earliest scientific speculations about the cause — he correctly hypothesized a substantial volcanic source somewhere in the high northern latitudes.

The 1783 sulphate aerosol blocked enough sunlight over the following two years to produce one of the coldest northern hemisphere winters on record (1783-84) and substantial agricultural disruption across Europe through 1784 and 1785. Grain prices in France rose approximately 75% over the period. Rural French food security progressively deteriorated through the late 1780s, particularly after a second crop failure in 1788.

The French Revolution

The direct connection between the Laki eruption and the French Revolution of 1789 is contested but plausible. The chain of causation runs through agricultural prices: Laki contributed to several years of substandard French harvests in the mid-1780s; the 1788 harvest was the worst in two decades; bread prices in Paris in spring 1789 reached approximately 88% of the average daily wage of a French urban worker; the bread riots of April 1789 and the political-revolutionary mobilization of May-July 1789 occurred against the immediate background of widespread urban food insecurity.

The traditional historiographical account of the Revolution emphasizes political and fiscal causes — the Bourbon monarchy’s bankruptcy, the political-constitutional crisis of the Estates-General, the failure of the Bourbon reform program — and treats the bad harvests as a secondary aggravating factor. Modern environmental-history work since the 2010s has produced a slightly more direct case: the Laki gas-cloud effects on European agriculture from 1783 to 1785 were among the more substantial natural-climate disruptions of the 18th century, and the specific French political-economic crisis of 1788-1789 took place against the background of agricultural recovery that was substantially slower than expected because of the lingering effects of the eruption.

The causal weight is impossible to settle definitively. What is not contested is the temporal correlation: Laki erupted in June 1783; the Estates-General convened in May 1789; the Bastille fell on 14 July 1789; Louis XVI was executed on 21 January 1793. The eruption and the political collapse are connected by approximately six years and several substantial agricultural and fiscal mediating events.

The site

The Lakagígar crater row is now part of the Vatnajökull National Park. The fissure itself, the volcanic cones along its length, and the substantial lava field that surrounds it are visible from the air and from a network of well-maintained Icelandic hiking trails. The eruption has been continuously studied as the largest historical analog for the kind of basaltic fissure eruption that has occurred several times in Icelandic prehistory — the Eldgjá eruption of 934, the Veiðivötn eruption of 1477, and several others — any of which would, if it recurred in the modern period, produce substantially similar atmospheric and agricultural effects across Europe. The 2010 eruption of Eyjafjallajökull (which closed European airspace for six days) was a much smaller event than Laki by a factor of approximately one hundred.