An eight-month catastrophe
On June 8, 1783, a 25-kilometer-long fissure in the Earth's crust tore open in southern Iceland. Part of the Grímsvötn volcanic system, this was not a mountain eruption. Instead, a series of over 130 vents, known as the Lakagígar craters, began to spew molten rock in an event called a basaltic fissure eruption. For eight months, until February 1784, the ground itself bled lava.
The sheer volume of the eruption is difficult to comprehend. It produced an estimated 14 cubic kilometers of basalt lava, enough to cover nearly 600 square kilometers of land. But the lava flows, which destroyed numerous farms, were the beginning of the disaster. The eruption also released colossal quantities of volcanic gases into the atmosphere. An estimated 120 million tons of sulfur dioxide (SO₂) and 8 million tons of hydrogen fluoride (HF) were ejected high into the troposphere and stratosphere.
A global winter and a deadly haze
The atmospheric effects were immediate and catastrophic. The sulfur dioxide reacted with water vapor to form a dense aerosol of sulfuric acid. This created a persistent "dry fog" or "Laki haze" that spread across the Northern Hemisphere, reaching the rest of Europe within weeks. Naturalist Gilbert White in England described the sun at noon as being "as blank as a clouded moon, and shed a rust-coloured ferruginous light on the ground."
This haze acted like a planetary sunshade, causing global temperatures to drop. The winter of 1783-1784 was exceptionally severe across Europe and North America, with reports of the Mississippi River freezing in New Orleans. In Iceland, the consequences, known as the Móðuharðindin or "Mist Hardships," were horrific. The hydrogen fluoride poisoned pastures, leading to skeletal fluorosis that killed over 50% of Iceland's cattle, 80% of its sheep, and 50% of its horses. The resulting famine killed an estimated 9,000 people, about 20-25% of the island's population.
The devastation spread far beyond Iceland. The acid haze damaged crops across Europe, and respiratory illnesses caused by the pollution led to tens of thousands of deaths. In Great Britain alone, an estimated 23,000 people died from inhaling the sulfurous compounds. The years of failed harvests and food shortages that followed are believed by many historians to have exacerbated social tensions in France, contributing to the conditions that led to the French Revolution in 1789.